The Effect of Valinomycin on the Ionic Permeability of Thin Lipid Membranes

Optically black membranes prepared from sheep red cell lipids have a high electrical resistance (1–3 x 10⁸ ohm-cm²). The ionic transference numbers (T_i) for cations (Na⁺ or K⁺) are equal to each other but at least four to five times greater than for Cl^-. The cyclic depsipeptide valinomycin produces a striking decrease in the membrane resistance when K⁺, but not when Na⁺ is in the solutions bathing the membrane. The ratio T_Na/T_K, estimated from membrane voltages in the presence of ionic concentration gradients, approaches zero. The order of membrane monovalent cation selectivity, in the presence of valinomycin, is H⁺ > Rb⁺ > K⁺ > Cs⁺ > Na⁺. Addition of the antibiotic to one side of a membrane which separates identical solutions of NaCl produces a substantial (up to 80 mV) membrane voltage (side opposite valinomycin negative). These data are consistent with the hypothesis that valinomycin can interact with appropriately sized cations (hydrated diameter ??? 6 A) to increase their membrane permeability, perhaps by forming hydrogen bonds between the solvation shell of the cations and carbonyl oxygens in the valinomycin molecule which are directed toward the aperture of the ring.