Inversion of Membrane Surface Charge by Trivalent Cations Probed with a Cation-Selective Channel

摘要

We demonstrate that the cation-selectivenchannel formed by gramicidin A can be used as a reliablensensor for studying the multivalent ion accumulation at thensurfaces of charged lipid membranes and the “chargeninversion” phenomenon. In asymmetrically charged membranesnwith the individual leaflets formed from pure negativenand positive lipids bathed by 0.1 M CsCl solutions the channelnexhibits current rectification, which is comparable to that of antypical n/p semiconductor diode. We show that even at thesenhighly asymmetrical conditions the channel conductance cannbe satisfactorily described by the electrodiffusion equation innthe constant field approximation but, due to predictable limitations, only when the applied voltages do not exceed 50 mV.nAnalysis of the changes in the voltage-dependent channel conductance upon addition of trivalent cations allows us to gauge theirninteractions with the membrane surface. The inversion of the sign of the effective surface charge takes place at the concentrations,nwhich correlate with the cation size. Specifically, these concentrations are close to 0.05 mM for lanthanum, 0.25 mM fornhexaamminecobalt, and 4 mM for spermidine.