Electrostatic interactions in a neutral model phospholipid bilayer by molecular dynamics simulations

The organization of the lipid headgroups in a neutral model membrane is studied by atomistic simulations in the fluid lamellar phase, L-alpha. In particular, we report the results obtained for a fully hydrated 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine lipid bilayer at room temperature. The orientational distribution of the lipid dipole moments with respect to the membrane normal presents a maximum at 70degrees (20degrees above the plane of the interface, pointing toward the water region). We also found another smaller peak at 110degrees (-20degrees with respect to the membrane plane). This preferential orientation of the lipid headgroup dipoles with respect to the bilayer normal obtained at 303 K is qualitatively different from previous calculations at higher temperatures in the fluid lamellar phase, where headgroup dipoles were uniformly distributed with orientations spanning 0degrees-135degrees. Despite their differences, both situations give rise to a similar mean orientation of similar to70degrees, which is in excellent agreement with experiment. The statistics of the main lipid-lipid interactions, the charge density profiles, the electrostatic potential along the bilayer normal, and the polarization of water molecules at the interfacial plane are also analyzed. (C) 2002 American Institute of Physics.