LIPID AND CELL-MEMBRANES IN THE PRESENCE OF GADOLINIUM AND OTHER IONSWITH THE HIGH-AFFINITY FOR LIPIDS - 2 - DIPOLE COMPONENT OF THE BOUNDARY POTENTIAL FOR MEMBRANES WITH DIFFERENT SURFACE CHARGES

Two experimental methods applied to liposomes and planar lipid bilayer membranes (BLM) were used for monitoring the changes in the boundary and surface potentials induced by Gd3+ adsorption at the membranes mad e from phosphatidylserine (PS) and phosphatidylcholine (PC). The surfa ce charge density of the membranes were controlled by the PS/PC mixtur e composition or pH within a range of 2,5-7,5. The theory of Gouy-Chap men-Stern (GCS) combined with the condition of material balance in the experimental cell was used for quantitative description of adsorption and potential drop in the diffuse part of the electrical double layer . The modified GCS theory is in a good agreement with the data of elec trokinetic studies in the liposome suspension and Gd3+ association con stants of 5 . 10(4) and 10(3) M-1 for PS and PC, respectively, found i n our previous studies (Ermakov Yu.A., Averbakh A.Z., Sukharev S.I. // Biol. Membrany (in Russian). 1997. V. 14. N 4. P. 434-445). This cont radicts the data on the boundary potentials obtained in planar BLM by the method of intramembranous field compensation. The difference of bo undary and surface potential changes was interpreted as the changes in the dipole potential that correlate with the presence of PS in the PS /PC mixtures and PS ionization at different pH: Dipole potential chang es became significant at about 80% or more of the surface charge maxim al values in both cases. The nature of structural changes of the membr ane/water interface induced by Gd3+ - PS interaction and possible lipi d clusterization are discussed with respect to their biological import ance.