Properties of lipid bilayer membranes separating two aqueous phases: The effects of Fe+3 on electrical properties

Abstract

Ferric ion has been found to alter the electrical properties of lecithincholesterol-decane bilayer membranes. Within minutes after the addition of microgram quantities of FeCl3 to the ambient aqueous phase, the resistance of the membrane falls by a factor of 10(5) to 10(6). No change in capacitance is observed. The resistance change is obtained with membranes made from synthetic lecithin (fully saturated fatty acids) as well as by those formed from egg lecithin. The conductance of the modified membrane exhibits both time and voltage dependent behavior; the time dependence of the current is similar to that of an inductance, and the voltage dependence of the current is exponential. Concomitant with the resistance change, the modified membrane becomes permselective, passing chloride almost to the complete exclusion of sodium. Anion selectivity can be converted to cation selectivity by the subsequent addition of certain chelating agents. Area-conductance measurements show the resistance change occurs in the thin film. The addition of a reducing agent causes the effect of the ferric ion to be reversed, and the conductance returns to that characteristic of unmodified membranes. When ferric ion is added to only one side of the membrane, the system rectifies with current ratios of up to 20∶1. It is concluded that the alteration of membrane properties owes its origin to the hydrolysis of membrane-bound ferric ion. The interaction of ferric ion with aqueous dispersions of lecithin has been investigated by several techniques, and evidence is presented that the dispersions bind charged species of iron and that this charge diminishes under conditions where iron hydrolysis occurs.