Two forms, α and β, are known for the racemic amino acid dl-me-thio-nine, C5H11NO2S. The phase transition between them, taking place around 326 K, is associated with sliding at the central inter-faces of the hydro-phobic 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 [R factor = 0.118, no H-atom coordinates; Taniguchi et 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 a gauche+ rather than a gauche- conformation for the N-C-C-C group. In the crystal of the title compound, N-H⋯O hydrogen bonds link the mol-ecules into (100) sheets.
Keywords: crystal structure; disorder; hydrogen bonding; phase transition; zwitterions.