Dynamics of water in the hydration layer of a self-assembled system and in bio-molecules

Abstract

Water molecules in the hydration layer of a bio-molecule (proteins and DNA) and of a self-assembled system (micelles. lipids) show many anomalous behaviours. In this article, we attempt to present a coherent picture of dynamics of water in the hydration layer of protein molecules. We will discuss a simple theoretical model (N Nandi and B Bagchi J Phys Chern 101 (1997) 10954) that assumes a dynamic equilibrium between the bound and the free states of the water molecules in the hydration layer. It is shown here that both the observed subdiffusive translational diffusion and the non-exponential orientational relaxation may arise from a distribution of binding energies of the surface water molecules. The dynamic equilibrium model (DEM) predicts a slow component (about 100 ps) in the orientational relaxation of water molecules in the hydration shell. The rate of the slow relaxation is determined by the rate of transition from the bound to the free state. DEM allows us to establish a relation between the residence time of the water molecules in the hydration layer and the observed slow component in solvation dynamics. It is further shown that the hydration shell of the protein molecule gives rise to enhanced rotational friction which results from the slow relaxation of water in the hydration shell and thus. may be correlated with the residence time. Finally, we argue that the ultra-slow component (of a few ns) might arise from the exchange of the probe (rather than the solvent) between different heterogeneous domains in a self-organized assembly.