Abstract
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 R4(4)(16) rings and to reduce S...S distances from 3.8457 (10) to 3.450 (4) angstroms. 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 degrees 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 angstroms. 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.
Highlights
The response of crystalline molecular solids to high hydrostatic pressure is a rapidly advancing area of interest in smallmolecule crystallography
We show in x3 that one effect of compression of l-cysteine-I is a substantial shortening of the SÁ Á ÁS distances
We note in passing that the headto-tail N1H7Á Á ÁO2 interaction is characteristic of many amino acid structures; we have studied its response to pressure in lserine, l-cystine and -glycine, and it is always amongst the least compressible of the NHÁ Á ÁO hydrogen bonds
Summary
The response of crystalline molecular solids to high hydrostatic pressure is a rapidly advancing area of interest in smallmolecule crystallography. The relative compressibility of crystals of serine and glycine, serine-II) at 4.8 GPa Hydrogen bonds and other intermolecular interactions both undergo single-crystal to single-crystal phase transitions formed along the directions in which the voids compress to high-pressure polymorphs. In other work on twinned crystals we have observed that anomalously low values of Tmin are sometimes observed after such integrations, even with ambient pressure data sets. This can be traced to the problems associated with peaks containing partially overlapped contributions from the different twin domains. The structure of the racemate, d,l-cysteine, was determined by Luger & Weber (1999)
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