Redetermined crystal structure of α-DL-methionine at 340 K

Abstract

Two forms, α and β, are known for the racemic amino acid DL-methionine, C5H11NO2S. The phase transition between them, taking place around 326 K, is associated with sliding at the central interfaces of the hydrophobic regions in the crystal, leaving the hydrogen-bonding pattern unperturbed. For the high-temperature α phase, only a structure of rather low quality has been available [Rfactor = 0.118, no H-atom coordinates; Taniguchiet al.(1980).Bull. Chem. Soc. Jpn,53, 803–804]. We here present accurate structural data for this polymorph [R(F) = 0.049], which are compared with other related amino acid structures with similar properties. We report for the first time that the side chain of this phase has a minor disorder component [occupancy 0.0491 (18)] with agauche+ rather than agauche− conformation for the N—C—C—C group. In the crystal of the title compound, N—H...O hydrogen bonds link the molecules into (100) sheets.