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An Understanding of the Modulation of Photophysical Properties of Curcumin inside a Micelle Formed by an Ionic Liquid: A New Possibility of Tunable Drug Delivery System

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Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India
*E-mail: [email protected]. Fax: 91-3222-255303.
Cite this: J. Phys. Chem. B 2012, 116, 10, 3369–3379
Publication Date (Web):February 10, 2012
https://doi.org/10.1021/jp211242c

Copyright © 2012 American Chemical Society. This publication is licensed under these Terms of Use.

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Abstract

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The present study reveals the modulation of photophysical properties of curcumin, an important drug for numerous reasons, inside a micellar environment formed by a surfactant-like ionic liquid (IL-micelle) in aqueous solution. Higher stability of the drug inside IL-micelle in the absence and presence of a simple salt (sodium chloride) as well as considerably large partition coefficient (Kp = 8.59 × 103) to the micellar phase from water make this system a well behaved drug loading vehicle. Remarkable change in fluorescence intensity with a strong blue-shift implies the gradual perturbation of intramolecular hydrogen bond (H-bond) present within the keto–enol group of curcumin along with considerable formation of intermolecular H-bond between curcumin and the headgroup of surfactant-like IL. Very fast nonradiative decay channels in curcumin mainly caused by the excited state intramolecular proton transfer (ESIPT) are thus depleted remarkably in the presence of IL-micelle of reduced polarity and as a result of restricted rotational and vibrational degrees of freedom when bound to the micelle. Moreover, time-resolved results confirm that not only the keto–enol group of curcumin is playing here but also the phenolic hydroxyl groups are also responsible for such modulation in photophysical properties. From a thermodynamic point of view, our system shows good correlation with its stability parameters (higher binding constant with very less hydrolytic degradation rate ∼1%) and higher negative value of binding enthalpy of interaction (−ΔH) than total free energy change (−ΔG) implies that the nature of binding interaction is enthalpy driven not entropy alone. Summarizing all the above observations, we have concluded that the modulation of the intramolecular proton transfer is due to the presence of both intermolecular proton transfer as well as strong hydrophobic interaction between curcumin and the IL-micelle.

1 Introduction

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Curcumin (diferuloylmethane), the Indian solid gold, the major active component of turmeric, Curcuma longa L. (Zingiberaceae), is used as a spice in curry and as a coloring agent in yellow mustards, cosmetics, pharmaceuticals, and hair dyes. (1) It has attracted great interest in recent years because of its antioxidant, anti-inflammatory, and potential cancer chemopreventive activities. (2-9) Recently, curcumin has also been found to bind to α-amyloid proteins in models of Alzheimer’s disease. (10) Moreover, curcumin can inhibit the metabolic action of aflatoxin B1, (11) aminopeptidase N, (12) lipoxygenase, (13) cycloxygenase, (14) and ornithine decarboxylase. (15) Finally, it seems to have a potential in the treatment of cystic fibrosis (16) and can be considered as a model substance for the treatment of HIV infection. (17-19) In considering potential mechanisms for its range of biological activities, it is important to acknowledge the inherent chemical features of the curcumin molecule. Although the systematic name for curcumin 1, 7-bis(4-hydroxy-3-methoxyphenyl)-1,6-hepatadiene-3,5-dione, implies that curcumin is a α-diketone tautomer. X-ray crystal structure analysis has established that curcumin and its bisacetoxy derivative exist as a keto–enol form. (20, 21) Recently, the 1H and 13C NMR studies have confirmed the presence of the enol form in nonpolar, polar, and protic solvents with NMR spectroscopy. (22) Theoretical results also support the existence of the enolic form having perfect resonance between the two phenolic rings, and the electron density is distributed on the entire molecule. (23-27) The enolic form can exist in different cis and trans isomeric forms depending on the temperature, polarity, or hydrogen bonding nature of the solvents. (28, 29) Formation of the cis keto–enol structure becomes preferred because of its large dipole moment (7.7 and 10.8 D in the gas and solution phases, respectively), which leads to formation of a strong intramolecular H-bond, as well as the extended conjugation of the molecular backbone compared with that of the diketo form. (23, 30) In fact, the keto–enol form can be considered as coexisting in two equivalent keto–enol tautomers which may interconvert between each other via an intramolecular hydrogen atom transfer (IHT) process. (26) Very recently, Adhikari et al. proposed that cyclocurcumin also undergoes photoinduced cis–trans isomerization and the trans form is more stable than the cis form. (31) Several recent studies in various solvents, (32-38) micelles, (36, 38, 39) vesicles, (35, 40, 41) etc., confirm the fastest nonradiative S1-decay kinetics of curcumin proceeds through an excited state intramolecular proton transfer (ESIPT) between the hydroxyl group and the keto group of curcumin followed by radiationless excited-tautomer decay in closed cis-enol tautomer. Support to the occurrence of radiationless decays of curcumin through ESIPT paths has been provided in a recent femtosecond fluorescence upconversion study. (42)
To better understand the physicochemical basis of interaction of curcumin with other biological/chemical molecules, it is necessary to study its fundamental spectroscopic and physicochemical properties. However, the major problem with curcumin is its extremely low solubility in aqueous solution (2.99 × 10–8 M) and its poor bioavailability, which limits its clinical efficacy. (42-44) Curcumin is stable at low pH in aqueous alcohol solutions but undergoes hydrolysis and chemical degradation at basic pH. (45, 46) Even under physiological pH conditions, degradation was significant. To resolve this serious issue, attempts have been made through encapsulation in polymeric micelles, liposomes, polymeric nanoparticles, lipid-based nanoparticles, and hydrogels to increase its aqueous solubility and bioavailability. (47-54) Encapsulation of hydrophobic drug molecules inside an aqueous nanoparticulate system has been attempted so as to deliver such drugs with their full potential. In our present work, we have used a micellar system formed by an ionic liquid (IL), 1-butyl-3-methylimidazolium octyl sulfate ([C4mim][C8SO4]), in aqueous solution. A special feature of this IL is that it can form micelles (CMC = 31–37 mM) in water. (55) In recent studies, there is a tremendous interest to investigate photophysical, chemical properties of many compounds in room temperature ionic liquids (RTILs). Moreover, the effect of addition of polar solvent to RTILs and RTILs containing microemulsions has received much attention. Several groups prepared and characterized RTILs containing micelles and microemulsions. (56-60) The self-aggregation behavior of RTILs in aqueous solution has recently attracted much attention due to structural similarities of RTILs with ionic surfactants, and the thermodynamics and phase equilibria of the ionic liquid water mixtures have been studied in detail (61) because this knowledge is of great importance to the development of extraction methods. Another reason for choosing [C4mim][C8SO4] is due to its greenness with respect to other commonly used RTILs such as bmimPF6, bmimBF4, etc. [C4mim][C8SO4] can undergo modest biodegradation, while other commonly used RTILs show negligible biodegradation. (62-64) Generally, the C4mim ionic liquids are treated as poor candidates in close bottle tests (OECD 301D) for their biodegradability. (65) 1-Butyl-3-methylimidazolium octylsulfate was the only ionic liquid that experienced a significant level of degradation (25%), but the level of achievement was 60%. (62) If the side chain of the imidazolium cation contains an ester group, then it was found to have higher biodegradability with all of the anions that were tested. The combination of such ester containing cation and the octyl sulfate anion shows the highest biodegradability in this series (in closed bottle test). (62) However, all these ILs have been proved that they are toxic enough for using in our body system and it is still now a debatable issue. (62, 66-71) However, there is now a growing interest in the materials applications of ILs which utilize novel tunable physical and chemical properties and more recently the chemical properties, the toxicity, a biological property has been one of the most highly debated topics in this field. (72) Indeed, toxicity of the ILs is also a tunable property of ILs, and given the similarities between many common IL building blocks and active pharmaceutical ingredients (APIs) or API precursors. (73) Research on modulation of molecular architecture of ionic liquids makes them a wide range of newly formed compounds having tunable active pharmaceutical ingredients with the advantage of higher solubility, bioavailability, modest stability, etc. For example, lidocaine docusate, ranitidine docusate, and didecyldimethylammonium ibuprofen are the newly synthesized drugs which have controlled and modified efficacy compared to their precursors. (74) An ionic liquid-in-oil microemulsion (Tween80/span20/dimethylimidazolium dimethylphosphate) has been used as a potential carrier of the sparingly soluble drug acyclovir (ACV) for excellent solubility and skin permeation enhancing effect. (75) Recently, it was observed that ionic liquid can act as dual functional antimicrobial agents and plasticizers in medical devices. (76)
Herein, we have carried out a spectroscopic investigation of curcumin in a micellar environment formed by an ionic liquid. We have also determined the partition coefficient of curcumin inside IL-micelle and their binding constant in aqueous solution. A temperature dependent study as well as a salt addition study has also been carried out to get an idea about the location of the drug molecule and the nature of interaction with the micelle. We also want to monitor the hydrolytic stability of this compound inside IL containing micelle which can be useful for its use as a drug carrier in the near future. Our main objective is to unravel the photophysical properties of curcumin and its nature of hydrogen bonding and hydrophobic interaction with RTILs containing confined environment. Another important aspect to understand the sensitivity of the fluorescence properties of curcumin with change in polarity around its microenvironment in micelle is also discussed here. More importantly, by using simple photophysical spectroscopy, here we want to get detailed knowledge about a stable drug–micelle system, which has a wide range of tunable properties to make it very special in pharmaceutical research.

2 Experimental Section

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2.1 Materials

[C4mim][C8SO4] was obtained from Fluka and purified according to the literature procedure. (77) Curcumin obtained from Sigma Aldrich (purity ∼81%) was purified by the column chromatographic technique, and the purity of the sample used (∼99%) was checked by the HPLC technique. (38) The structures of the IL and curcumin are given in Scheme 1. Milli-Q water was used to prepare all solutions. The concentration of curcumin in the experiment was kept at 4 × 10–6 M.

Scheme 1

Scheme 1. Structure of Curcumin and the Ionic Liquid
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Instruments and Methods

The absorption and emission spectrum was measured with a Shimadzu (model UV 2450) UV–vis spectrophotometer and Jobin Yvon-fluoromax-3. All the fluorescence emission spectra and steady state anisotropy at a particular wavelength were corrected for the wavelength sensitivity of the detection system. Steady state anisotropy is defined as (78)(1)where G is the correction factor. IVV and IVH are the fluorescence decays polarized parallel and perpendicular to the polarization of the excitation light, respectively.
Fluorescence lifetimes were obtained from a time-correlated single photon counting (TCSPC) spectrometer using nano-LED (IBH, U.K.) as the light source at 408 nm. The experimental setup for picosecond time correlated single photon counting (TCSPC) has been described elsewhere. (79) Briefly, the samples were excited at 408 nm using a picosecond laser diode (IBH, Nanoled), and the signals were collected at the magic angle (54.7°) using a Hamamatsu microchannel plate photomultiplier tube (3809U). The instrument response function of our setup is 90 ps.
The average fluorescence lifetimes for the decay curves were calculated from the decay times and the relative contribution of the components using the following equation (78)(2)where τ1 and τ2 are the first and second components of decay time of curcumin and a1 and a2 are the corresponding relative weightage of these components.
We have calculated the quantum yield of curcumin with the following equation (78)(3)where ΦX and ΦST are the quantum yield of the experimental solution and standard solution taken. AX and AST are the integrated area under the fluorescence curve, and AbsX and AbsST are the absorbances. Quinine sulfate in 0.1 N H2SO4 solutions was taken as a standard solution.
For size measurement, we have done dynamic light scattering measurement using a Malvern NanoZS employing a 4 mW He–Ne laser (λ = 632.8 nm). Temperature was maintained by using a circulating temperature bath.

