Interactions of hydration water and biological membranes studied by neutron scattering

To elucidate general features of structural and dynamical properties of hydration water and the influence of hydration water on the dynamical behavior of biomembranes, purple membranes from halobacteria and disk membranes from bovine retinae have been studied by neutron scattering techniques. Hydrated films of oriented multilamellar membrane stacks were used to measure lamellar diffraction patterns and quasielastic incoherent neutron scattering as a function of hydration level, of temperature, and of the protein/lipid ratio. These measurements revealed a strong interaction of a "first hydration layer" with the membrane surface and a reduced self-diffusion of aqueous solvent parallel to the membrane surface (the self-diffusion coefficient is about 5 times smaller as compared to excess water). The picosecond internal molecular motions of the protein/lipid complex are strongly affected by the amount of solvent interacting with the lipids and the membrane proteins. In particular, the lipids and their ability to attract solvent molecules play an important role for "hydration-induced flexibility" of biomembranes. On the basis of these measurements, the impact of the hydration process on the function of biomembranes is discussed for the light-driven proton pump bacteriorhodopsin in purple membranes.