Determination of association parameters for the adsorption of mono- and divalent cations at the lipid membrane surface

Abstract

The Gouy-Chapman-Stern (GCS) theory has two parameters - binding site density S and association constant K - to describe the adsorption of ions at the surface of lipid membranes and the screening effect. Despite the many assumptions used in the theory, it is well supported by the results of experiments.' Most authors used a value of S derived from calculations based on surface area per lipid molecule, but not from measurement of their state of ionization, which may depend on lipid organization (different in crystals, monolayers and membranes). The correct test of the hypothesis for cation adsorption presumed a fitting procedure for the direct determination of S and K from the experimental data obtained with the same lipid system - liposomes or planar bilayer lipid membranes (BLM). Only these parameters are suitable for comparing the behavior of different kinds of electrolyte. In order to study the question of whether there are some interactions of ions with lipids not considered by the GCS theory we used two approaches: the fitting procedure for electrokinetic measurements of liposomes suggested in Ref. 2 for monovalent electrolytes, and the comparison of zeta potentials with the surface potentials measured on BLMs by the method of membrane capacity minimization. The practical realization of this method in our laboratory3 used the 2-nd harmonic of BLM capacity current to compensate the inner membrane electric field - IMFC-method. Electrokinetic measurements were carried out by laser light scattering applied to electrophoresis (Zetasizer-2, MALVERN Instr., UK). Liposomes and BLM were made from bovine phosphatidyl serine (PS), egg lecithin (PC) and dimyristoyl phosphatidyl lecithin (DMPC, Avaty Co.). PS and PC were obtained from the Laboratory of Lipid Chemistry at Vladivostok State University.