Dipole potentials indicate restructuring of the membrane interface induced by gadolinium and beryllium ions

The dipole component of the membrane boundary potential, phi (d), is an integral parameter that may report on the conformational state of the lipid headgroups and their hydration. In this work, we describe an experimental approach to measurements of the dipole potential changes, Delta phi (d), and apply it in studies of Be2+ and Gd3+ interactions with membranes composed of phosphatidylserine (PS), phosphatidylcholine (PC), and their mixtures. Delta phi (d), is determined as the difference between the changes of the total boundary potential, phi (b), measured by the IFC method in planar lipid membranes and the surface potential, phi (s), determined from the electrophoretic mobility of liposomes. The Gouy-Chapman-Stern formalism, combined with the condition of mass balance, well describes the ion equilibria for these high-affinity cations. For the adsorption of Be2+ and Gd3+ to PC membranes, and of Mg2+ to PS membranes, the values of Delta phi (b) and Delta phi (s) are the same, indicative of no change of phi (d). Binding of Gd3+ to PS-containing membranes induces changes of phi (d) of opposite signs depending on the density of ionized PS headgroups in the bilayer, At low density, the induced Delta phi (d) is negative (-30 mV), consistent with the effect of dehydration of the surface. At maximal density (pure PS, neutral pH), adsorption of Be2+ or Gd3+ induces an increase of phi (d) of 35 or 140 mV, respectively. The onset of the strong positive dipole effect on PS membranes with Gd3+ is observed near the zero charge point and correlates with a six-fold increase of membrane tension. The observed phenomena may reflect concerted reorientation of dipole moments of PS headgroups as a result of ion adsorption and lipid condensation. Their possible implications to in-vivo effects of these high-affinity ions are discussed.