Phase transitions in the crystals of L- and DL-cysteine on cooling: the role of the hydrogen-bond distortions and the side-chain motions. 2. DL-cysteine.

Structural strain and a first-order phase transition in the crystalline DL-cysteine on cooling and on reverse heating were followed by Raman spectroscopy and X-ray diffraction. The transition is reversible and has a large hysteresis (over 100 K). The temperature at which the transition is observed depends strongly on the cooling/heating rate. The phase transition is accompanied by crystal fragmentation. The low-temperature phase could be obtained not only as a result of the solid-state transformation in situ as a polycrystalline sample (with strong preferred orientation, or without it, depending on the preparative technique), but also (using an original crystallization technique) as a single crystal of the quality suitable for structural analysis. For the first time, the crystal structure of the low-temperature phase was solved independently by powder and by single-crystal diffraction techniques. The spectral changes were correlated with the precise diffraction data on the intramolecular conformations and the intermolecular hydrogen bonding before and after the phase transition. The role of the distortion of the intermolecular hydrogen bonds and of the motions of the -CH(2)SH side chains in the phase transition is discussed in a comparison with the low-temperature phase transition in L-cysteine, which is of a different type and preserves the single crystals intact (Kolesov et al. J. Phys. Chem. B, 2008, 112 (40), 12827-12839).