Capacitance and conductance as tools for the measurement of asymmetric surface potentials and energy barriers of lipid bilayer membranes

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Abstract
A simple method for the determination of asymmetric surface potentials in lipid bilayers is described. The method is based on the dependence of bilayer capacitance on transmembrane voltage. The capacitance is measured by rectifying the 90° component of an applied alternating current signal. A superimposed slow triangular wave results in a hysteresis-like time course of capacitance. The center of the hysteresis figure is shifted along the voltage axis by an amount equal to the difference of the dipole plus surfacecharge potentials on the two sides of the bilayer (capacitance minimization potential). Alternatively, such bilayer asymmetry was studied by using the current-voltage characteristics in the presence of nonactin as a carrier. This analysis was based on the integrated Nernst-Planck equation, assuming a trapezoidal energy barrier and equilibrium of the surface reactions. The two methods gave consistent results for the surface potentials of phosphatidyl serine membranes asymmetrically shielded with calcium. In addition, the current analysis yields the positions of the corners of the barrier, found to be set in 13% for this lipid.