High‐pressure polymorphism in l ‐cysteine: the crystal structures of l ‐cysteine‐III and l ‐cysteine‐IV

The crystal structure of the orthorhombic phase of l ‐cysteine (hereafter l ‐cysteine‐I) consists of chains of molecules linked via NH⋯O hydrogen bonds. The chains are linked into a layer by other NH⋯O hydrogen bonds, forming ring motifs. The layers are linked by further NH⋯O and disordered SH⋯S/SH⋯O interactions. The main effects of compression to 1.8 GPa are to contract voids in the middle of the rings and to reduce S⋯S distances from 3.8457 (10) to 3.450 (4) Å. The latter is at the lower limit for S⋯S distances and we suggest that strain about the S atom is responsible for the formation of a new phase of l ‐cysteine, l ‐cysteine‐III, above 1.8 GPa. The phase transition is accompanied by a change in the NCCS torsion angle from ca 60 to ca −60° and small positional displacements, but with no major changes in the orientations of the molecules. The structure of l ‐cysteine‐III contains similar R ‐type ring motifs to l ‐cysteine‐I, but there are no S⋯S contacts within 3.6 Å. l ‐Cysteine‐III was found to be stable to at least 4.2 GPa. On decompression to 1.7 GPa, another single‐crystal to single‐crystal phase transition formed another previously uncharacterized phase, l ‐cysteine‐IV. This phase is not observed on increasing pressure. The structure consists of two crystallographically independent cysteine molecules in the same conformations as those found in l ‐cysteine‐I and l ‐cysteine‐III. The structure separates into zones with are alternately phase I‐like and phase III‐like. l ‐Cysteine‐IV can therefore be thought of as an unusual example of an intermediate phase. Further decompression to ambient pressure generates l ‐cysteine‐I.