Dynamical properties of interfacial water and its role for biomolecular dynamics

Doctoral thesis, 2006

Water is the foundation of life, and without it life as we know it would not exist. An organism consists to a large extent of water, and apart from a few larger reservoirs, a large fraction of the water in a body is closely associated with surfaces of biomolecules of different kinds. This water is denoted biological water. The biological water is known to affect the dynamics of biomaterials such as proteins, which in turn is necessary for their functions. However, how and why the surrounding environment affects the dynamics of proteins and other biomolecules is still not fully understood. Therefore, it is of importance to investigate not only the dynamics of the biomolecules but also the dynamics of their surrounding solvent and how they are related. We have investigated systems with water associated to surfaces of different kinds, both a model system and in various biological systems. In addition, measurements on proteins in solvents of different viscosities and dynamical properties have been performed in order to investigate the coupling between protein and solvent dynamics, for a better understanding of the role of water in biological systems. From the results we can conclude that the dynamics of confined water in various systems change its character from a low temperature Arrhenius behaviour to high temperature non-Arrhenius behaviour at a certain temperature, which suggests that only local motions are present in confined water at low temperatures, whereas the dynamics is of more global character at higher temperatures. The results furthermore indicates that the most local protein motions are determined by the local motions in the surrounding solvent, whereas more large scale protein motions are driven by cooperative motions in the solvent. Thus, a similar temperature behaviour is observed for both the solvent and the protein dynamics. This implies that our findings seem to support the recently suggested solvent-slaving idea, which suggests that local and more global protein motions are slaved by the local -relaxations and the more large-scale cooperative -relaxation in the surrounding solvent, respectively.