Image-charge forces in phospholipid bilayer systems

Abstract

The image-charge forces between zwitterionic phospholipid bilayers are analysed in a dielectric model for both the bilayer and the aqueous region. When a second bilayer approaches surface charges are induced by the ionic groups and a repulsive force is generated. The molecular origin of the repulsion is the long-range character of the polar headgroup solvation and the second bilayer excludes some of the solvating water molecules. The effect is analysed quantitatively in a combined statistical-mechanical–electrostatic formulation. It is found that the range and the strength of the force depend in a crucial way on the zwitterionic correlations within a bilayer. For strong correlations as in a lattice the force decays exponentially with short decay lengths ( < 1 Å), while in the completely uncorrelated case the leading term follows a power law. The calculated magnitude of the repulsion is of the same order of magnitude as those found experimentally. We conclude that the image-charge mechanism should be considered as a possible source of the hydration force found in these systems. The explanation is appealing since its source is simply the hydrophilicity of the polar groups.