Electrostatic interactions in lecithin-bilayer systems A Monte Carlo study

Abstract

Monte Carlo simulations have been performed to study the electrostatic interaction between two planar zwitterionic surfaces. The mobile and orientable zwitterions are imbedded in an inhomogeneous dielectric medium and image charges have been used to account for the dielectric discontinuities. This is a simple model system for lecithin bilayers, emphasizing the molecular nature of the headgroups, while treating the intermediate aqueous region and the apotar interior of the bilayer as continua. The calculated lateral distribution functions and the inter-bilayer force are shown to be sensitive to the location of the dielectric discontinuities. For a structured system, which is the case when the zwitterions are confined to the apolar region, the inter-bilayer pressure is strongly repulsive. With zwitterions in the aqueous phase, the pressure is found to be attractive. The results are discussed in terms of an extension to the traditional Lifshitz theory of van der Waals forces.