title: The polymorphs of L-phenylalanine.
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.
authors.affiliation: Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo (Norway).
authors.forename: Franziska Stefanie
authors.initials: FS
authors.lastname: Ihlefeldt
authors.forename: Fredrik Bjarte
authors.initials: FB
authors.lastname: Pettersen
authors.forename: Aidan
authors.initials: A
authors.lastname: von Bonin
authors.forename: Malgorzata
authors.initials: M
authors.lastname: Zawadzka
authors.forename: Carl Henrik
authors.initials: CH
authors.lastname: Görbitz
mesh_terms: D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008968:Molecular Conformation; D010649:Phenylalanine / Q000737:chemistry*
chemical_list: D010649:Phenylalanine
keywords: amino acids; hydrogen bonds; polymorphism; pseudosymmetry; racemic twins