Water dynamics in the hydration layer around proteins and micelles

An overview of the dynamics of water molecules in the hydration layer surrounding self-assemblies and proteins in aqueous solutions is given. It is shown that relaxation in the hydration layer is in general nonexponential. The single particle properties, like translational diffusion and orientational relaxation, are quite different in the layer from that in the bulk. Number density fluctuation in the layer slows down significantly, signaling increased rigidity of the layer. The hydrogen bond lifetime between the polar/charged groups of the protein or surfactant and the surface water molecule is longer than that between two water molecules in the bulk. The ultraslow time scales observed in some experiments might not arise from surface water molecules at all but rather from motion of the probe itself or from protein side chain motion or even from buried water. The hydration layer is expected to participate in many biological and natural processes, ranging from protein folding to molecular recognition and catalysis. Finally, it appears that all of the different experimental approaches are now converging to provide a unified view of water dynamics on protein and micellar surfaces.