The Polymorphs of L-Phenylalanine†
This work was supported by the Department of Chemistry,University of Oslo
Graphical Abstract
It was long considered impossible to prepare crystals of L-phenylalanine, the prototypical aromatic amino acid. High-quality crystals have now been obtained that provide solid-state data for two polymorphs. For one of these polymorphs the C2 space group previously assigned was revised to P21 (with Z′=4). Good-quality crystals can also be grown from solutions of dl-phenylalanine, but these are not proper racemic crystals.
Abstract
The solid-state structure of the amino acid phenylalanine (Phe) offers a potential key to understanding the behavior of a large class of important aromatic compounds. Obtaining good single crystals is, however, notoriously difficult. The structure of the common polymorph of Phe, form I, was first reported by Weissbuch et al. (as D-Phe) in 1990, but the correctness of the published C2 unit cell with two disordered molecules in the asymmetric unit was later questioned and other space groups suggested. The identity of form I of L-Phe is here established to be P21 with Z′=4, based on data from a well-diffracting single crystal grown from an acetic acid solution of the amino acid. A second new polymorph, form IV, together with the two recently described forms II and III provide unprecedented information on the structural complexity of this essential amino acid. It is furthermore documented that the racemate, dl-Phe, does not grow proper single crystals.
