Structure and dynamics of a fluid phase bilayer on a solid support as observed by a molecular dynamics computer simulation

Simulations of a 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine lipid bilayer interacting with a solid surface of hydroxylated nanoporous amorphous silica have been carried out over a range of lipid-solid substrate distances. The porous solid surface allowed the water layer to dynamically adjust its thickness, maintaining equal pressures above and below the membrane bilayer. Qualitative estimates of the force between the surfaces leads to an estimated lipid-silicon distance in very good agreement with the results of neutron scattering experiments. Detailed analysis of the simulation at the separation suggested by experiment shows that for this type of solid support the water layer between surfaces is very narrow, consisting only of bound waters hydrating the lipid head groups and hydrophilic silica surface. The reduced hydration, however, has only minor effects on the head group hydration, the orientation of water molecules at the interface, and the membrane dipole potential. Whereas these structural properties were not sensitive to the presence of the solid substrate, the calculated diffusion coefficient for translation of the lipid molecules was altered significantly by the silica surface.