3 Results and Discussion

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3.1 Steady State Results

3.1.1 Effect of IL Concentration

3.1.1.a UV-Fluorescence Results
Although curcumin is barely soluble in water, the solubility is considerably increased in the presence of a surfactant-like long chain ionic liquid containing aqueous solution. The presence of the micellar phase is the reason for this increment, and it indicates that the partition of curcumin to the micellar phase from the bulk water is large enough. Absorption and fluorescence spectra of curcumin have already been well characterized by different groups in different kinds of solvents and media. (8, 38, 80-82) Curcumin shows strong absorption bands at 426, 423, and 432 nm in TX-100, DTAB, and SDS micelles, respectively. (83) In our study, we have obtained a structured and intense peak at 431 nm with another peak at 350 nm in water, but it gradually blue-shifted to 423 nm upon addition of IL (Figures 1 and 2 in the Supporting Information). Curcumin absorption is π–π* type in nature, and gradual increment in absorbance is observed with addition of IL in aqueous solution. The presence of an additional peak at 350 nm and absence of any shoulder at 442 nm in pure water and the absence of the peak at 350 nm and presence of the shoulder peak at 442 nm after addition of the IL suggests that curcumin is present in a vastly different environment in the presence of IL in water. Close inspection of the UV spectra shows that it is very similar to the UV spectra that were observed inside TX-100 and DTAB micelle and in methanol and DMSO. (84) However, such a shoulder in the absorption peak is absent for SDS micelle and such observation was explained by the presence of strong interaction of water with curcumin located in the Stern layer. This information helps us to consider that in our case curcumin is located far inside from the hydrodynamic shear surface of the micelle.
Similar to absorption spectra, a large blue-shift of 30 nm is obtained for fluorescence spectra upon addition of the ionic liquid up to 60 mM (Figure 1).

Figure 1

Figure 1. Fluorescence spectra of curcumin (left) with change in [bmimOs] and (right) with change in [NaCl]. Plots of variation of emission intensity and emission maxima of curcumin are given in the inset.

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Measurement of the fluorescence Stokes shift of a probe molecule is a very useful technique to measure reorganization of the probe’s local environment. Because of changes in the chromophore’s properties (charge distribution, polarizability, size, etc.), the excited-state energy minimizes at a different value of the reorganization coordinate compared to the ground state. Absorption occurs from molecules equilibrated to a distribution about the ground-state minimum. If there is no environmental relaxation, the fluorescence occurs at the same frequency as the absorption, and there is no Stokes shift. If the environment relaxes completely, the fluorescence is red-shifted, and the full equilibrium Stokes shift is observed. Moreover, Stokes shifts are highly sensitive to local motions, weakly sensitive to collective internal motions, and completely insensitive to overall tumbling. Herein, the absorption and emission spectra of curcumin show a Stokes shift of 4716 cm–1 and it is shifted to 3423 cm–1 in 60.8 mM IL-micellar solution (Table 1). Emission maxima of curcumin inside TX-100 and DTAB micelle (83) are at ∼500 nm which is very close to that in our system. Similar values of Stokes shift of curcumin were found for TX-100, DTAB micelle, (83) and also in the presence of HSA (85) compared to our system. It was reported that curcumin is strongly bound in the hydrophobic pocket in the proteins and situated inside the palisade layer of TX-100, DTAB (83) micelle. Accordingly, it can be guessed that the microenvironment around curcumin inside IL-micelle is hydrophobic in nature. Upon gradual addition of IL, the emission maximum of curcumin is blue-shifted with steep increment as the polarity around curcumin molecule is continuously changed. Moreover, spectral widths of fluorescence spectra also decrease considerably with increase in IL-concentration. This absorption and emission behavior of curcumin suggests that the S1 state has large intramolecular charge transfer character inside the IL-micellar aqueous solution. We got an apparent idea about the polarity of the micellar environment by using a plot of Stokes shift versus ET(30) of a series of alcohols (CnOH, where n = 1, 2, 3, 4, 5, 6, 8) as a reference scale (Figure 2a). Increment in fluorescence intensity with decrease in ET(30) upon addition of IL clearly proves that curcumin undergoes a favorable transfer from water-rich region to more hydrophobic region of the micelle, and a linear plot of Stokes shift with the solvent polarity parameter ET(30) values of the surroundings of curcumin proves the presence of strong hydrogen bonding interaction present in this IL micelle–curcumin system (Table 1, Figure 2b). Strong blue shift in emission spectra with considerable increase in quantum yield (0.051) in the IL micellar phase clearly indicates that the polarity around the drug is less than that in the bulk water phase. This can be explained by the fact that weaker H-bonding interaction in an environment having lower ET(30) reduces the efficiency of the nonradiative deactivation process. Increment in lifetime is also observed with a decrease in ET(30) value which is discussed in a forthcoming section.

Figure 2

Figure 2. (a) Plot of Stokes shift vs ET(30) of a series of alcohols (CnOH, where n = 1, 2, 3, 4, 5, 6, 8). (b) Plot of Stokes shift vs ET(30) of the surrounding environment of curcumin at different concentrations of IL.

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Table 1. Stokes Shift and ET(30) Values of the Curcumin–bmimOs System
curcumin in ET(30) Stokes shift (cm–1)
7.6 mM bmimOs 49.34 4427
15.2 mM 48.44 4242
22.8 mM 47.65 4053
30.4 mM 47.02 3938
38.0 mM 46.52 3815
45.6 mM 45.95 3703
53.2 mM 45.39 3584
60.8 mM 44.87 3452
3.1.1.b Determination of Partition Coefficient
To establish curcumin and the IL-micelle as a potential drug loaded system, one should have the quantitative idea of penetration of the drug into the micellar system when normally dissolved in a solvent. Strong support of the existence of curcumin inside micelle was confirmed by determination of the partition coefficient. By using fluorescence data in eq 4, one can get the quantitative estimation of the extent of penetration of curcumin and also a qualitative idea about the interaction of curcumin with the surfactant-like IL. (86-88)(4)where I∞, It, and I0 are the fluorescence intensities of curcumin at saturated IL concentration, an intermediate IL concentration, and in the absence of IL, respectively. Kp is the partition coefficient of curcumin to the micellar phase from the bulk water. From the slope of the (I∞ – I0)/(It – I0) vs [surfactant]−1 plot, we got the value of Kp = 8.59 × 103 which is quite large for such a newly studied system (Figure 3). Such considerable partitioning of the drug inside IL micelle indicates the favorable interaction between the drug and the micelles leading to a stable system.

Figure 3

Figure 3. Plot of (I∞ – I0)/(It – I0) vs [bmimOs]–1 for partition coefficient determination.

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3.1.1.c Determination of Steady State Anisotropy (r0)
Moreover, we found another way to monitor the transfer of curcumin by measuring its degree of rotation which is the reflection of restriction imposed by the surfactant assembly. From a systematic observation of the change in steady state anisotropy with addition of surfactant, one can get fruitful information about the microenvironment around the probe molecule. (78, 89-92) Steady state anisotropy is thus a direct proof of the firmness of the drug moiety, and we got gradual increment in anisotropy (r0) upon addition of IL followed by a plateau at higher concentration by using eq 1. Around the CMC, there is an inflection indicating the change in microenvironment sensed by the drug molecule inside the micelle. A higher value of anisotropy (Figure 4) above the critical micellar concentration (CMC) implies that the drug molecule is rigidly held inside the micellar region. Gradual increase in anisotropy reflects gradual increase in rigidity of the drug inside micelle, which in turn means that the drug is going to a more and more restricted region inside the micelle. This fact was corroborated by a huge blue shift in the steady state emission spectra.

Figure 4

Figure 4. Steady state anisotropy vs concentration of bmimOs.

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3.1.1.d Determination of Binding Constant (Kb)
The organized micellar media can serve to protect the compound from external perturbations in solution; also, entrapment within the micellar cavity may result in appreciable modulation in rates of various physical and/or chemical processes. To what extent it can serve as a quite good carrier of any compound, one should know the stability or the binding constant of that compound with the surfactant in solution. Quantitative estimation of binding ability between curcumin and the IL micelles in water was determined by the method as described by Almgren et al. (93) This method can be described by the following equation(5)where I0, It, and I∞ are the emission intensities of curcumin in the absence of IL, at an intermediate concentration of IL, and at a condition of saturation, respectively, and Kb is the binding constant with the micellar system when [M] is described by the following way(6)in which [S] is the IL-surfactant concentration and Nagg is the aggregation number of the micellar system. Variation in (I∞ – I0)/(It – I0) vs [M]−1 gives a linear plot from which we have easily determined the binding constant. From Figure 5, we have obtained a value of 2.08 × 104 M–1 for the binding constant which is large enough and support for a favorable interactive system. Such high value of binding constant can be attributed to some specific type of interactions such as strong van der Waals interaction between the hydrophobic octyl chain of the IL and two phenyl moieties of curcumin or maybe π-stacking between the aromatic headgroups of the micelle and the two phenyl moieties of curcumin. The pH of our micellar solution is 6.88 in which curcumin is in neutral form, which means the participation of the hydroxyl group of the two phenyl moiety as a H-bonding site cannot be ruled out.

Figure 5

Figure 5. Plot of (I∞ – I0)/(It – I0) vs [bmimOs]−1 for binding constant determination.

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3.1.2 Effect of Salt

A monotonic increase in emission spectra is observed for NaCl addition to the micellar solution (Figure 1). During salt addition, curcumin undergoes considerable blue-shift from 512 to 496 nm but very negligible shift is observed in absorption spectra. This clearly indicates that the presence of salt has no effect on ground state but a profound effect on excited state phenomenon; i.e., ESIPT of curcumin is largely influenced by the addition of a high molar concentration of salt. It is quite expected because salts are known as a good candidate for H-bond breaking and H-bond making substances. The presence of NaCl facilitates the formation of intermolecular H-bonding and slows down the ESIPT process. It was reported that bmim+ and Os– show strong H-bonding interaction and addition of salt affect their network. (94) As a consequence, a new network of H-bonding can be formed between salt and surfactant. Moreover, in the absence of strong H-bonding between surfactant cation and anion, the chance of formation of intermolecular H-bonding between curcumin and surfactant anion also cannot be ruled out.
We have successfully determined the CMC of the IL-surfactant by using curcumin fluorescence in the presence of different salt concentrations, and it was found that the CMC is drastically decreased in the presence of a high concentration of NaCl (Figure 6). The CMC is lowered to 21 mM from 34 mM in the presence of 1.5 M NaCl, which means that the micellar transition of curcumin is more favorable and more stable in the presence of salt. Now the reason behind the monotonic increase in fluorescence spectra of curcumin with the help of the aggregation behavior of bmimOs molecules in the presence of various amounts of salt can be clearly understandable. We have used pyrene quenching study by cetylpyridinium chloride (CPC) in the absence as well as in the presence of NaCl and determined the aggregation number of bmimOs micelle by using the following equation (95)(7)where I0 and IQ are the fluorescence intensities of pyrene in the absence of cytlpyridinium chloride (CPC), ⟨n⟩ is the mean occupancy number of the quencher molecule and in the presence of CPC, respectively, Nagg is the aggregation number, and CMC is the critical micellar concentration of bmimOs micelles. By using the slope of the straight line obtained from the plot of ln(I0/IQ) vs [CPC] (Figure 3, Supporting Information), we have calculated the aggregation number or bmimOs micelle at different salt concentrations. The aggregation numbers of bmimOs micelle are 39, 54, and 84 in the presence of 0.0, 1.0, and 1.5 M NaCl, whereas CMC values are 34, 30, and 21 mM, respectively. The overall result of change in CMC and aggregation is an increase in the number of bmimOs micelle in the solution (using eq 6) from 0.15 to 0.22 mM. Moreover, the size of the micelle is also getting increased in the presence of salt and it was obtained by using dynamic light scattering study (DLS). The size of the micelle changes from 2.0 to 5.00 nm almost 3-fold after addition of salt up to 1.5 M NaCl. As a result, the number of curcumin molecules partitioned to the micelles is also increased. We have also checked the nature of binding in the presence of salt, and a high binding constant (6.98 × 104 M–1) was found at a concentration above the CMC in the presence of 1 M NaCl. We have also monitored the absorption spectra of curcumin inside IL-micelle in the presence of 1 M NaCl with a certain time interval, and we found that the drug is stable enough inside the micellar environment, as depicted in Figure 7. In the presence of salt, the rate of degradation of the curcumin–micelle system is 0.399%/h, which is quite low compared to that in absence of salt, 1.22%. The higher stability of curcumin inside bmimOs micelle in the presence of salt concentration suggests the usefulness of this drug–micellar system.

Figure 6

Figure 6. Plot of fluorescence intensity vs [bmimOs] at different salt concentrations (differential plots are given in the inset).

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Figure 7

Figure 7. Absorption spectra of curcumin in the presence of 1 M salt with a gradual time interval. Variation of absorbance with time is given in the inset.

