Slow dynamics of water molecules on the surface of a globular protein

Water is essential for the stability and function of biological macromolecules. High-resolution quasi-elastic neutron scattering studies of the translational dynamics of water molecules on the surface of a deuteriated protein are presented. The quasi-elastic spectra from the interfacial H2O are analysed by a confined diffusion model to obtain the elastic incoherent structure factor (EISF), the short-time self-diffusion constant (D) and the residence time, τ0, as functions of coverage and temperature. The combined effects of the hydration level and temperature on the retardation of the single-particle motions are discussed in the light of available NMR relaxation data and of a well known model of α-relaxation from the theory of kinetic glass transitions in dense supercooled fluids. The vibrational density of states of interfacial water is presented as a function of temperature and for two levels of hydration of the protein.