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Molecular effects of a nanocrystalline quartz support upon planar lipid bilayers.

Supported lipid bilayer membranes play a vital role in a number of applications from biosensors to fundamental studies of membrane proteins. It is widely understood that the underlying solid support in such assemblies causes large perturbations to the lipid bilayer as compared with black lipid membranes, but the exact nature of these effects on the membrane by the solid support is less understood. Here, all-atom molecular dynamics simulations of DLPC, DMPC, POPC, and DEPC on a hydroxylated nanocrystalline alpha-quartz (011) slab have revealed a pronounced thinning effect. It is shown that this thinning effect proceeds by one of two mechanisms; the first is through a curling of the terminal methyl groups at the interface of opposing leaflets, and the second is through increased interdigitation of the alkyl chains. In all cases, it is shown that the thinning effect is accompanied by a commensurate spreading of the lipid membrane across the quartz substrate. Also, with the introduction of the solid support, a marked asymmetry in a number of structural properties is reported. These asymmetries include (a) the surface areas per lipid, (b) the electron probabilities of the polar headgroups, (c) the radial distributions of the choline groups, and (d) the average orientation of water surrounding the membranes. Finally, asymmetries associated with the different interaction energies within each system studied are reported. These unequal interaction energies lead to a net force holding the membrane to the surface of the support. It was found that direct membrane-substrate interactions play only a minor role in holding the membrane to the surface and it is the interstitial water that dominates these interactions. This is due to the fact that the water throughout the interstitial region displays an average orientational preference that is more favorable (attractive to the membrane and yields a higher number of hydrogen bonds) than water in the external region of the assembly.