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

We report molecular dynamics simulations of water hydrating a lipid (d imyristoylphosphatidylcholine) monolayer under conditions chosen to el iminate simulation artifacts. These simulations provide a description of the behavior of the membrane-water interface that agrees with recen t experimental studies. In particular, we find that the hydrating wate r orients to contribute the positive end of its dipole to the substant ially positive electrostatic potential of the membrane interior, consi stent with interpretations of recent experiments. In addition, recent experiments show that this water reorients rapidly on the NMR time sca le. 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 clath rate. The motion of the hydrating water was found to be slower than th at 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 wa ter 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 inter actions. 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 employ ed but moved faster than bulk water when small cutoffs were used and t he residence times for water about the lipids were twofold-fivefold la rger using large cutoffs. In general it was found that the lipids sign ificantly influence water out to several hydration shells, and that wa ter hydrating the lipids behaves differently than bulk water.