Measurements of Electrostatic Interactions between Charged Membranes

Abstract

Neighboring lipid membranes experience attractive and repulsive forces. The attractive (van der Waals) force is due to induced change fluctuations and it acts even between neutral membranes. Repulsive forces arise from the tendency of lipids to stay hydrated and can include electrostatic interactions due to accumulation of membrane surface charge. When the attractive vdW force is balanced by repulsive forces, an equilibrium spacing between lamellae is established, as measured by X-ray scattering. This is often the case for neutral membranes. By contrast, lipid bilayers made entirely of charged lipid species such as the negatively charge phosphatidylserine (PS) swell indefinitely (“unbind”) in the absence of salt indicating the dominance of electrostatic repulsion. To measure the strength of electrostatic interactions in solution, we manipulate membrane charge in two simple ways: by adsorbing “sticky” ions such as bromide onto neutral phosphatidylcholine lipids, and by mixing-in charged lipid species. We will present results and discuss in what conditions membrane electrostatics can lead to unbinding.