Proton/hydroxide conductance through lipid bilayer membranes

Abstract

A simple method of measuring proton/hydroxide conductance (G _H/OH) through planar lipid bilayer membranes is described. First the total conductance (G _m ) is measured electrically. Then the H⁺/OH⁻ transference number (T _H/OH) is estimated from the diffusion potential (V _m ) produced by a transmembrane pH gradient. The pH gradient is produced by a pair of buffered solutions which have identical concentrations of all ions except H⁺ and OH⁻. Thus,V _m is due entirely to H⁺/OH⁻ diffusion andG _H/OH can be calculated from the relations,V _m =T _H/OH E _H/OH andG _H/OH=T _H/OH G _m , whereE _H/OH is the equilibrium potential for H⁺ and OH⁻. In bilayers made from bacterial phosphatidylethanolamine (PE) inn-decane,G _H/OH is nearly independent of pH, ranging from about 10⁻⁹ S cm⁻² at pH 1.6 to 10⁻⁸ S cm⁻² at pH 10.5. BecauseG _H/OH is nearly independent of pH, the calculated permeability coefficients to H⁺ and/or OH⁻ are extremely pH dependent, which partly explains the wide range of values reported for phospholipid vesicles and biological membranes.G _H/OH appears to be independent of the membrane surface charge, because titrating either the phosphate or the amino group of PE has little effect onG _H/OH.G _H/OH is reduced about 10-fold when the water activity is reduced 33% by replacement with glycerol. Although the mechanism of H⁺/OH⁻ conductance is not known, the relation betweenG _H/OH and water activity suggests that several water molecules are involved in the H⁺/OH⁻ transport process.