Acta Crystallographica Section B

Structural Science

Volume 62, Part 2 (April 2006)

research papers

Acta Cryst. (2006). B62, 296-309 [ doi:10.1107/S0108768105038802 ]

High-pressure polymorphism in L-cysteine: the crystal structures of L-cysteine-III and L-cysteine-IV

S. A. Moggach, D. R. Allan, S. J. Clark, M. J. Gutmann, S. Parsons, C. R. Pulham and L. Sawyer

Abstract: The crystal structure of the orthorhombic phase of L-cysteine (hereafter L-cysteine-I) consists of chains of molecules linked via NHO hydrogen bonds. The chains are linked into a layer by other NHO hydrogen bonds, forming ring motifs. The layers are linked by further NHO and disordered SHS/SHO interactions. The main effects of compression to 1.8 GPa are to contract voids in the middle of the rings and to reduce SS distances from 3.8457 (10) to 3.450 (4) Å. The latter is at the lower limit for SS 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 SS 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.

Keywords: compression; conformation; chain molecules; hydrogen bonds.


	Structure factor file (CIF format) (67.3 kbytes) [ doi:10.1107/S0108768105038802/bs5023cys100sup2.hkl ] Contains datablock cys100

	Structure factor file (CIF format) (21.1 kbytes) [ doi:10.1107/S0108768105038802/bs5023cys118sup3.hkl ] Contains datablock cys118

	Structure factor file (CIF format) (19.5 kbytes) [ doi:10.1107/S0108768105038802/bs5023cys326sup4.hkl ] Contains datablock cys326

	Structure factor file (CIF format) (19.3 kbytes) [ doi:10.1107/S0108768105038802/bs5023cys342sup5.hkl ] Contains datablock cys342

	Structure factor file (CIF format) (31.8 kbytes) [ doi:10.1107/S0108768105038802/bs5023cys417sup6.hkl ] Contains datablock cys417

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