Heterogeneity of Water Dynamics of Hydrated Lipid Bilayers in Atomistic MD Simulations

Abstract

Probing dynamics of water molecules interacting with polar headgroups of hydrated lipid membranes is vital in understanding general properties of membrane systems. Recent terahertz spectroscopy experiments provided new insights into dipolar relaxation and dynamics of water molecule reorientation in lipid bilayers with decreasing hydration level[1]. We perform molecular dynamics simulations of DOPC with varied levels of hydration. Our simulation models reproduce the experimental therahertz spectroscopy results with reasonable accuracy. Previously, three different types of water molecules were proposed that were described as irrotational water, bulk water, and fast water with distinct relaxation dynamics. We analyze single molecule dipole correlations in detail to study reorientational dynamics of water molecules in our simulated systems. Our results provide us with distributions of relaxation properties as a function of hydration level. We identify a population of water molecules which are tightly bound to lipid headgroups and exhibit relatively very slow relaxation dynamics. The remaining water molecules in the simulated systems, whose reorientational dynamics can be probed on the timescale of our simulations exhibit a broad heterogenous distribution of dynamical properties. This result suggests that models used to interpret experiments probing the reorientational dynamics of water molecules in a hydrated lipid bilayer should be based on a proper description of this distribution instead of isolated populations of water molecules with distinct properties.This work is supported by grants from the NIH (GM74637) and NSF (CHE-0750175), and Teragrid resources provided by the NSF-supported TACC. This research used ShaRCS, UC Shared Research Computing Services Cluster.[1] K.J. Tielrooj, D. Papro, L. Piatkowski, H.J. Bakker, and M. Bonn, Biophysical Journal (2009), 2484-2492