Structural and Dynamical Properties of POPC Bilayers Supported on Nanoporous Substrates

Abstract

The structure, dynamics and organization of lipid bilayers depend on a variety of environmental factors. Here we examine, using molecular dynamics (MD) simulations, the specific effects of nanoporous substrates on palmitoyl-oleyl phosphatidylcholine (POPC) bilayers. We expose POPC bilayers unilaterally and separately to various model Lennard-Jones (LJ) solid substrates differing in surface chemistry. In an earlier study, we had found that substrates with surface hydroxyl densities in the range 10-20% kept POPC bilayers juxtaposed to the substrates. While the bilayers did not interact directly with these hydroxylated substrates, they were separated from substrates by thin (< 0.6 nm) layers of water. Additionally, despite such buffered interactions, these supported bilayers exhibited physical properties notably different from unsupported bilayers. Recent experiments show that zwitterionic lipid bilayers supported on partially charged, nanoporous silica wafers also do not interact with silica directly. However, in such cases the bilayer is separated from the silica substrate by a relative thicker (∼1.5 nm) layer of solvent. We report here results from our MD simulation that reconcile these two seemingly disparate observations. We find that LJ nano-substrates whose porosity and surface hydroxyl density match those of nanoporous silica wafers do not increase the substrate-bilayer distance. Instead, the introduction of partial charge on the LJ substrate increases the substrate-bilayer distance to 1.5 nm. The negative charge on the substrate attracts hydrated ions, creating an electric double layer, which is a sufficient condition to increase the substrate-bilayer distance. While the properties of the bilayer supported on a charged substrate are also different from those of unsupported bilayers, the extent of the perturbation is not as large compared to that induced by uncharged hydroxylated substrates.