LIPID AND CELL-MEMBRANES IN THE PRESENCE OF GADOLINIUM AND OTHER IONSWITH HIGH-AFFINITY FOR LIPIDS .1. DIPOLE AND DIFFUSE COMPONENTS OF THE BOUNDARY POTENTIAL

Two methods were used for monitoring the changes in the boundary and s urface potentials induced by Gd3+ adsorption - the method of intramemb rane field compensation applied to planar BLM and electrophoresis of l iposomes, respectively. The data of both methods agree well in the cas e of membranes made from phosphatidylcholine (PC), in contrast to ones made from phosphatidylserine (PS). In the latter case, they show sign ificant discrepancy between potential amplitudes even withing the micr omolar range of Gd3+ concentration because of cation induced changes i n the dipole component of the boundary potential. The adsorption ot th ese 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 limit ed volumes of real systems. The electrokinetic data for PS and PC lipo somes are in a good agreement with the modified theory and correlate w ell with the Gd3+ association constants of 5.10(4) and 10(3) M-1, resp ectively. The same theory was used to extract the changes in the dipol e component from the measured changes in the PS membrane boundary pote ntial. The dependence of this component on the number of binding sites 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 memb ranes. This potential corresponds to the structural changes in the lip id bilayers which are discussed with respect to the Gd3+ blocking effe cts on the mechanosensitive channels.