Lipid and cell membranes in the presence of gadolinium and other ions with high affinity to lipids. 1. Dipole and diffuse components of the boundary potential.

Two methods were used for monitoring the changes in the boundary and surface potentials induced by Gd3+ adsorption--the method of intramembranous field compensation applied to planar BLM and electrophoresis of liposomes, respectively. The data of both methods agree well in the case of membranes made from phosphatidylcholine (PC), in contrast to those made from phosphatidylserine (PS). In the latter case, they show a significant discrepancy between potential amplitudes even within the micromolar range of Gd3+ concentrations because of cation-induced changes in the dipole component of the boundary potential. The adsorption of these cations with extremely high affinities to lipids was described by the theory of diffuse double layer (Gouy-Chapman-Stern) combined with the condition of mass balance, which is essential in the case of limited volumes of real systems. The electrokinetic data for PS and PC liposomes are in good agreement with the modified theory and correlate well with the Gd3+ association constants of 5.10(4) and 10(3) M-1, respectively. The same theory was used to extract the changes in the dipole component from the measured changes in the PS membrane boundary potential. The dependence of this component on the number of binding sites occupied by cation was shown to be nonlinear but cooperative with the step-like increase of dipole potential up to about 140 mV (positive inside the membrane) at the Gd3+ concentration around the zero charge point of the PS membranes. This potential corresponds to the structural changes in the lipid bilayers which are discussed with respect to the Gd3+ blocking effects on the mechanosensitive channels.