Structural Stability Of L-Cystine Under Extreme Conditions

Cystine is important in sustaining the three-dimensional structures of proteins, and hence it might have played an important role in the origin of life as a prebiotic molecule. As harsh environments may have been involved in producing the "prebiotic soup" either from chemical syntheses or sourced from extraterrestrial objects, understanding the structural stability of cystine in extreme environments is essential. Nevertheless, investigations of the stability of cystine at extreme conditions are very limited. In this work, using high-pressure X-ray diffraction (XRD) and Raman and infrared spectroscopies, we studied the structural stability of a hexagonal L-cystine crystal. Results show that the L-cystine crystal is structurally stable at pressures up to similar to 25 GPa at room temperature. However, there are changes in the intermolecular hydrogen bonding and/or atomic bond rotations/torsions at three transition pressures of similar to 1 to 3, 5-6, and 13-15 GPa. The L-cystine crystal is highly compressible, with a bulk modulus of 32.2 GPa (with B-0(') = 4) or 18.6 GPa (with B-0(') = 6.9) at an average size of similar to 20 to 40 nm. When the compression is above similar to 32 GPa, the structural relaxation is retarded in decompression, yet the original structures can still be largely recovered if enough time is given after full decompression. It was also found that even at pressures up to similar to 20 GPa and temperatures up to 150 degrees C, the L-cystine crystal is still stable in essence. These findings would have important implications to postulate and rationalize credible theories on the origin of life at extreme conditions.