The energy barriers to ion transport by nonactin across thin lipid membranes

The experimental steady-state current-voltage relations for low concentrations of a neutral carrier and an ion may be fitted theoretically either by assuming a form for the potential dependence of the rate of transfer of complex across the membrane and adjusting the proposed nature of the association-dissociation reactions or by assuming equilibrium for the association and adjusting the potential dependence of the transfer process. Different dependences for the rate of transfer correspond, at least formally, to different shapes for the potential energy barrier which the complex must cross. By comparing measurements of the current-voltage relations for non-actin with Na⁺, K⁺, and NH₄⁺, it is possible to distinguish between the effeects of the various rates. For black lipid membranes made from glycerolmonooleate+$\text{n-}\text{hexadecane}$, the potential energy barrier is high with a narrow top, but the rate of association still becomes increasingly limiting for Na⁺, K⁺ and NH₄⁺, in the order given. For bacterial phosphatidylethanolamine, with $\text{n-}\text{decane}$ the barrier is much wider and no effect of the rate of association can be detected.