The Origin of the Hydration Interaction of Lipid Bilayers

The results of four molecular dynamics simulations of dipalmitoylphosphatidylcholine (DPPC)/water systems are reported. To investigate the origin of the hydration force we follow the experiments performed recently by McIntosh and Simon. We study the DPPClwater system in both the gel phase and the liquid crystalline phase at two different water contents, namely at 11 and 20.5 water moleculesflipid. Long ranged Coulomb interactions are treated with the so-called particle mesh Ewald method. The polarization profile of the water is calculated. We find that the polarization decays smoothly within 6 A. The smoothly decaying polarization profile is a result of the roughness of the bilayer surface even in the gel phase. Furthermore we characterize the structure around the lipid head group and find a layer ofwater molecules with stronger hydrogen bonds. At low water contents we observe that a significant fraction of head groups of opposing bilayers is either in close contact or separated by just one or two water layers. In our interpretation, the region of water molecules with stronger hydrogen bonds keeps the opposing head group pairs separated by one or two water layers. Both situations, close contact and separation by one or two water layers, possibly give rise to the so-called hydration force.