Effects of lipid structure on the kinetics of carrier-mediated ion transport.

The mechanism of alkali-ion transport mediated by valinomycin (or similar macrocyclic carriers) may be studied using artificial planar bilayer membranes. The rate constants of the single transport steps (association and dissociation of the ion-carrier complex, translocation of free and complexed carrier) can be determined from electrical relaxation experiments. The turnover number of valinomycin which may be calculated from the rate constants is found to be 10(4)-10(5) s-1. Carriers of the valnomycin-type offer the possibility of studying the relationship between membrane structure and transport kinetics. Increasing the chain-length of the lipid strongly reduces the translocation rate constants of the free and the loaded carrier, and also (in the case of lecithin membranes) the association rate constant. Increasing the number of double bonds in the fatty-acid residue of the lipid leads to an inrease of the translocation rate constants. These effects are discussed in terms of microviscosity of the membrane. Addition of cholesterol to monoglyceride membranes seems to affect both the microviscosity and the dipolar potential at the membrane-solution interface.