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 R₄ ⁴(16) 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 R₄₄(16) 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.