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3.1.3 Effect of Temperature

Upon an increase in temperature, the fluorescence intensity of curcumin decreases sharply, as shown in Figure 9 (inset). The nature of interaction between curcumin and IL surfactant was characterized by measuring the change in thermodynamic parameters with temperature. We have increased the temperature form 10 to 60 °C at a constant IL concentration (35 mM). Binding constant (Kb) values are decreasing with an increase in temperature, and we have determined the change in enthalpy (ΔH) and change in entropy (ΔS) by plotting ln Kb vs 1/T (Figure 8) using the van’t Hoff equation [ln Kb = (−ΔH/R)/T + ΔS/R]. Change in Gibbs free energy was calculated by using the following equation: ΔG = –RT ln Kb, where R is the universal gas constant and T is the absolute temperature (Table 2). The calculated binding enthalpy and entropy are ΔH (kcal/mol) = −10.17 and ΔS (cal/mol) = −13.72 over this temperature range. From Table 2, we can see that the value of change in enthalpy is greater than the total free energy change during this interaction. Similar negative values of thermodynamic parameters were noticed for curcumin–lipid (35, 40) and curcumin–HSA (85) interactions in which it was concluded that the binding is enthalpy driven and binding is guided by both H-bonding and hydrophobic interactions. In our study, the binding enthalpy (−ΔH) value for transfer or binding of curcumin to micelle from water is distinctly negative and larger than the total free energy change (−ΔG), indicating the binding interaction is an enthalpy driven process which is governed by H-bonding interaction between the keto–enol group of curcumin and the negative group of the surfactant as well as hydrophobic interaction between aromatic rings of curcumin and the long alkyl chain of IL.

Figure 8

Figure 8. van’t Hoff plot of curcumin in bmimOs micelle at different temperatures.

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Table 2. Change in Binding Constant and Free Energy (ΔG) at Different Temperaturesa
temperature (35.0 mM bmimOs) Kb/104 (M–1) ln Kb –ΔG (kcal/mol)
283 K 5.97 10.99 6.29
293 K 3.58 10.49 6.15
303 K 2.08 9.94 6.01
313 K 1.35 9.51 5.88
323 K 0.83 9.02 5.76
333 K 0.42 8.37 5.60
a

ΔH (kcal/mol) = −10.17 and ΔS (cal/mol) = −13.72 in this temperature range.

4 Time-Resolved Results

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The change in fluorescence lifetime helps to get a clear idea about the modulation of photophysical properties such as radiative and nonradiative deactivation channels brought about by encapsulation of the probe molecule. Unlike the steady state fluorescence measurements, there are not many reports on the fluorescence lifetimes of curcumin. The competing nonradiative processes shortened the fluorescence lifetimes considerably. The fluorescence decay profiles, obtained from time correlated single photon counting (TCSPC) studies, in most of the organic solvents showed a multiexponential fit, and the fluorescence lifetime values as well as their relative amplitudes varied significantly with the nature of the solvent. (38, 80, 96-98) Due to the complex nature, in most of the studies, average fluorescence lifetimes (τAV) were only compared where τAV can be expressed by eq 2.
All the studies were focused on excited-state intramolecular hydrogen atom transfer (ESIPT) of curcumin because this phenomenon has been associated with the medicinal properties of other pigment molecules. It was reported that the fluorescence lifetime of curcumin in methanol has a dominant component with a time constant of roughly 130 ps. (38, 80) This decay component, however, was assigned to different molecular processes, i.e., solvation (80) and ESIPT (38) in the two studies. Recently, it was observed that ESIPT occurs with a time scale of approximately 100 ps. Furthermore, the similarity between the time scale of ESIPT and the measured fluorescence lifetime indicates that ESIPT is a major photophysical process in the deactivation of the excited state. In our work, we have also given attention to the average lifetime of curcumin, giving thrust only to the proton transfer mechanism. The lifetime of curcumin was detected at different concentrations of IL and was found gradual increment from 103 to 158 ps (Table 3). There was also an increment upon gradual addition of NaCl to the micellar solution (Table 4). Similar to steady-state, time-resolved decay also shows a decrement in lifetime with increase in temperature (Table 5). The decays in all cases were biexponential in nature, and we got considerable change in lifetime, which confirms the modulations of photophysical character of curcumin when encapsulated inside IL-micelle. Previously, many studies have shown that the ESIPT rate of curcumin can be modulated in the presence of a surfactant assembly (83, 38, 99) and it was found that the curcumin lifetime is shortened mainly due to its fast ESIPT process. Moreover, it was also concluded in those works that intermolecular H-bonding occurs between curcumin and water inside micellar layers, and curcumin and surfactant. In our study, we have obtained quite similar values of time constants that were obtained for TX-100 micelle. For TX-100 micelle, it was concluded that the presence of strong intermolecular H-bonding between oxygen of the C–O bonds in TX-100 and curcumin slows down the intramolecular proton transfer rate within the keto–enol group. Methanol, ethanol like protic polar solvents also provide a similar environment by forming an intermolecular H-bond with the keto–enol group of curcumin, and retardation of intramolecular proton transfer makes the lifetime longer compared to nonpolar solvent like cyclohexane. Longer time constants of curcumin inside IL-micelles also suggest that there is a strong perturbation in the intramolecular H-bond in between the keto–enol group by forming strong intermolecular H-bonding with the anion of the IL. Moreover, it was reported that DMSO, DMF like H-bond accepting solvents show a much lower lifetime (155–175 ps) than that of ethyl acetate, benzonitrile, and glycerol triacetate like aprotic polar solvents (230–250 ps). In latter solvents, dipole–dipole interaction perturbed the intramolecular H-bond, but the in case of the former solvents, existence of multiple intermolecular H-bonds between the keto–enol group and two phenolic OH groups of curcumin and the surfactant headgroups is possible. The present study shows the lifetime of curcumin is very close (110–180 ps) to that of DMSO, DMF, and it implies that formation of multiple intermolecular H-bonds induces significant nonradiative deactivation through hydrogen stretching vibrations.
Table 3. Fluorescence Lifetime, Quantum Yield, Non-Radiative Rate and Radiative Rate Constants of Curcumin in bmimOs Micelle at Different Concentrations
curcumin in τ1/ps (a1) τ2/ps (a2) τAVERAGE/ps quantum yield (ΦX) Knr (s–1)/109 Kr (s–1)/108
0.0 mM bmimOs            
7.6 mM 61 (0.73) 215 (0.27) 103 0.0118 9.59 1.10
15.2 mM 79 (0.79) 224 (0.21) 110 0.0178 8.92 1.62
22.8 mM 89 (0.82) 238 (0.18) 115 0.0255 8.47 2.21
30.4 mM 94 (0.84) 260 (0.16) 124 0.0345 7.79 2.77
38.0 mM 104 (0.86) 290 (0.14) 130 0.0398 7.39 2.98
45.6 mM 118 (0.87) 298 (0.13) 141 0.0435 6.78 3.08
53.2 mM 126 (0.88) 305 (0.12) 148 0.0461 6.44 3.11
60.8 mM 138 (0.89) 318 (0.11) 158 0.0507 5.93 3.20
Table 4. Fluorescence Lifetime, Quantum Yield, Non-Radiative Rate and Radiative Rate Constants of Curcumin in bmimOs Micelle at Different Salt Concentrations
curcumin in 35.0 mM bmimOs τ1/ps (a1) τ2/ps (a2) τAVERAGE/ps quantum yield (ΦX) Knr (s–1)/109 Kr (s–1)/108
0.0 M 76 (0.60) 193 (0.40) 123 0.034 7.85 2.76
0.13 M 96 (0.65) 195 (0.35) 130 0.038 7.40 2.92
0.39 M 111 (0.68) 196 (0.32) 137 0.041 7.00 2.99
0.64 M 124 (0.73) 199 (0.27) 145 0.048 6.57 3.31
0.89 M 136 (0.76) 200 (0.24) 152 0.053 6.23 3.49
1.02 M 147 (0.80) 199 (0.20) 158 0.058 5.96 3.67
1.15 M 161 (0.82) 205 (0.18) 169 0.063 5.55 3.72
1.28 M 171 (0.85) 212 (0.15) 178 0.067 5.24 3.76
1.408 M 178 (0.86) 218 (0.14) 184 0.073 5.03 3.96
Table 5. Fluorescence Lifetime, Quantum Yield, Non-Radiative Rate and Radiative Rate Constants of Curcumin in bmimOs Micelle at Different Temperatures
temperature (35.0 mM bmimOs) τ1/ps (a1) τ2/ps (a2) τAVERAGE/ps quantum yield (ΦX) Knr (s–1)/109 Kr (s–1)/108
283 K 120 (0.67) 260 (0.33) 166 0.051 5.71 3.07
293 K 111 (0.72) 230 (0.28) 140 0.041 6.85 2.92
303 K 83 (0.87) 209 (0.23) 120 0.035 8.04 2.91
313 K 78 (0.85) 254 (0.15) 105 0.028 9.26 2.67
323 K 70 (0.88) 272 (0.12) 94 0.020 10.40 2.12
333 K 65 (0.92) 268 (0.08) 82 0.010 12.04 1.22
In analogy to literature reports of modulations of excited state photophysics of the curcumin in constrained environments of proteins, micelles, and cyclodextrin, the increase of lifetime associated with increasing IL concentration (Figure 9) seems attributable to diminution of nonradiative decay channels through reduction of rotational/vibrational degrees of freedom resulting from its encapsulated state inside micellar environments. To further construe the modulations in excited state behavior of curcumin, we have calculated the radiative (kr) and nonradiative (knr) decay rate constants using the following two equations:where kr and knr are the radiative and nonradiative rate constants, respectively, and ϕF is the fluorescence quantum yield of curcumin. From Table 1, we can see reduction in the nonradiative decay path accompanied by increment in the radiative decay path for encapsulated curcumin upon gradual addition of IL (Figure 10). Similar results were also observed in the case of salt addition to micellar solution. The nonradiative rate constant of curcumin is highest in cyclohexane (1.5 × 1010 s–1) which is larger than the value observed inside our system. However, in strong hydrogen–bond donating/accepting solvent like methanol, DMF, and DMSO, (36, 38) the nonradiative rate constants are quite close to each other and also similar to knr inside IL–micelle, indicating our previous supposition that the excited state of curcumin is perturbed by the presence of intermolecular H-bonding interaction between IL-micelle and curcumin. In other words, this fact established that the excited state proton transfer is the main photophysical process for the nonradiative deactivation of the excited curcumin molecule.

Figure 9

Figure 9. Time-resolved decay plot of curcumin (left) with change in NaCl concentration, (middle) with change in concentration, and (right) with change in temperature.

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Figure 10

Figure 10. Variation of nonradiative rate constant (black points) and variation of quantum yield (blue points) with polarity of the medium in ET(30) scale in IL-micelle.

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In summary, we have successfully observed the modulation of the photophysical properties of curcumin inside an IL-micelle, a promising drug delivery system under various conditions, and also obtained useful information about the stability of this drug–micelle system. A high value of partition coefficient as well as binding constant of curcumin from water to micellar phase confirms the preferential location of the drug inside the micelle. Considerable increase in anisotropy value with increase in IL concentration supports the encapsulation of the drug. In our study, we have used curcumin for a special job to determine the CMC of the IL in water by using its very sensitive fluorescence properties with change in polarity around its microenvironment and we have successfully determined the value of the CMC of the IL in the absence of salt and in the presence of salt. Considerable blue-shift in absorption spectra as well as in emission spectra having a structural change points toward the existence of a strong H-bonding environment. Remarkable enhancement in steady state fluorescence intensity inside IL micelle seems to be the manifestation of the effect of perturbation of intramolecular H-bond in a microenvironment having reduced polarity compared to bulk water. A longer lifetime inside IL-micelle in the absence or presence of salt also supports the formation of an intermolecular H-bond between the keto–enol group of curcumin and the surfactant headgroup. The salt effect and temperature effect also give the idea about the mechanism and nature of interaction which is attributed mainly to hydrogen bond as well as hydrophobic interaction. A higher negative value of binding enthalpy proves the process is mostly enthalpy driven. Significant depletion of nonradiative decay channels of curcumin inside IL-micelle matches very well with other examined confined systems like surfactant micelle, (83, 100) membrane, (35, 40) proteins, (85) cyclodextrin, (101, 102) supramolecular assembly, (103) etc., and provides a huge area of possibility of curcumin-IL system to be studied as a successful drug carrier vehicle.

