Properties of the conductance induced in lecithin bilayer membranes by alamethicin

Current-voltage relations have been measured across lecithin bilayers doped with alamethicin molecules. The results show that there are two aspects of the induced conductances, a voltage-dependent and a voltage-independent conductance. Both have been characterized as a function of alamethicin and KCl concentration. The two aspects of the conductances do not show the same changes with those two variables. The voltage-independent conductance is affected very little by changes in KCl concentration, and its dependance on alamethicin concentration reveals that it is produced by two or three alamethicin molecules. The voltage-dependent conductance is shifted by the changes in KCl concentration only when the concentrations are greater than or equal to 100 mM; below 100 mM KCl the slope of the log conductance-voltage curve is also reduced. The effect of changing alamethicin concentration reveals that nine or ten molecules are involved for KCl concentrations larger than 100 mM; if the KCl concentration is less than 100 mM, the effect of changing the alamethicin concentration is reduced. Time-dependent measurements have also been performed; only one time constant was found and it is strongly voltage-dependent. Also a very slow voltage-dependent absorption process is found. These results can be explained if it is assumed that pores are formed of a mixture of charged and uncharged alamethicin molecules when a voltage is applied and that uncharged alamethicin can also form pores without applying a voltage, once the absorption process has been started by previously applied voltages. The voltage dependence of the time constant seems to indicate that the voltage-dependent pore formation is produced by aggregates of charged alamethicin rather than independent molecules.