Power-Law Trappings Cause Anomalous Diffusions of Water Molecules on Membrane Surfaces

Abstract

Cell membranes provide unique local environments for biological reactions, where diffusion of biomolecules as well as water molecules plays critical roles. In this study, molecular dynamics simulations for a system of water molecules / lipid bilayer were performed at temperatures from 250 K to 350 K to examine dynamics of water molecules around the surface of the lipid bilayer. Our analysis introduces a mean exit time approach which allows characterizing diffusive properties of water molecules around the surface of lipid bilayers. Using this method, we show that translational motions of water molecules around the surface of lipid bilayers are slower than those in bulk. Moreover, we find that trapping times of water molecules onto membrane surfaces are distributed according to power-law distributions depending on temperature and that water molecules on the membrane surfaces exhibit subdiffusions in translational as well as rotational motions. We provide evidence that not only an enhancement of the viscosity but also subdiffusions of water molecules on membrane surfaces originates from power-law trappings in translational motions on membrane surfaces.