Low temperature/high pressure polymorphism in dl-cysteine

We compare the response of the crystalline dl-cysteine to cooling and to increasing pressure. The structure undergoes a low-temperature phase transition into an isosymmetric polymorph, dl-cysteine-II, with the conformation of zwitterion changing from gauche- to gauche+. The first pressure-induced transition at 0.1 GPa (the lowest pressure reported for a phase transition in a crystalline amino acid thus far) gives the same polymorph. Further compression of dl-cysteine-II proceeds differently on cooling and with increasing hydrostatic pressure. dl-cysteine-II is preserved down to 3 K, but undergoes phase transitions on compression at about 1.55 GPa, and 6.20 GPa. The changes in the hydrogen bond network preceding the phase transition in dl-cysteine-II in the range 0.25-0.85 GPa differ from those observed on cooling the same structure, but resemble those preceding pressure-induced phase transitions in β- and γ-glycine.