Asymmetry of the gramicidin channel in bilayers of asymmetric lipid composition: II. Voltage dependence of dimerization.

The asymmetric current-voltage relationship of gramicidin-doped asymmetric bilayers made by the Montal-Mueller technique was investigated in current relaxation experiments. It was shown that, in addition to the contribution of the asymmetric single channel conductance to the asymmetry of the steady-state current-voltage relationship, there is an asymmetric voltage dependence of the step which leads to the formation of the conducting channel. This asymmetric voltage dependence could be simulated in a model assuming a membrane-internal electrical potential drop or an equivalent potential, called asymmetry potential, which could be compensated by externally applying an offset potential. Significant asymmetry potentials were found in asymmetric bilayers made of charged lipids or only of neutral lipids. The asymmetry potential was dependent on the salt composition in the aqueous phase. The factors responsible for the asymmetry potential do not appear to be of electrostatic origin. Several lines of evidence suggest that the dimerization step which leads to the conducting ion channel may be a complex series of reactions which are influenced by one or more membrane structural properties not yet characterized, in addition to the effects of the externally applied electric field.