The diffusion of small ions through phospholipid bilayers

The passage of the alkali metals Li, Na, K, Rb, and Cs through saturated phosphatidylcholine membranes has been measured using particle induced gamma ray and x‐ray emission to observe the ions. Simultaneous measurements of these five cations has not been possible with more traditional methods involving ion specific electrodes or radioactive tracers. To the authors’ knowledge this is the first time this technique has been used in lipid bilayer research. The ion leakage was determined for an incubation period of 30 min at several temperatures which spanned the bilayer melting temperature. The dependence of this permeability on temperature, acyl chain length, and ion size is described theoretically in terms of the density fluctuations in the bilayer. A leakage rate which depends on the mass of the ion was observed. This dependence shows that the heavier the ion the faster it diffuses out of the vesicle. The effect of this selectivity is shown to be most pronounced near the melting temperature of the lipid. M...