Effect of surface charge density on the kinetics of transmembrane lipid migration in asymmetric bilayers

An asymmetric bilayer consisting of neutral and negatively-charged lipid molecules experiences an internal electric field due to the difference in the surface densities of the charged lipid molecules in the two surfaces of the bilayer. Considering both an uncoupled and a tightly-coupled model of transmembrane migration (flip-flop) of the lipid head groups, the importance of the internal field of the kinetics of flip-flop is discussed. It is shown that as the equilibrium asymmetry due to different solutions bathing each side of the membrane becomes less pronounced, the fitting of the kinetics by an uncoupled model neglecting the electric field becomes better, although the rate constants that can be extracted from this procedure are meaningless from a physical point of view.