Carrier-mediated ion transport through black membranes of lipid mixtures and its coupling to Ca⁺⁺-induced phase separation

Summary

Voltage jump-current relaxation experiments have been performed with valinomycin-doped membranes of mixtures of 1,2-dipentadecylmethylidene-glycero-3-phosphorylcholine (PC) and charged-phosphatidic acid (PA). Both relaxation processes predicted by a simple carrier model could be resolved which allowed the calculation of the rate constants of the Rb⁺ transport. The dependence of the rate constants on the membrane composition indicates that (i) the lipids in the mixed membranes are homogeneously distributed and that (ii) no major difference exists between the composition of the membrane and that of the torus. The analysis of the stationary conductance data, however, shows that the valinomycin content of the mixed membranes depends strongly on their lipid composition. Addition of Ca⁺⁺ ions to a 1∶1 mixture induces a phase separation into PA domains of very low conductivity and PC-enriched regions of high conductivity. Half saturation is reached atc _ca=5×10⁻⁴ m. At 10⁻² m Ca⁺⁺ in the aqueous phase, the rate constants clearly indicate that all PA molecules are electrically “passivated” and only pure PC domains contribute to the membrane current. A detailed picture is thus derived of the coupling of a model transport system to the externally triggered membrane reorganization.