EVIDENCE IN FAVOR OF THE EXISTENCE OF A KINETIC BARRIER FOR PROTON-TRANSFER FROM A SURFACE OF BILAYER PHOSPHOLIPID MEMBRANE TO BULK WATER

When the hydrogen-ion flux is induced by nigericin across the planar bilayer lipid membrane (BLM) with bulk pH values being equal at the opposite sides of the BLM, formation of a difference in boundary potentials (DELTAphi(b)) on the membrane is observed by the method of inner membrane field compensation. pH gradients are titrated routinely by the addition of sodium acetate at one side of the membrane. The increase in buffer concentration (citrate, phosphate, Mes) leads to a decrease in DELTAphi(b). DELTAphi(b) forms in the presence of phosphatidylserine in the membrane-forming solution only. It is concluded that the steady-state difference of the hydrogen ion binding to the opposite surfaces of the membrane (HIBD) is created under the conditions of equal pH values near surfaces of the BLM. The model of the processes implies that nigericin transfers proton predominantly from interface to interface while acetate transfers the proton from bulk phase to bulk phase. In the other series of experiments the monensin-mediated formation of the HIBD leads to the formation of an potassium-ion gradient in the presence of nigericin. Thus, a possibility of performing a work due to the formation of HIBD is demonstrated. Owing to these properties the hydrogen-ion binding difference can be interpreted in a first approximation as a difference of surface hydrogen-ion concentration at the opposite sides of the membrane, arising due to the existence of a kinetic barrier for the proton transfer at the membrane interfaces. These findings can be significant for the mechanism of energy transduction in membrane phosphorylation in mitochondria and chloroplasts.