Structural characteristics in protein hydration investigated by cryogenic X-ray crystal structure analyses

Cryogenic X-ray crystallography has heen applied to investigate the hydration structures of proteins. The amount of hydration water molecules identified at cryogenic temperature is more than twice those at ambient temperature, and the structural models of proteins with a lot of hydration water molecules have provided much information to elucidate the static and dynamical characteristics of hydration structures of proteins. On protein surface, hydration water molecules distribute non-randomly and still retain the tetrahedral hydrogen-bond geometry as well as in bulk solvent. In addition, water molecules form clathrate-like arrangements to cover the hydrophobic residues exposed to solvent. The standard interaction geometry enables the three-dimensional extension of hydrogen-bond networks around protein molecules and, simultaneously, ensures the concerted reorganization of hydration structures during the dynamical motion of proteins at work. The hydration structure analyses at cryogenic temperatures may contribute to understanding physical principles governing the dynamics of 'molecular machines' in aqueous environment.