The role of water in the hydration force - molecular dynamics simulations

To understand the contribution of water to the repulsive force acting between phospholipid membrane molecules we performed molecular dynamics computer simulations on lamellar systems of phospholipid bilayers in water. Four simulations were performed. Two simulations were done on dilauroylphosphatidylethanolamine (DLPE) in water systems and two on dipalmitoylphosphatidylcholine (DPPC) in water systems. The simulations differed by the amount of water per phospholipid headgroup. From the simulations we concluded that even at the hydration limit the headgroups of the opposing membranes come in to close proximity, so that they are separated by only one or two water layers. Since the water structure in the first solvation shell is distinctly different from bulk water, the hydration force is likely to be due to the solvation layer of water around the headgroups, which are separated by up to two water layers, rather than due to a long range perturbation of the water structure. We also observed that different solvation patterns exist for water around the phosphatidylcholine (PC) and phosphatidylethanolamine (PE) headgroups. We propose that this solvation pattern is connected to the difference in the hydration of DPPC and DLPE membranes.