THE LIMITING BEHAVIOR OF WATER HYDRATING A PHOSPHOLIPID MONOLAYER - A COMPUTER-SIMULATION STUDY

We report molecular dynamics simulations of water hydrating a lipid (dimyristoylphosphatidylcholine) monolayer under conditions chosen to eliminate simulation artifacts. These simulations provide a description of the behavior of the membrane-water interface that agrees with recent experimental studies. In particular, we find that the hydrating water orients to contribute the positive end of its dipole to the substantially positive electrostatic potential of the membrane interior, consistent with interpretations of recent experiments. In addition, recent experiments show that this water reorients rapidly on the NMR time scale. Our results concur, however the relatively rapid water motion does not preclude the preferential ordering that we observe. The limiting behavior of the system shows three hydration shells about the lipid PC headgroups and significant hydrogen bonding of water to the phosphate groups. The choline group experiences different environments, and the structure of the first hydration shell clearly corresponds to a clathrate. The motion of the hydrating water was found to be slower than that of bulk water, and the computed residence times for water about the lipids (20 ps about choline, 10 ps about phosphate) were in excellent agreement with results of NMR experiments. This further shows that water resides in a clathrate shell longer than in a shell about ions. In addition, we show that the structure and dynamics of water hydrating the lipids are very sensitive to the treatment of the long-range interactions. In particular, the radial structure sharpens considerably, a third hydration shell about the phosphate was observed only with large cutoffs, and hydrogen bonding of water to the lipids increased by 25%. The water moved more slowly than bulk when large cutoffs were employed but moved faster than bulk water when small cutoffs were used and the residence times for water about the lipids were twofold-fivefold larger using large cutoffs. In general it was found that the lipids significantly influence water out to several hydration shells, and that water hydrating the lipids behaves differently than bulk water.