Ionic Permeability of Thin Lipid Membranes

Abstract

Ultrathin (black) lipid membranes were made from sheep red cell lipids dissolved in n-decane. The presence of aliphatic alcohols in the aqueous solutions bathing these membranes produced reversible changes in the ionic permeability, but not the osomotic permeability. Heptanol (8 mM), for example, caused the membrane resistance (R_m) to decrease from >10⁸ to about 10⁵ ohm-cm² and caused a marked increase in the permeability to cations, especially potassium. In terms of ionic transference numbers, deduced from measurements of the membrane potential at zero current, T_cat/T_Cl increased from about 6 to 21 and T_K/T_Na increased from about 3 to 21. The addition of long-chain (C₈ndash;C₁₀) alcohols to the lipid solutions from which membranes were made produced similar effects on the ionic permeability. A plot of log R_m vs. log alcohol concentration was linear over the range of maximum change in R_m, and the slope was -3 to -5 for C₂ through C₇ alcohols, suggesting that a complex of several alcohol molecules is responsible for the increase in ionic permeability. Membrane permselectivity changed from cationic to anionic when thorium or ferric iron (10^-4 M) was present in the aqueous phase or when a secondary amine (Amberlite LA-2) was added to the lipid solutions from which membranes were made. When membranes containing the secondary amine were exposed to heptanol, R_m became very low (10³–10⁴ ohm-cm²) and the membranes became perfectly anion-selective, developing chloride diffusion potentials up to 150 mv.