PASSIVE ION TRANSPORT IN ARTIFICIAL LIPID BILAYER MEMBRANES

Passive ion transport across artificial lipid bilayer membranes has been studied. Two mechanisms, the carrier mechanism and the pore mechanism, are described. A special case of the carrier mechanism--hydrophobic ion transport, also known as self carrier--is treated in detail. Two electrical properties of hydrophobic ion transport are calculated. One is the relaxation current after a sudden imposition of an external voltage step; the other is the steady state noise spectrum. Our results extend those previously obtained since the model treated is more general than those described in previous work: diffusive coupling with the membrane processes and interior binding sites are both considered. Our results are best applied to systems for which there is strong coupling of diffusion with membrane processes. Previous analysis can only be applied if either the diffusion coupling is weak or the binding sites are at the water-membrane interface (but not both). Therefore they are limiting cases of our more general results. The result of the relaxation current calculation indicates that the relaxation current can be used to establish the positions of the binding sites under favorable conditions. We use a formalism developed by Frehland to compute steady state noise. However, we have modified his formalism since the binding sites at the interface are not discrete as is required in his approach. The result of steady state noise calculations shows that diffusive coupling with the membrane processes does not lead to a rigorous 1/f dependence in the hydrophobic ion case. The total noise power has two components, one of them is equilibrium-like noise, the other is called excess noise. In some cases the excess noise power increases as the frequency drops. This excess noise power has structure characteristics of the transport system. Separation of the equilibrium-like noise from the total noise is possible in some favorable circumstances. This leads to another way of characterizing the transport system by excess noise power. The results of ion pore relaxation current calculation exhibits similar trends. They can also be used to correlate the binding sites in some favorable cases.