Conclusion

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We have presented detailed information about the modulation of the photophysical properties of a potent ESIPT molecule, namely, curcumin, upon interaction with surfactant-like ionic liquid, bmimOs, a new class of surfactant in aqueous solution. In particular, the remarkable enhancement in fluorescence spectra with a distinct blue-shift along with considerable change in time-resolved spectra dictates the sensitivity of the interaction of curcumin with the IL-micelle. By using this very property, we have successfully calculated the CMC of the IL in water. Higher encapsulation efficiency was also determined along with an enthalpy guided favorable binding interaction in the presence of salt and in the absence of salt. A very low value of the hydrolytic degradation rate of curcumin encapsulated inside micelle makes it a smart nanocompartment to handle it with ease. Our study opens a promising area of research that can provide a wide range of designed nanocompartments having tunable properties only by changing the cation or the anion which helps to monitor some specific interaction present within that system.

Supporting Information

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Absorption spectra of curcumin inside IL–micelle upon addition of IL and salt. This material is available free of charge via the Internet at http://pubs.acs.org.

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Author Information

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  • Corresponding Author
    • Nilmoni Sarkar - Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India;  Email: [email protected]
  • Authors
    • Chiranjib Ghatak - Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India
    • Vishal Govind Rao - Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India
    • Sarthak Mandal - Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India
    • Surajit Ghosh - Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

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N.S. is thankful to the Board of Research in Nuclear Sciences (BRNS), Council of Scientific and Industrial Research (CSIR), Government of India, for generous research grants. C.G., V.G.R., S.M., and S.G. are thankful to CSIR for research fellowships.

References

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    Excited-state intramol. proton transfer reactions are reviewed with 224 refs.. Special emphasis is given to the intrinsic processes and to the mechanisms of proton transfers in relation to the nature of the intramol. hydrogen bond ring.
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    In order to obtain detailed information on ground and excited states of curcumin and curcuminoids, as well as to understand the photobiol. characteristics of them, their spectral and photophys. behaviors are investigated in various conditions. Various curcuminoids were obtained and their structures were detd. by spectroscopic methods. In n-hexane, the absorption and fluorescence spectra of these compds. contain some structures which disappear in more polar solvents such as methanol. The fluorescence intensities of curcumin and dimethylated curcumin decrease as the concn. of water increases. The intensities also decrease as the solvent varies from neutral to extremely acidic (lower than pH 1.5) or to basic (higher than pH 8.0) conditions. These results indicate that the spectral and photophys. properties of both of curcumin and curcuminoids are strongly influenced by solvent, water, and pH.
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    A sensitive and rapid spectrofluorometric method for detn. of microamounts of curcumin in curcumin spices and related flavors involves dissolving samples in dry Me2CO, irradiating the resulting clear soln. at λE = 424 nm, and measuring the stable intense green-yellow fluorescence at λF = 504 nm. The fluorescent soln. shows no change in λE or λF or in fluorescence intensity for ≥1 mo under ambient conditions. Beer's law is followed over the range 0.0-500 ppb of curcumin. The sensitivity and detection limit (S/N = 2) are 4.7 and 0.34 ppb of curcumin per fluorescence unit, resp. The relative std. deviation and recovery for a series of concns. (0.01-0.3 ppm) are 1.13-2.03 and 98.96-100%, resp. Temp. control is needed; pH adjustment and O2 removal from test soln. are unnecessary. Under the specified conditions, water is the only quencher for curcumin fluorescence. Direct calibration detn. is satisfactory; therefore, there is no need to use a rather lengthy std. addns. technique.
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    Curcumin, the yellow pigment from the rhizoma of Curcuma longa, is a widely studied polyphenolic compd. which has a variety of biol. activity as anti-inflammatory and antioxidative agent. Genistein one of the flavonoids found in soybean and chickpeas inhibits DNA strand breaks acting as a direct scavenger of reactive oxygen species. Human serum albumin (HSA) with high affinity binding sites is a major transporter for delivering several endogenous compds. and drugs in vivo. The aim of this study was to examine the interactions of curcumin and genistein with human serum albumin at physiol. conditions, using const. protein concn. and various pigment contents. FTIR, UV-Visible, CD and fluorescence spectroscopic methods were used to analyze drug binding mode, the binding const. and the effects of pigment complexation on HSA stability and conformation. Structural anal. showed that curcumin and genistein bind HSA via polypeptide polar groups with overall binding consts. of K curcumin = 5.5 (±0.8) × 104 M-1 and K genistein = 2.4 (±0.40) × 104 M-1. The no. of bound pigment (n) is 1.33 for curcumin and 1.49 for genistein. The HSA conformation was altered by pigment complexation with redn. of α-helix and increase of random coil and turn structures suggesting a partial protein unfolding.
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    Stability of curcumin in buffer solutions and characterization of its degradation products
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    The degrdn. kinetics of curcumin under various pH conditions and the stability of curcumin in physiol. matrixes were investigated. When curcumin was incubated in 0.1 M phosphate buffer and serum-free medium, pH 7.2 at 37°C, about 90% decompd. within 30 min. A series of pH conditions ranging from 3 to 10 were tested and the result showed that decompn. was pH-dependent and occurred faster at neutral-basic conditions. It is more stable in cell culture medium contg. 10% fetal calf serum and in human blood; less than 20% of curcumin decompd. within 1 h, and after incubation for 8 h, about 50% of curcumin is still remained. Trans-6-(4'-hydroxy-3'-methoxyphenyl)-2,4-dioxo-5-hexenal was predicted as major degrdn. product and vanillin, ferulic acid, feruloyl methane were identified as minor degrdn. products. The amt. of vanillin increased with incubation time.
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    Ma, Zengshuan; Haddadi, Azita; Molavi, Ommoleila; Lavasanifar, Afsaneh; Lai, Raymond; Samuel, John
    Journal of Biomedical Materials Research, Part A (2008), 86A (2), 300-310CODEN: JBMRCH; ISSN:1549-3296. (John Wiley & Sons, Inc.)
    Curcumin is recognized as a potential chemotherapeutic agent against a variety of tumors. However, the clin. application of curcumin is hindered due to its poor water soly. and fast degrdn. The objective of this study was to investigate amphiphilic block copolymer micelles of poly(ethylene oxide)-b-poly(ε-caprolactone) (PEO-PCL) as vehicles for the solubilization, stabilization, and controlled delivery of curcumin. Curcumin-loaded PEO-PCL micelles were prepd. by a cosolvent evapn. technique. PEO-PCL micelles were able to solubilize curcumin effectively, protect the encapsulated curcumin from hydrolytic degrdn. in physiol. matrix, and control the release of curcumin over a few days. The characteristics of resultant micelles were found to depend on the polymn. degrees of ε-caprolactone. Among different PEO-PCL micelles, PEO5000-PCL24500 was the most efficient in solubilizing curcumin while PEO5000-PCL13000 was the best carrier in reducing its release rate. PEO-PCL micelle-encapsulated curcumin retained its cytotoxicity in B16-F10, a mouse melanoma cell line, and SP-53, Mino, and JeKo-1 human mantle cell lymphoma cell lines. These results demonstrated the potential of PEO-PCL micelles as an injectable formulation for efficient solubilization, stabilization, and controlled delivery of curcumin.
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    Liposomal curcumin with and without oxaliplatin: effects on cell growth, apoptosis, and angiogenesis in colorectal cancer
    Li, Lan; Ahmed, Bilal; Mehta, Kapil; Kurzrock, Razelle
    Molecular Cancer Therapeutics (2007), 6 (4), 1276-1282CODEN: MCTOCF; ISSN:1535-7163. (American Association for Cancer Research)
    The role of curcumin (diferuloylmethane), a proapoptotic compd., for the treatment of cancer has been an area of growing interest. Curcumin in its free form is poorly absorbed in the gastrointestinal tract and therefore may be limited in its clin. efficacy. Liposome encapsulation of this compd. would allow systemic administration. The current study evaluated the preclin. antitumor activity of liposomal curcumin in colorectal cancer. We also compared the efficacy of liposomal curcumin with oxaliplatin, a std. chemotherapy for this malignancy. In vitro treatment with liposomal curcumin induced a dose-dependent growth inhibition [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium salt] and apoptosis [poly(ADP-ribose) polymerase] in the two human colorectal cancer cell lines tested (LoVo and Colo205 cells). There was also synergism between liposomal curcumin and oxaliplatin at a ratio of 4:1 in LoVo cells in vitro. In vivo, significant tumor growth inhibition was obsd. in Colo205 and LoVo xenografts, and the growth inhibition by liposomal curcumin was greater than that for oxaliplatin (P < 0.05) in Colo205 cells. Tumors from animals treated with liposomal curcumin showed an antiangiogenic effect, including attenuation of CD31 (an endothelial marker), vascular endothelial growth factor, and interleukin-8 expression by immunohistochem. This study establishes the comparable or greater growth-inhibitory and apoptotic effects of liposomal curcumin with oxaliplatin both in vitro and in vivo in colorectal cancer. We are currently developing liposomal curcumin for introduction into the clin. setting.
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    Curcumin (1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, Cm) is a natural compd. which possesses antioxidant, anti-inflammatory and anti-tumor ability. Here, phospholipid vesicles or lipid-nanospheres embedding Cm (CmVe or CmLn) were formulated to deliver Cm into tissue macrophages through i.v. injection. Cm could be solubilized in hydrophobic regions of these particles to form nanoparticle dispersions, and these formulations showed ability to scavenge reactive oxygen species as antioxidants in dispersions. At 6 h after i.v. injection in rats via the tail vein (2 mg Cm/kg body wt.), confocal microscopic observations of tissue sections showed that Cm was massively distributed in cells assumed as macrophages into the bone marrow and spleen. Taken together, these results indicate that the lipid-based nanoparticulates provide improved i.v. delivery of Cm to tissues macrophages, specifically bone marrow and splenic macrophages in present formulation, which has therapeutic potential as an antioxidant and anti-inflammatory.
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    Curcumin, a natural constituent of Curcuma longa was formulated as prolonged release biodegradable microspheres for treatment of inflammation. Natural biodegradable polymers, namely, bovine serum albumin and chitosan were used to encapsulate curcumin to form a depot forming drug delivery system. Microspheres were prepd. by emulsion-solvent evapn. method coupled with chem. crosslinking of the natural polymers. Curcumin could be encapsulated into the biodegradable carriers up to an extent of 79.49 and 39.66% resp. with albumin and chitosan. Different drug:polymer ratios did not affect the mean particle size or particle size distribution significantly. However, the concn. of the crosslinking agent had remarkable influence on the drug release. In-vitro release studies indicated a biphasic drug release pattern, characterized by a typical burst-effect followed by a slow release which continued for several days. Evaluation of antiinflammatory activity using Freund's adjuvant induced arthritic model in Wistar rats revealed significant difference between both the formulations, albumin microspheres and chitosan microspheres as well as against control. It was evident from the present study that the curcumin biodegradable microspheres could be successfully employed as prolonged release drug delivery system for better therapeutic management of inflammation as compared to oral or s.c. route.
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    A new cyclodextrin-based carrier for active targeting of low sol. and degradable drugs has been synthesized and characterized. β-Cyclodextrins were first reacted with excess hexamethylene diisocyanate and the resulting CD-(C6-NCO)5 deriv. was reacted with 700 Da diamino-PEG to yield CD-(C6-PEG-NH2)5. About one out of five free amino groups of PEG were functionalized with folic acid (FA) as a tumor targeting moiety. The chem. structures of the intermediates as well as the final product, CD-(C6-PEG)5-FA, were characterized by 1H and 13C NMR, reverse phase and gel permeation chromatog., and UV-Vis spectroscopy. After modification, the hemolytic activity of β-cyclodextrins decreased by about 70%. In the presence of the new carrier, the β-estradiol soly. increased by more than 300-fold and the chlorambucil degrdn. rate decreased by 50-60%. CD-(C6-PEG)5-FA formed an inclusion complex with curcumin displaying an assocn. const. of 954,732 M-1. The new carrier increased the curcumin soly. by about 3200-fold as compared to native β-cyclodextrins and reduced its degrdn. rate at pH 6.5 and 7.2 by 10 and 45-fold, resp. FA receptor-overexpressing human nasopharyngeal tumor KB cell lines and nonfolic acid receptor-expressing human breast cancer MCF7 cells were used to evaluate the targeting properties of the new drug delivery system. The in vitro studies demonstrate that the new carrier possesses potential selectivity for the folate receptor-overexpressing tumor cells as ED50 values of 52 μM, 58 μM and 21 μM were obtained with curcumin-loaded CD-(C6-PEG-NH2)5, curcumin in fetal serum medium and CD-(C6-PEG)5-FA, resp.
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    A review. The dramatic growth in ionic liq. research over the past decade has resulted in the development of a huge no. of novel ionic liqs., as well as many assocd. applications. The perceived environmentally friendly nature of ionic liqs., which results from their negligible vapor pressure, is now under scrutiny since although they will not evap. into air, it is not possible to guarantee that they will never enter the environment. Toxicity research studies including ecotoxicity have recently received broad attention and the commonly accepted notion that ionic liqs. have low toxicity has been shown to be incorrect. This review attempts to highlight the progress of ionic liq. toxicity research as well as the development of degradable and biorenewable ionic liqs.
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  • Abstract

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    Scheme 1

    Scheme 1. Structure of Curcumin and the Ionic Liquid
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    Figure 1

    Figure 1. Fluorescence spectra of curcumin (left) with change in [bmimOs] and (right) with change in [NaCl]. Plots of variation of emission intensity and emission maxima of curcumin are given in the inset.

