Inversion of Lipid Bilayer Surface Charge by Trivalent Cations: Probing with Single-channel Conductance and Kinetics

Abstract

The conductance of gramicidin A is sensitive to the charge of the lipid bilayer in which it resides. We used this property to probe the effects of lanthanum3+, hexaamminecobalt3+, and spermidine3+ on the surface charge of phosphatidylserine (PS) bilayers. Addition of trivalent cations to negatively-charged PS bilayers reduced gramicidin conductance below the conductance seen for neutral phosphatidylcholine bilayers, to a level nearly as low as for positively charged trimethylammonium propane bilayers. This suggests that trivalent cations can overcompensate the negative surface charge of the PS bilayer. Complementary zeta-potential measurements of PS liposomes with trivalent cations also suggested charge inversion. There were differences in the concentrations required to invert charge among the different cations, with lanthanum3+ the most potent and spermidine3+ the least potent. We also find that the rate of channel formation is sensitive to the surface concentration of permeating ions. Our interpretation is that gramicidin monomers in a bilayer exist in different configurations and that the equilibrium between these configurations depends on the cation binding within monomers. The difference in occupancy of monomers by cations makes channel formation dependent on the surface potential.