Surface potential and conductance induced in lipid bilayers by the negatively charged ionophore Br-X537A

The adsorption of Br-X537Z and its effect on the surface potential of monoolein lipid bilayers was measured using the nonactin conductances as a probe to determine the surface charge density. Because of the pH dependence of this adsorption, it was concluded that not only the negatively charged molecules X- could induce a surface charge but also a dimer HX2- made from X- and the neutral molecule HX. Also an important bilayer conductance was induced by Br-X537A. From the BrX537A concentration dependence of this conductance, the effect of pH, and the induced surface potential, it was found that two charged complexes are transported across the bilayer depending on pH. At pH greater than or equal to 7 the conducting molecule is X- and at pH less than or equal to 5 the complex is H2X3-. A quantitative model is obtained to calculate both the induced surface potential and the conductance.