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    Figure 2

    Figure 2. (a) Plot of Stokes shift vs ET(30) of a series of alcohols (CnOH, where n = 1, 2, 3, 4, 5, 6, 8). (b) Plot of Stokes shift vs ET(30) of the surrounding environment of curcumin at different concentrations of IL.

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    Figure 3

    Figure 3. Plot of (I∞ – I0)/(It – I0) vs [bmimOs]–1 for partition coefficient determination.

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    Figure 4

    Figure 4. Steady state anisotropy vs concentration of bmimOs.

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    Figure 5

    Figure 5. Plot of (I∞ – I0)/(It – I0) vs [bmimOs]−1 for binding constant determination.

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    Figure 6

    Figure 6. Plot of fluorescence intensity vs [bmimOs] at different salt concentrations (differential plots are given in the inset).

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    Figure 7

    Figure 7. Absorption spectra of curcumin in the presence of 1 M salt with a gradual time interval. Variation of absorbance with time is given in the inset.

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    Figure 8

    Figure 8. van’t Hoff plot of curcumin in bmimOs micelle at different temperatures.

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    Figure 9

    Figure 9. Time-resolved decay plot of curcumin (left) with change in NaCl concentration, (middle) with change in concentration, and (right) with change in temperature.

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    Figure 10

    Figure 10. Variation of nonradiative rate constant (black points) and variation of quantum yield (blue points) with polarity of the medium in ET(30) scale in IL-micelle.

