CONTROL OF ION-TRANSPORT ACROSS BILAYER LIPID-MEMBRANES BY ADJUSTMENT OF SURFACE-CHARGE ASSOCIATED WITH PHASE DOMAIN-STRUCTURES

Interest continues in the use of immobilized lipid membranes of mixed lipid composition to prepare chemically modified electrodes. Fundamental investigations of the structure and mechanism of the analytical function of such systems are scarce. In this work ion conductivities of bilayer lipid membranes formed from mixtures of egg phosphatidylcholine and dipalmitoylphosphatidic acid were evaluated to determine the effect of surface charge and phase domain formation on the process of ion translocation. Ion conductivity was controlled by the surface distribution of ions at the membrane solution interface as predicted from electrical double-layer theory. It was found that the conductivity of the membranes could be approximated as a linear function of the weight percentage composition of the charged lipid. The conductivity was observed to alter drastically at a lipid composition containing a minimum of 25% phosphatidic acid as this component within the membrane was increased. This was attributed to the presence of a phase transition induced by the phosphatidic acid. At concentrations of the acid less than 25%, ion conduction occurred through zones that were enriched in the charged lipid. At higher concentrations of the acid, the average surface charge was the predominant factor which determined the magnitude of ion conductivity. The adjustment of pH to control the degree of ionization of the phosphatidic acid had a similar effect to the variation of the amount of the acidic phospholipid within the membrane for experiments done at fixed pH. An electrochemical method based on ion permeability is proposed for the determination of the pK(a) value for a charged lipid within a planar bilayer lipid membrane.