The effect of pressure on the crystal structure of hexagonal L-cystine

The crystal structure of hexagonal L-cystine has been determined at room temperature at pressures between 0.4 and 3.7 GPa; unit-cell dimensions were measured up to 6.4 GPa. The structure of this phase consists of molecules in their zwitterionic form, and crystallizes in the hexagonal space group P6 ₁22. The structure consists of hydrogen-bonded layers which are strongly reminiscent of those seen in a-glycine, and consist of R ₄⁴(16) hydrogen-bonded ring motifs. These layers are connected on one side by the disulflde bridges within the cystine molecules, and on the other by NH⋯O hydrogen bonds to other glycine-like layers. The most compressible unit-cell dimension, and the direction of greatest strain in the structure, is along the c-axis, and application of pressure pushes the layers closer together. The compression occurs approximately equally in the regions of the interlayer hydrogen bonds and the disulflde bridges; in the latter, changes in the C-S-S-C torsion angles allow the cystine molecules to act like springs. The effects of pressure can be interpreted in terms of closing-up of voids in the structure, and this leads to (i) a lessening of the N-C-C-O and C-S-S-C torsional angles, (ii) shortening of the N-H⋯O hydrogen bonds by 0.10-0.60 Å and (iii) a further shortening of an already short S⋯S contact from 3.444 (4) Å to 3.264 (4) Å.