MOLECULAR-DYNAMICS STUDY OF A MEMBRANE WATER INTERFACE

A 200 ps molecular dynamics simulation of a membrane bilayer consisting of 202 dilauroylphosphatidyl-ethanolamine molecules and 8108 water molecules at 315 K is conducted. Distribution functions of lipid groups, order parameters, and other properties of the lipid bilayer are calculated and compared with experimental measurements. A detailed analysis is conducted for the structure at the membrane-water interface. Water polarization profile, membrane dipole potential profile, and susceptibility profile are calculated. Simulation results suggest that the polarization of water is determined mainly by the distribution of lipid head groups in the interfacial region. The membrane dipole potential is mainly due to the ester groups linked to the glycerol backbone, while the contribution due to the phosphatidylethanolamine head groups is almost completely cancelled by the contribution due to oriented water molecules. The susceptibility profile suggests a dielectric constant around 30 for the head group-water interface and a dielectric constant around 10 for the ester group region. The ammonium groups of the DLPE membrane are found to form hydrogen bonds with water molecules, while no orientational preference is observed for water molecules around the choline groups of a previously simulated POPC (1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphatidylcholine) membrane. Correlations of the membrane surface charge density are also analyzed. The simulations which involved 32 808 atoms included Coulombic forces between all atom pairs evaluated by means of the fast multipole algorithm. Effects of cutting off Coulombic forces at a distance of 8 Angstrom are discussed.