Dynamics of hydration water in proteins

Thermodynamic and transport properties of liquid water are determined essentially by inter-molecular hydrogen bonds and their dynamics. Because the molecular dynamics depends mostly on geometric constrains and dynamics of hydrogen bonds, it is shown that a simple statistics on the number of bonds, including at the vicinity of hydrophilic substrates describes well the temperature dependence of water diffusion on the surface of a small peptide. Experiments were performed by incoherent quasi-elastic neutron scattering giving information about the hydrogen bond lifetime and the residence time of the water molecules. In order to study the interactions between water molecules and a small hydrophobic peptide (an assembly of five monomers of alanine), the hydration level is changed step by step. The only motion of the first single water molecule of the structure is due to hydrogen libration. The first two added water molecules bind with the hydrophilic site of the peptide and have only the dynamics of the bond breaking. The other hydration molecules show fast diffusive motions on the surface of the peptide.