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  • References

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      Srimal, R. C.; Dhawan, B. N.
      Journal of Pharmacy and Pharmacology (1973), 25 (6), 447-52CODEN: JPPMAB; ISSN:0022-3573.
      Curcumin (diferuloyl methane) (I) [458-37-7] showed a significant antiinflammatory activity in acute and chronic models of inflammation in rats and mice. I was as potent as phenylbutazone in the carrageenin edema test but only half as potent in chronic tests. I prevented the increases in serum glutamic-oxalacetic transaminase [9000-97-9] and glutamic-pyruvic transaminase [9000-86-6] seen in inflammation. I had a lower ulcerogenic index than phenylbutazone. I had no analgesic and antipyretic effects in mice and rats and no cardiovascular effects in cats. The oral LD50 of I in mice was >2.0 g/kg.
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    8. 8
      Priyadarsini, K. I. Free Radical Biol. Med. 1997, 23, 838– 843
      8
      Free radical reactions of curcumin in membrane models
      Priyadarsini, K. Indira
      Free Radical Biology & Medicine (1997), 23 (6), 838-843CODEN: FRBMEH; ISSN:0891-5849. (Elsevier)
      Free radical reactions of curcumin, a lipid sol. antioxidant from turmeric (Curcuma longa), have been studied with a variety of oxidants using TX 100 micelle as a model membrane. The phenoxyl radicals of curcumin generated by one electron oxidizing azide radicals in acetonitrile-water mixt. and TX 100 micelles show very similar spectral behavior. However, in membrane models the radical lifetimes and the molar extinction coeffs. are significantly different from the homogeneous solns. Micellized curcumin reacts with haloperoxyl radicals, superoxide, and lipid peroxyl radicals with rate consts. of 5 × 108, 4.6 × 104, and 5.3 × 105 M-1 s-1, resp. Curcumin derived phenoxyl radicals decay by radical-radical reactions in homogeneous solns., while in the micelles, radical decay is mostly first order when the av. occupancy of the micelle is less than 1. Implications of these results in evaluating curcumin as an antioxidant is discussed.
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    9. 9
      Khopde, S. M.; Priyadarsini, K. I; Venkatesan, P.; Rao, M. N. A. Biophys. Chem. 1999, 80, 85– 91
      There is no corresponding record for this reference.
    10. 10
      Yang, F.; Lim, G. P.; Begum, A. N.; Ubeda, O. J.; Simmons, M. R.; Ambegaokar, S. S.; Chen, P.; Kayed, R.; Glabe, C. G.; Frautschy, S. A.; Cole, G. M. J. Biol. Chem. 2005, 280, 5892
      There is no corresponding record for this reference.
    11. 11
      Lee, S. E.; Campbell, B. C.; Molyneux, R. J.; Hasegawa., S.; Lee, H. S. J. Agric. Food Chem. 2001, 49, 5171– 5177
      There is no corresponding record for this reference.
    12. 12
      Shim, J. S.; Kim, J. H.; Cho, H. Y.; Yum, Y. N.; Kim, S. H.; Park, H. J.; Shim, B. S.; Choi, S. H.; Kwon, H. J. Chem. Biol. 2003, 10, 695– 704
      12
      Irreversible Inhibition of CD13/Aminopeptidase N by the Antiangiogenic Agent Curcumin
      Shim, Joong Sup; Kim, Jin Hee; Cho, Hyun Young; Yum, Young Na; Kim, Seung Hee; Park, Hyun-Ju; Shim, Bum Sang; Choi, Seung Hoon; Kwon, Ho Jeong
      Chemistry & Biology (2003), 10 (8), 695-704CODEN: CBOLE2; ISSN:1074-5521. (Cell Press)
      CD13/aminopeptidase N (APN) is a membrane-bound, zinc-dependent metalloproteinase that plays a key role in tumor invasion and angiogenesis. Here, we show that curcumin, a phenolic natural product, binds to APN and irreversibly inhibits its activity. The direct interaction between curcumin with APN was confirmed both in vitro and in vivo by surface plasmon resonance anal. and an APN-specific antibody competition assay, resp. Moreover, curcumin and other known APN inhibitors strongly inhibited APN-pos. tumor cell invasion and basic fibroblast growth factor-induced angiogenesis. However, curcumin did not inhibit the invasion of APN-neg. tumor cells, suggesting that the antiinvasive activity of curcumin against tumor cells is attributable to the inhibition of APN. Taken together, our study revealed that curcumin is a novel irreversible inhibitor of APN that binds to curcumin resulting in inhibition of angiogenesis.
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    13. 13
      Flynn, D. L.; Rafferty, M. F. Prostaglandins, Leukotrienes Med. 1986, 22, 357– 360
      13
      Inhibition of 5-hydroxyeicosatetraenoic acid (5-HETE) formation in intact human neutrophils by naturally occurring diarylheptanoids: inhibitory activities of curcuminoids and yakuchinones
      Flynn, Daniel L.; Rafferty, Michael F.; Boctor, Amal M.
      Prostaglandins, Leukotrienes and Medicine (1986), 22 (3), 357-60CODEN: PLMEDD; ISSN:0262-1746.
      Various diarylheptanoids including curcumin [458-37-7], bis(3,4-dihydroxycinnamoyl)methane [104061-29-2], and yakuchinones A [78954-23-1] and B [81840-57-5] were screened and found to be potent inhibitors of 5-HETE [71030-39-2] prodn. by intact human neutrophils, with resp. 50% inhibitory concns. of 8.0, 4.4, 5.4, and 4.0 μM. These diarylheptanoids were more potent than BW-755C [66000-40-6], phenidone [92-43-3], and AA-861 [80809-81-0]. Also several of these diarylheptanoids inhibited cyclooxygenase [39391-18-9]. Thus, curcuminoids and yakuchinones are more accurately characterized as dual inhibitors of arachidonic acid [506-32-1] metab.
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    14. 14
      Huang, M.T..; Lysz, T.; Ferraro, T.; Abidi, T. F.; Laskin, J. D.; Conney, A. H. Cancer Res. 1991, 51, 813– 819
      There is no corresponding record for this reference.
    15. 15
      Huang, M. T.; Smart, R. C.; Wong, C. Q.; Conney, A. H. Cancer Res. 1988, 48, 5941– 5946
      There is no corresponding record for this reference.
    16. 16
      Egan, M. E.; Pearson, M.; Weiner, S. A.; Rajendarn, V.; Rubin, D.; Glochner-Pagel, J.; Canney, S.; Du, K.; Lukacs, G. L.; Caplan, M. F. Science 2004, 304, 600– 602
      There is no corresponding record for this reference.
    17. 17
      Aggrawal, B. B.; Sundaram, C.; Malani, N.; Ichikawa, H. Adv. Exp. Med. Biol. 2007, 595, 1– 75
      There is no corresponding record for this reference.
    18. 18
      Mazumder, A.; Neamati, N.; Sunder, S.; Schulz, J.; Perez, H.; Aich, E.; Pommier, Y. J. Med. Chem. 1997, 40, 3057– 3063
      There is no corresponding record for this reference.
    19. 19
      Sui, Z.; Salto, R.; Li, J.; Craik, C.; Ortiz de Montellano, P. R. Bioorg. Med. Chem. 1993, 1, 415– 422
      There is no corresponding record for this reference.
    20. 20
      Tonnesen, H. H.; Karlsen, J.; Mostad, A. Acta Chem. Scand., Ser. B 1982, 36, 475– 480
      There is no corresponding record for this reference.
    21. 21
      Parimita, S. P.; Ramshankar, Y. V.; Suresh, S.; Guru, R. T. N. Acta Crystallogr., Sect. E 2007, 63, o860– o862
      There is no corresponding record for this reference.
    22. 22
      Payton, F.; Sandusky, P.; Alworth, W. L. J. Nat. Prod. 2007, 143– 146
      There is no corresponding record for this reference.
    23. 23
      Shen, L.; Ji, H.-F. Spectrochim. Acta, Part A 2007, 67, 619– 623
      23
      Theoretical study on physicochemical properties of curcumin
      Shen, Liang; Ji, Hong-Fang
      Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (2007), 67A (3-4), 619-623CODEN: SAMCAS; ISSN:1386-1425. (Elsevier B.V.)
      Curcumin is a yellow-orange pigment, which has attracted considerable attention due to its wide spectrum of biol. and pharmacol. activities. In spite of much effort devoted on curcumin, there still exist some open questions concerning its fundamental physicochem. properties. The present study suggests that the DFT and TD-DFT calcns. are useful to answer these questions. Firstly, the thermodn. as well as spectral parameters support that curcumin exists predominantly in enol form in soln. Secondly, the calcd. absorption spectra of curcumin anions provides direct evidence that the lowest pKa of curcumin corresponds to the dissocn. of enolic proton, which not only reconciles the controversy on this topic, but also has important implications on the proton-transfer/dissocn.-assocd. radical-scavenging mechanisms of curcumin.
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    24. 24
      Shen, L.; Ji, H. F.; Zhang, H. Y. Chem. Phys. Lett. 2005, 409, 300– 303
      There is no corresponding record for this reference.
    25. 25
      Shen, L.; Zhang, H. Y.; Ji, H. F. Org. Lett. 2005, 7, 243– 246
      There is no corresponding record for this reference.
    26. 26
      Balasubramanian, K. J. Agric. Food Chem. 2006, 54, 3512– 3520
      There is no corresponding record for this reference.
    27. 27
      Kong, L.; Priyadarsini, K. I.; Zhang, H.-Y. J. Mol. Struct.: THEOCHEM 2004, 688, 111– 116
      There is no corresponding record for this reference.
    28. 28
      Arnaut, L. G.; Formosinno, S. J. J. Photochem. Photobiol., A 1993, 75, 1– 20
      28
      Excited-state proton transfer reactions. I. Fundamentals and intermolecular reactions
      Arnaut, Luis G.; Formosinho, Sebastiao J.
      Journal of Photochemistry and Photobiology, A: Chemistry (1993), 75 (1), 1-20CODEN: JPPCEJ; ISSN:1010-6030.
      Theor. models that have been proposed and applied to proton transfer reactions are reviewed in this work. Simple models, like the Eigen model, Marcus theory and the intersecting state model, are applied to excited-state intermol. proton transfers. The kinetics and thermodn. of proton transfers occurring in the singlet states of arom. mols. with -OH, -NH3+, -NH2 and :CO substituents are reviewed;228 refs.
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    29. 29
      Arnaut, L. G.; Formosinno, S. J. J. Photochem. Photobiol, A 1993, 75, 21– 48
      29
      Excited-state proton transfer reactions. II. Intramolecular reactions
      Formosinho, Sebastiao J.; Arnaut, Luis G.
      Journal of Photochemistry and Photobiology, A: Chemistry (1993), 75 (1), 21-48CODEN: JPPCEJ; ISSN:1010-6030.
      Excited-state intramol. proton transfer reactions are reviewed with 224 refs.. Special emphasis is given to the intrinsic processes and to the mechanisms of proton transfers in relation to the nature of the intramol. hydrogen bond ring.
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    30. 30
      Bong, P. H. Bull. Korean Chem. Soc. 2000, 21, 81– 86
      30
      Spectral and photophysical behaviors of curcumin and curcuminoids
      Bong, Pill-Hoon
      Bulletin of the Korean Chemical Society (2000), 21 (1), 81-86CODEN: BKCSDE; ISSN:0253-2964. (Korean Chemical Society)
      In order to obtain detailed information on ground and excited states of curcumin and curcuminoids, as well as to understand the photobiol. characteristics of them, their spectral and photophys. behaviors are investigated in various conditions. Various curcuminoids were obtained and their structures were detd. by spectroscopic methods. In n-hexane, the absorption and fluorescence spectra of these compds. contain some structures which disappear in more polar solvents such as methanol. The fluorescence intensities of curcumin and dimethylated curcumin decrease as the concn. of water increases. The intensities also decrease as the solvent varies from neutral to extremely acidic (lower than pH 1.5) or to basic (higher than pH 8.0) conditions. These results indicate that the spectral and photophys. properties of both of curcumin and curcuminoids are strongly influenced by solvent, water, and pH.
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    31. 31
      Adhikary, R.; Barnes, C. A.; Trampel, R. L.; Wallace, S. J.; Kee, T. W.; Petrich, J. W. J. Phys. Chem. B 2011, 115, 10707– 10714
      There is no corresponding record for this reference.
    32. 32
      Jasim, F.; Alim, F. Microchem. J. 1992, 46, 209– 214
      32
      A novel and rapid method for the spectrofluorometric determination of curcumin in curcumin spices and flavors
      Jasim, Fadhil; Ali, Fatima
      Microchemical Journal (1992), 46 (2), 209-14CODEN: MICJAN; ISSN:0026-265X.
      A sensitive and rapid spectrofluorometric method for detn. of microamounts of curcumin in curcumin spices and related flavors involves dissolving samples in dry Me2CO, irradiating the resulting clear soln. at λE = 424 nm, and measuring the stable intense green-yellow fluorescence at λF = 504 nm. The fluorescent soln. shows no change in λE or λF or in fluorescence intensity for ≥1 mo under ambient conditions. Beer's law is followed over the range 0.0-500 ppb of curcumin. The sensitivity and detection limit (S/N = 2) are 4.7 and 0.34 ppb of curcumin per fluorescence unit, resp. The relative std. deviation and recovery for a series of concns. (0.01-0.3 ppm) are 1.13-2.03 and 98.96-100%, resp. Temp. control is needed; pH adjustment and O2 removal from test soln. are unnecessary. Under the specified conditions, water is the only quencher for curcumin fluorescence. Direct calibration detn. is satisfactory; therefore, there is no need to use a rather lengthy std. addns. technique.
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    33. 33
      Zsila, F.; Bikadi, Z.; Simonyi, M. Tetrahedron: Asymmetry 2003, 14, 2433– 2444
      There is no corresponding record for this reference.
    34. 34
      Mandeville, J. S.; Froehlich, E.; Tajmir-Riahi, H. A. J. Pharm. Biomed. Anal. 2009, 49, 468– 474
      34
      Study of curcumin and genistein interactions with human serum albumin
      Mandeville, Jean-Sebastien; Froehlich, Emilie; Tajmir-Riahi, H. A.
      Journal of Pharmaceutical and Biomedical Analysis (2009), 49 (2), 468-474CODEN: JPBADA; ISSN:0731-7085. (Elsevier B.V.)
      Curcumin, the yellow pigment from the rhizoma of Curcuma longa, is a widely studied polyphenolic compd. which has a variety of biol. activity as anti-inflammatory and antioxidative agent. Genistein one of the flavonoids found in soybean and chickpeas inhibits DNA strand breaks acting as a direct scavenger of reactive oxygen species. Human serum albumin (HSA) with high affinity binding sites is a major transporter for delivering several endogenous compds. and drugs in vivo. The aim of this study was to examine the interactions of curcumin and genistein with human serum albumin at physiol. conditions, using const. protein concn. and various pigment contents. FTIR, UV-Visible, CD and fluorescence spectroscopic methods were used to analyze drug binding mode, the binding const. and the effects of pigment complexation on HSA stability and conformation. Structural anal. showed that curcumin and genistein bind HSA via polypeptide polar groups with overall binding consts. of K curcumin = 5.5 (±0.8) × 104 M-1 and K genistein = 2.4 (±0.40) × 104 M-1. The no. of bound pigment (n) is 1.33 for curcumin and 1.49 for genistein. The HSA conformation was altered by pigment complexation with redn. of α-helix and increase of random coil and turn structures suggesting a partial protein unfolding.
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    35. 35
      Began., G.; Sudharshan, E.; Udaya, S. K.; Appu Rao, A. G. J. Agric. Food Chem. 1999, 47, 4992– 4997
      There is no corresponding record for this reference.
    36. 36
      Chignell, C. F.; Bilski, P.; Reszka, K. J.; Motten, A. N.; Sik, R. H.; Dhal, T. A. Photochem. Photobiol. 1994, 59, 295– 302
      There is no corresponding record for this reference.
    37. 37
      Tonnesen, H. H.; Arrieta, A. F.; Lerner, D. Pharmazie 1995, 50, 689– 693
      There is no corresponding record for this reference.
    38. 38
      Khopde, S. M.; Priyadarsini, K. I.; Palit, D. K.; Mukherjee, T. Photochem. Photobiol. 2000, 72, 625– 631
      There is no corresponding record for this reference.
    39. 39
      Kapoor, S.; Priyadarsini, K. I. Biophys. Chem. 2001, 92, 119– 126
      There is no corresponding record for this reference.
    40. 40
      Kunwar, A.; Barik, A.; Pandey, R.; Priyadarsini, K. I. Biochim. Biophys. Acta 2006, 1760, 1513– 1520
