Computational investigation of lipid hydration water of Lα 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine at three hydration levels

Abstract

Atomistic-scale simulations of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayer systems were performed at three hydration levels from 12 water per lipid to 41 water per lipid. The structural and dynamical properties of water at each hydration level are the focus of the current study. The water properties are reported as a function of slab position relative to the lipid central plane using a floating slab model. Water properties depend more strongly on the relative slab position to the lipid than on the hydration level of the lipid. Water hydrogen bonds strengthen as water enters into the lipid, reaching a maximum at the phosphorus density maximum. The ratio of lateral and normal diffusion coefficients of water varies as water enters into the lipid and demonstrates an interesting crossover phenomenon at the phosphorus density maximum regardless of the overall hydration level. While most properties do not change beyond the commonly established excess water point, the water diffusion coefficient still increases upon further hydration. We propose a scheme that classifies lipid hydration water into buried water, interface water, and bulk-like water.