      There is no corresponding record for this reference.
    41. 41
      Sun, Y.; Lee, C. C.; Hung, W. C.; Chen, F. Y.; Lee, M. T.; Huang, H. W. Biophys. J. 2008, 95, 2318– 2324
      There is no corresponding record for this reference.
    42. 42
      Adhikary, R.; Mukherjee, P.; Kee, T. W.; Petrich, J. W. J. Phys. Chem. B 2009, 113, 5255– 5261
      There is no corresponding record for this reference.
    43. 43
      Letchford, K.; Liggins, R.; Burt, H. J. Pharm. Sci. 2008, 97, 1179– 1190
      There is no corresponding record for this reference.
    44. 44
      Anand, P.; Kunnumakkara, A. B.; Newman, R. A.; Aggarwal, B. B. Mol. Pharmacol. 2007, 4, 807– 818
      There is no corresponding record for this reference.
    45. 45
      Price, L. C.; Buescher, R. W. J. Food Sci. 1997, 62, 267– 269
      There is no corresponding record for this reference.
    46. 46
      Wang, Y. J.; Pan, M. H.; Cheng, A. L.; Lin, L. I.; Ho, Y. S.; Hseieh, C. Y.; Lin, J. K. J. Pharm. Biomed. Anal. 1997, 15, 1867– 1876
      46
      Stability of curcumin in buffer solutions and characterization of its degradation products
      Wang, Ying-Jan; Pan, Min-Hsiung; Cheng, Ann-Lii; Lin, Liang-In; Ho, Yuan-Soon; Hsieh, Chang-Yao; Lin, Jen-Kun
      Journal of Pharmaceutical and Biomedical Analysis (1997), 15 (12), 1867-1876CODEN: JPBADA; ISSN:0731-7085. (Elsevier)
      The degrdn. kinetics of curcumin under various pH conditions and the stability of curcumin in physiol. matrixes were investigated. When curcumin was incubated in 0.1 M phosphate buffer and serum-free medium, pH 7.2 at 37°C, about 90% decompd. within 30 min. A series of pH conditions ranging from 3 to 10 were tested and the result showed that decompn. was pH-dependent and occurred faster at neutral-basic conditions. It is more stable in cell culture medium contg. 10% fetal calf serum and in human blood; less than 20% of curcumin decompd. within 1 h, and after incubation for 8 h, about 50% of curcumin is still remained. Trans-6-(4'-hydroxy-3'-methoxyphenyl)-2,4-dioxo-5-hexenal was predicted as major degrdn. product and vanillin, ferulic acid, feruloyl methane were identified as minor degrdn. products. The amt. of vanillin increased with incubation time.
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    47. 47
      Sahu, A.; Bora, U.; Kasoju, N.; Goswami, P. Acta Biomater. 2008, 4, 1752– 61
      There is no corresponding record for this reference.
    48. 48
      Ma, Z.; Haddadi, A.; Molavi, O.; Lavasanifar, A.; Lai, R.; Samuel, J. J. Biomed. Mater. Res., Part A 2008, 86, 300– 310
      48
      Micelles of poly(ethylene oxide)-b-poly(ε-caprolactone) as vehicles for the solubilization, stabilization, and controlled delivery of curcumin
      Ma, Zengshuan; Haddadi, Azita; Molavi, Ommoleila; Lavasanifar, Afsaneh; Lai, Raymond; Samuel, John
      Journal of Biomedical Materials Research, Part A (2008), 86A (2), 300-310CODEN: JBMRCH; ISSN:1549-3296. (John Wiley & Sons, Inc.)
      Curcumin is recognized as a potential chemotherapeutic agent against a variety of tumors. However, the clin. application of curcumin is hindered due to its poor water soly. and fast degrdn. The objective of this study was to investigate amphiphilic block copolymer micelles of poly(ethylene oxide)-b-poly(ε-caprolactone) (PEO-PCL) as vehicles for the solubilization, stabilization, and controlled delivery of curcumin. Curcumin-loaded PEO-PCL micelles were prepd. by a cosolvent evapn. technique. PEO-PCL micelles were able to solubilize curcumin effectively, protect the encapsulated curcumin from hydrolytic degrdn. in physiol. matrix, and control the release of curcumin over a few days. The characteristics of resultant micelles were found to depend on the polymn. degrees of ε-caprolactone. Among different PEO-PCL micelles, PEO5000-PCL24500 was the most efficient in solubilizing curcumin while PEO5000-PCL13000 was the best carrier in reducing its release rate. PEO-PCL micelle-encapsulated curcumin retained its cytotoxicity in B16-F10, a mouse melanoma cell line, and SP-53, Mino, and JeKo-1 human mantle cell lymphoma cell lines. These results demonstrated the potential of PEO-PCL micelles as an injectable formulation for efficient solubilization, stabilization, and controlled delivery of curcumin.
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    49. 49
      Li, L.; Ahmed, B.; Mehta, K.; Kurzrock, R. Mol. Cancer Ther. 2007, 6, 1276– 1282
      49
      Liposomal curcumin with and without oxaliplatin: effects on cell growth, apoptosis, and angiogenesis in colorectal cancer
      Li, Lan; Ahmed, Bilal; Mehta, Kapil; Kurzrock, Razelle
      Molecular Cancer Therapeutics (2007), 6 (4), 1276-1282CODEN: MCTOCF; ISSN:1535-7163. (American Association for Cancer Research)
      The role of curcumin (diferuloylmethane), a proapoptotic compd., for the treatment of cancer has been an area of growing interest. Curcumin in its free form is poorly absorbed in the gastrointestinal tract and therefore may be limited in its clin. efficacy. Liposome encapsulation of this compd. would allow systemic administration. The current study evaluated the preclin. antitumor activity of liposomal curcumin in colorectal cancer. We also compared the efficacy of liposomal curcumin with oxaliplatin, a std. chemotherapy for this malignancy. In vitro treatment with liposomal curcumin induced a dose-dependent growth inhibition [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium salt] and apoptosis [poly(ADP-ribose) polymerase] in the two human colorectal cancer cell lines tested (LoVo and Colo205 cells). There was also synergism between liposomal curcumin and oxaliplatin at a ratio of 4:1 in LoVo cells in vitro. In vivo, significant tumor growth inhibition was obsd. in Colo205 and LoVo xenografts, and the growth inhibition by liposomal curcumin was greater than that for oxaliplatin (P < 0.05) in Colo205 cells. Tumors from animals treated with liposomal curcumin showed an antiangiogenic effect, including attenuation of CD31 (an endothelial marker), vascular endothelial growth factor, and interleukin-8 expression by immunohistochem. This study establishes the comparable or greater growth-inhibitory and apoptotic effects of liposomal curcumin with oxaliplatin both in vitro and in vivo in colorectal cancer. We are currently developing liposomal curcumin for introduction into the clin. setting.
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    50. 50
      Bisht, S.; Feldmann, G.; Soni, S.; Ravi, R.; Karikar, C.; Maitra, A.; Maitra, A. J. Nanobiotechnol. 2007, 5:3, 1– 18
      There is no corresponding record for this reference.
    51. 51
      Sou, K.; Inenaga, S.; Takeoka, S.; Tsuchida, E. Int. J. Pharm. 2008, 352, 287– 293
      51
      Loading of curcumin into macrophages using lipid-based nanoparticles
      Sou, Keitaro; Inenaga, Shunsuke; Takeoka, Shinji; Tsuchida, Eishun
      International Journal of Pharmaceutics (2008), 352 (1-2), 287-293CODEN: IJPHDE; ISSN:0378-5173. (Elsevier B.V.)
      Curcumin (1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, Cm) is a natural compd. which possesses antioxidant, anti-inflammatory and anti-tumor ability. Here, phospholipid vesicles or lipid-nanospheres embedding Cm (CmVe or CmLn) were formulated to deliver Cm into tissue macrophages through i.v. injection. Cm could be solubilized in hydrophobic regions of these particles to form nanoparticle dispersions, and these formulations showed ability to scavenge reactive oxygen species as antioxidants in dispersions. At 6 h after i.v. injection in rats via the tail vein (2 mg Cm/kg body wt.), confocal microscopic observations of tissue sections showed that Cm was massively distributed in cells assumed as macrophages into the bone marrow and spleen. Taken together, these results indicate that the lipid-based nanoparticulates provide improved i.v. delivery of Cm to tissues macrophages, specifically bone marrow and splenic macrophages in present formulation, which has therapeutic potential as an antioxidant and anti-inflammatory.
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    52. 52
      Vemula, P. K.; Li, J.; John, G. J. Am. Chem. Soc. 2006, 128, 8932– 8938
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    53. 53
      Kumar, V.; Lewis, S. A.; Mutalik, S.; Shenoy, D. B.; Venkatesh, U. N. Indian J. Physiol. Pharmacol. 2002, 46, 209– 217
      53
      Biodegradable microspheres of curcumin for treatment of inflammation
      Kumar, Virender; Lewis, Shaila Angela; Mutalik, Srinivas; Shenoy, Dinesh B.; Venkatesh; Udupa, N.
      Indian Journal of Physiology and Pharmacology (2002), 46 (2), 209-217CODEN: IJPPAZ; ISSN:0019-5499. (Association of Physiologists and Pharmacologists of India)
      Curcumin, a natural constituent of Curcuma longa was formulated as prolonged release biodegradable microspheres for treatment of inflammation. Natural biodegradable polymers, namely, bovine serum albumin and chitosan were used to encapsulate curcumin to form a depot forming drug delivery system. Microspheres were prepd. by emulsion-solvent evapn. method coupled with chem. crosslinking of the natural polymers. Curcumin could be encapsulated into the biodegradable carriers up to an extent of 79.49 and 39.66% resp. with albumin and chitosan. Different drug:polymer ratios did not affect the mean particle size or particle size distribution significantly. However, the concn. of the crosslinking agent had remarkable influence on the drug release. In-vitro release studies indicated a biphasic drug release pattern, characterized by a typical burst-effect followed by a slow release which continued for several days. Evaluation of antiinflammatory activity using Freund's adjuvant induced arthritic model in Wistar rats revealed significant difference between both the formulations, albumin microspheres and chitosan microspheres as well as against control. It was evident from the present study that the curcumin biodegradable microspheres could be successfully employed as prolonged release drug delivery system for better therapeutic management of inflammation as compared to oral or s.c. route.
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    54. 54
      Salmaso, S.; Bersani, S.; Semenzato, A.; Caliceti, P. J. Drug Targeting 2007, 15, 379– 390
      54
      New cyclodextrin bioconjugates for active tumour targeting
      Salmaso, Stefano; Bersani, Sara; Semenzato, Alessandra; Caliceti, Paolo
      Journal of Drug Targeting (2007), 15 (6), 379-390CODEN: JDTAEH; ISSN:1061-186X. (Informa Healthcare)
      A new cyclodextrin-based carrier for active targeting of low sol. and degradable drugs has been synthesized and characterized. β-Cyclodextrins were first reacted with excess hexamethylene diisocyanate and the resulting CD-(C6-NCO)5 deriv. was reacted with 700 Da diamino-PEG to yield CD-(C6-PEG-NH2)5. About one out of five free amino groups of PEG were functionalized with folic acid (FA) as a tumor targeting moiety. The chem. structures of the intermediates as well as the final product, CD-(C6-PEG)5-FA, were characterized by 1H and 13C NMR, reverse phase and gel permeation chromatog., and UV-Vis spectroscopy. After modification, the hemolytic activity of β-cyclodextrins decreased by about 70%. In the presence of the new carrier, the β-estradiol soly. increased by more than 300-fold and the chlorambucil degrdn. rate decreased by 50-60%. CD-(C6-PEG)5-FA formed an inclusion complex with curcumin displaying an assocn. const. of 954,732 M-1. The new carrier increased the curcumin soly. by about 3200-fold as compared to native β-cyclodextrins and reduced its degrdn. rate at pH 6.5 and 7.2 by 10 and 45-fold, resp. FA receptor-overexpressing human nasopharyngeal tumor KB cell lines and nonfolic acid receptor-expressing human breast cancer MCF7 cells were used to evaluate the targeting properties of the new drug delivery system. The in vitro studies demonstrate that the new carrier possesses potential selectivity for the folate receptor-overexpressing tumor cells as ED50 values of 52 μM, 58 μM and 21 μM were obtained with curcumin-loaded CD-(C6-PEG-NH2)5, curcumin in fetal serum medium and CD-(C6-PEG)5-FA, resp.
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      Heintza, A.; Lehmanna, J. K.; Kozlovab, S. A.; Balantsevac, E. V.; Bazylevad, A. B.; Ondoe., D. Fluid Phase Equilib. 2010, 294, 187– 196
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      Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33– 36
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      Gao, H.; Li, J.; Han, B.; Chen, W.; Zhang, J.; Zhang, R.; Yan, D. Phys. Chem. Chem. Phys. 2004, 6, 2914– 2916
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      Gao, Y.; Han, S.; Han, B.; Li, G.; Shen, D.; Li, Z.; Du, J.; Hou, W.; Zhang, G. Langmuir 2005, 21, 5681– 5684
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      Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K.; Grillo, I. J. Am. Chem. Soc. 2005, 127, 7302– 7303
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      Patrascu, C.; Gauffre, F.; Nallet, F.; Bordes, R.; Oberdisse, J.; de Lauth- Viguerie, N.; Mingotaud, C. ChemPhysChem 2006, 7, 99– 101
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      Wei, G.-T.; Yang, Z.; Lee, C.-Y.; Yang, H. Y.; Wang, C. R. J. Am. Chem. Soc. 2004, 126, 5036– 5037
      61
      Aqueous-Organic Phase Transfer of Gold Nanoparticles and Gold Nanorods Using an Ionic Liquid
      Wei, Guor-Tzo; Yang, Zusing; Lee, Chia-Ying; Yang, Hsiao-Yen; Wang, C. R. Chris
      Journal of the American Chemical Society (2004), 126 (16), 5036-5037CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The water-immiscible ionic liq., [C4MIM][PF6], is a solvent medium that allows complete transfer of gold nanoparticles from an aq. phase into an org. phase. Both spherical and rod-shaped gold nanoparticles are efficiently transferred from an aq. soln. into the org. phase without requiring the use of thiols. The sizes and shapes of the gold nanoparticles were preserved during the phase-transfer process when a surfactant was added to the ionic liq. This process offers a simple approach for obtaining solns. of differently sized and shaped gold nanoparticles in ionic liqs.
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      Stolte, S.; Matzke, M.; Arning, J.; Boeschen, A.; Pitner, W. R.; Welz-Biermann, U.; Jastorff, B.; Ranke, J. Green Chem. 2007, 9, 1170– 1779
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      Zhao, D.; Liao, Y.; Zhang, Z. Clean: Soil, Air, Water 2007, 35, 42– 48
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      Toxicity of ionic liquids
      Zhao, Dongbin; Liao, Yongcheng; Zhang, Ziding
      Clean: Soil, Air, Water (2007), 35 (1), 42-48CODEN: CSAWAC; ISSN:1863-0650. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The dramatic growth in ionic liq. research over the past decade has resulted in the development of a huge no. of novel ionic liqs., as well as many assocd. applications. The perceived environmentally friendly nature of ionic liqs., which results from their negligible vapor pressure, is now under scrutiny since although they will not evap. into air, it is not possible to guarantee that they will never enter the environment. Toxicity research studies including ecotoxicity have recently received broad attention and the commonly accepted notion that ionic liqs. have low toxicity has been shown to be incorrect. This review attempts to highlight the progress of ionic liq. toxicity research as well as the development of degradable and biorenewable ionic liqs.
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      Peplow, M. Nature 2005,  DOI: 10.1038/news051031-8
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      Carter, E. B.; Culver, S. L.; Fox, P. A.; Goode, R. D.; Ntai, I.; Tickell, M. D.; Traylor, R. K.; Hoffman, N. W.; Davis, J. H., Jr. Chem. Commun. 2004, 630– 631
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      Hough, W. L.; Smiglak, M.; Rodrıguez, H.; Swatloski, R. P.; Spear, S. K.; Daly, D. T.; Pernak, J.; Grisel, J. E.; Carliss, R. D.; Soutullo, M. D., Jr; Davis, J. H.; Rogers, R. D. New J. Chem. 2007, 31, 1429– 1436
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      Choi, S. Y.; Rodríguez, H.; Mirjafari, A.; Gilpin, D. F.; McGrath, S.; Malcolm, K. R.; Tunney, M. M.; Rogers, R. D.; McNally, T. Green Chem. 2011, 13, 1527– 1535
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      Miskolczy, Z.; Sebok-Nagy, K.; Biczok, L.; Gökturk, S. Chem. Phys. Lett. 2004, 400, 296– 300
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      Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum: New York, 1999; Vol. 2.
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      Nardo, L.; Paderno, R.; Andreoni, A.; Masson, M.; Haukvik, T.; Tonnesen, H. H. Spectroscopy 2008, 22, 187– 198
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      Nardo, L.; Andreoni, A.; Bondani, M.; Masson, M.; Tonnesen, H. H. J. Photochem. Photobiol., B 2009, 97, 77– 86
      81
      Studies on curcumin and curcuminoids. XXXIV. Photophysical properties of a symmetrical, non-substituted curcumin analogue
      Nardo, Luca; Andreoni, Alessandra; Bondani, Maria; Masson, Mar; Hjorth Tonnesen, Hanne
      Journal of Photochemistry and Photobiology, B: Biology (2009), 97 (2), 77-86CODEN: JPPBEG; ISSN:1011-1344. (Elsevier B.V.)
      Curcumin is the main constituent of curry. In its ground state it shows chemo-preventive, chemo-therapeutic, anti-inflammatory and immune stimulating effects, and it is considered as a drug or drug model in the treatment of AIDS and cystic fibrosis. Further biol. activity is induced in curcumin by light exposure: cytotoxicity is enhanced and photosensitized antibacterial effects are achieved. For the curcumin cis enol conformer, the fastest deactivation mechanism of the first excited singlet state is an excited-state intra-mol. proton transfer, which brings curcumin back to the ground state. This mechanism, as well as reketonization, interaction with the solvent and photodegrdn., compete with the phototherapeutic action. The native compd. curcumin carries phenolic hydroxyl and methoxy groups that influence the mol. charge distribution and hence the excited-state intra-mol. proton transfer rate in an unpredictable way. In this work we study static and time-resolved spectroscopic properties of a nonsubstituted curcuminoid that lacks both the phenolic hydroxyl and the phenolic methoxy groups. The photophys. properties of this compd. are compared to those of native curcumin, in order to provide a rationale to the design of curcuminoids with mol. structures optimized for a photosensitizer.
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    82. 82
      Caselli, M.; Ferrari, E.; Imbriano, C.; Pignedoli, F.; Saladini, M.; Ponterini, G. J. Photochem. Photobiol., A 2010, 210, 115– 124
      82
      Probing solute-solvent hydrogen bonding with fluorescent water-soluble curcuminoids
      Caselli, Monica; Ferrari, Erika; Imbriano, Carol; Pignedoli, Francesca; Saladini, Monica; Ponterini, Glauco
      Journal of Photochemistry and Photobiology, A: Chemistry (2010), 210 (2-3), 115-124CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
      Glycosylated water-sol. curcuminoids (C1-3, first scheme of this article) differing in the 3,3'-ring substituents (-OH, -OCH3 and H) equilibrate between the di-keto and the keto-enol forms. The former are well observable in the absorption spectra in water, but their emissions are always negligible. The keto-enol forms of C1-3 exhibit a broad range of fluorescence quantum yields in different solvents, org. and water: formation of solute-solvent hydrogen bonds through the 3,3'-ring substituents may change the radiationless S1-state decay const. by up to a factor 200. Such a fluorescence quenching mechanism is extremely efficient in water and, for C1, in accepting org. media. On the contrary, no effects traceable to intermol. hydrogen bonds involving the central β-dicarbonyl moiety have been obsd. So, fluorescence of these curcuminoids may probe the hydrogen bonding ability, particularly as acceptor, of their microenvironments, including hydrophilic/hydrophobic domains in complex biol. systems. Interaction of C1 and C2 with bovine serum albumin results in emission enhancements inverse to the quantum yields of the curcuminoids in water. The observations support the idea that, although the curcuminoid microenvironment within its complex with the protein is less polar and hydrogen bonding than water itself, residual water/ligand hydrogen bonds are active in enhancing radiationless transitions. Finally, fluorescence confocal images of HCT116 cells treated with C1-3 suggest the apparently small structural differences to affect, besides their fluorescence behavior, their interactions and fate within living cells.
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    83. 83
      Adhikary, R.; Carlson, P. J.; Kee, T. W.; Petrich, J. W. J. Phys. Chem. B 2010, 114, 2997– 3004
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      Ghosh, R.; Mondal, J. A.; Palit, D. K. J. Phys. Chem. B 2010, 114, 12129– 12143
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      Mandy, H. M.; Leung., Tak W. K. Langmuir 2009, 25, 5773– 5777
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      Das, P.; Sarkar, D.; Chattopadhyay, N. Chem. Phys. Lipids 2008, 158, 38– 45
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      Sarkar, D.; Bose, D.; Mahato, A.; Ghosh, D.; Chattopadhyay, N. J. Phys. Chem. B 2010, 114, 2261– 2269
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      Rodrigues, C.; Gamerio, P.; Reis, S.; Lima, J. L. F. C.; Castro, B. D. Langmuir 2002, 18, 10231– 10236
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      Das, P.; Chakrabarty, A.; Haldar, B.; Mallick, A.; Chattopadhyay, N. J. Phys. Chem. B 2007, 111, 7401– 7408
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      Mallick, A.; Haldar, B.; Maiti, S.; Bera, S. C.; Chattopadhyay, N. J. Phys. Chem. B 2005, 109, 14675– 14682
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      Paul, B. K.; Samanta, A.; Guchhait, N. Langmuir 2010, 26, 3214– 3224
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      Mukherjee, S.; Chattopadhyay, A. Langmuir 2005, 21, 287– 293
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      Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279– 291
      93
      Dynamic and static aspects of solubilization of neutral arenes in ionic micellar solutions
      Almgren, Mats; Grieser, Franz; Thomas, J. Kerry
      Journal of the American Chemical Society (1979), 101 (2), 279-91CODEN: JACSAT; ISSN:0002-7863.
      The kinetics of solubilization and the solubilities of neutral arenes in ionic micellar systems were measured by using phosphorescence, fluorescence, and steady-state absorption techniques. The exit rates for the arenes naphthalene, biphenyl, and 1-methylnaphthalene, measured by using their phosphorescence as a monitor, are >5 × 104 s-1. The exit rate of 1-bromonaphthalene, from NaLS micelles, is 2.5 × 104 s-1. For this mol. the entrance rate is 5-8 × 109 M-1 s-1, which can be considered as a representative value for other arenes. Distribution consts. between arenes and micelles were measured at low and satn. probe to micelle ratios. The distribution consts. obtained at the different probe to micelle ratios are in approx. agreement, indicating that the arenes are dispersed among the micelles consistent with a Poisson distribution. An empirical model for the solubilization process is presented which incorporates the site of solubilization of the arene and the factors which govern the exit and entrance rates from the micelle.
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      Dávila, M. J.; Aparicio, S.; Alcalde, R.; García, B.; Leal, J. M. Green Chem. 2007, 9, 221– 232
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      Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951– 5952
      95
      Luminescent probes for detergent solutions. A simple procedure for determination of the mean aggregation number of micelles
      Turro, Nicholas J.; Yekta, Ahmad
      Journal of the American Chemical Society (1978), 100 (18), 5951-2CODEN: JACSAT; ISSN:0002-7863.
      A procedure for detn. of the mean aggregation no. of detergent solns. is proposed and tested for the Na dodecyl sulfate system. The method uses simple experimentation to measure quenching of a luminescent probe by a micelle assocd. quencher. The results agree with those obtained by membrane osmometry and classical light scattering studies, but the advantage is that the mean aggregation no. of highly concd. detergent solns. can be measured.
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    96. 96
      Gorman, A. A.; Hamblett, I.; Srinivasan, V. S.; Wood, P. D. Photochem. Photobiol. 1994, 59, 389– 398
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      Barik, A.; Priyadarsini, K. I.; Mohan, H. Orient. J. Chem. 2002, 18, 427– 432
      97
      Excited state photophysical properties of curcumin and its methoxy derivative in benzene
      Barik, Atanu; Priyadarsini, Indira; Mohan, Hari
      Oriental Journal of Chemistry (2002), 18 (3), 427-432CODEN: OJCHEG; ISSN:0970-020X. (Oriental Scientific Publishing Co.)
      Absorption and fluorescence spectra of curcumin (C) and its methoxy deriv. (TMC) were recorded in benzene. The fluorescence quantum yields were detd. using coumarin-153 as std. On 355 nm picosecond (35 ps) laser excitation, the singlet-singlet absorption spectra of curcumin and TMC showed absorption band at 555 and 550 nm resp. The triplet excited states were populated by intersystem crossing from the singlet excited states in benzene. The triplets exhibit a broad absorption band in 500-700 nm region. The triplet-triplet absorption spectra in benzene were also recorded by energy transfer from pulse radiolytically generated biphenyl triplet. The rates of intersystem crossing, nonradiative decay, extinction coeffs. of the triplet excited states, their half-lives were reported. Probably both curcumin and its methoxy deriv. show similar photophys. properties in the excited state.
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    98. 98
      Barik, A.; Goel, N. K.; Priyadarsini, K. I.; Mohan, H. J. Photosci. 2004, 11, 95– 99
      98
      Effect of deuterated solvents on the excited state photophysical properties of curcumin
      Barik, A.; Goel, N. K.; Priyadarsini, K. I.; Mohan, Hari
      Journal of Photoscience (2004), 11 (3), 95-99CODEN: JOPHFS; ISSN:1225-8555. (Korean Society of Photoscience)
      Optical absorption and emission studies have been carried out to understand the effect of deuterium on the solvent dependent photophys. properties of curcumin in deuterated solvents such as CDCl3, (CD3)2SO, (CD3)2CO, CD3OD and CD3CN. Optical absorption spectral studies showed that there is no significant shift in absorption maxima compared to the non-deuterated solvent. The fluorescence maxima shows significant shift with polarity of solvent but not much affected by the deuteration. The fluorescence quantum yield of curcumin increased marginally in almost all the deuterated solvents, indicating redn. in the non-radiative pathways. The fluorescence decay was biexponential in all the solvents and the av. fluorescence lifetime was not much affected with deuteration, but showed decrease with increasing solvent polarity. Based on these studies, it is concluded that intermol. hydrogen transfer is only partially responsible for the excited state deactivation of curcumin.
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    99. 99
      Silva, A. M. S.; Filipe, P.; Seixas, R.; Pinto, D.; Patterson, L. K.; Hug, G. L.; Cavaleiro, J. A. S.; Maziere, J. C.; Santus, R.; Morliere, P. J. Phys. Chem. B 2008, 112, 11456– 11461
      There is no corresponding record for this reference.
    100. 100
      Kohara., N.; Sano, C.; Ikuno, H.; Magoshi, Y.; Becker, M. A.; Yatagai, M.; Saito, M. ACS Symp. Ser. 2001, 779, 74– 85
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      Bagole, K. N.; Boland, P. G.; Wagner, B. D. J. Photochem. Photobiol., A 2005, 173, 230– 237
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      Swaroop, S.; Mishra, B.; Priyadarsni, K. I. Proc. Natl. Acad. Sci., India 2007, 77, 205– 211
      102
      Studies on β-cyclodextrin inclusion complex of curcumin
      Swaroop, S.; Mishra, Beena; Priyadarsini, K. Indira
      Proceedings of the National Academy of Sciences, India, Section A: Physical Sciences (2007), 77 (3), 205-211CODEN: PAIAA3; ISSN:0369-8203. (National Academy of Sciences, India)
      Formation of inclusion complex of curcumin with β-cyclodextrin (β-CD) has been characterized by change in the absorption spectrum, blue shifted fluorescence spectrum and increase in fluorescence intensity. The spectral changes could be fitted to 1:1 complex formation with estd. binding const. of 2.6 ± 0.3 × 102 M-1. At high concns. (> 4 mM) the data indicated formation of 1:2 complex. Fluorescence measurements in the temp. range from 293 to 323 K showed a steady decrease in binding const. with increase in temp., from which the thermodn. parameters ΔH and ΔS, for the 1:1 complexation have been estd. to be -8.5 kJ/mol and -0.02 kJ/mol/K resp. Kinetics of binding was studied by stopped flow technique from which the binding consts. for 1:1 and 1:2 complex formation could be resolved as 6.87 × 102 M-1 and 6.8 × 104 M-2. Further, influence of such inclusion complex on change in superoxide radical scavenging property of curcumin was examd. using xanthine/xanthine oxidase assay.
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    103. 103
      Rankin, M. A.; Wagner, B. D. Supramol. Chem. 2004, 16, 513– 519
      103
      Fluorescence enhancement of curcumin upon inclusion into cucurbituril
      Rankin, Matthew A.; Wagner, Brian D.
      Supramolecular Chemistry (2004), 16 (7), 513-519CODEN: SCHEER; ISSN:1061-0278. (Taylor & Francis Ltd.)
      The effect of the macrocyclic host compds. cucurbit[n]urils (Qn), with n = 5-7, on the fluorescence of the biol. active compd. curcumin was studied. Curcumin, the main constituent of the Indian spice turmeric, is of growing interest because of its wide-ranging pharmaceutical properties. This compd. forms strong 2:1 host-guest inclusion complexes with Q6 (the original cucurbituril), with an overall equil. const. of (1.9 ± 0.8) × 104 M-2. It is postulated that a Q6 host partially encapsulates each of the two Ph groups at the ends of the curcumin mol. The difference in magnitude of the equil. consts. K1 (72 ± 2 M-1) and K2 (260 ± 120 M-1) for stepwise encapsulation of the two ends of the curcumin mol. indicates that encapsulation by the first Q6 significantly alters its entire electronic structure, resulting in a more favorable second encapsulation. A very large enhancement of the fluorescence of curcumin results from this complex formation, on the order of 5.0; this is a significant fraction of the polarity sensitivity factor (PSF) of 39 measured for curcumin, that is the ratio of fluorescence intensity in ethanol vs. water. Surprisingly, no such enhancement could be obsd. in the case of Q7, indicating that the interactions between the guest and the host cavity are not favorable in this case, contrary to expectations. Similarly, no enhancement was obsd. in the case of Q5, which is not unexpected, because of the extremely small size of the host cavity and portal in this case.
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    Absorption spectra of curcumin inside IL–micelle upon addition of IL and salt. This material is available free of charge via the Internet at http://pubs.acs.org.


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