The transfer of protons between the surface of lipid membrane and water can be slowed down by the presence of a high potential barrier, which affects the functioning of proton-transporting proteins. To evaluate the rate of the proton transfer across the barrier, the photoactivatable compounds that can adsorb on the membrane boundary and release protons upon excitation are used. One of these compounds, which we studied earlier, sodium salt of 2-methoxy-5-nitrophenylsulfate (MNPS), was used in this work. The molecule of MNPS can adsorb on the bilayer lipid membrane (BLM) as anion and release sulfate and proton upon excitation with UV light, becoming an electroneutral product. Upon illumination of the BLM, on one side of which MNPS anions were adsorbed, changes in the electrostatic potential at the membrane–water interface were observed. The slow changes of the potential were measured by the intramembrane field compensation method and the fast changes, by the operational amplifier as an electrometer. When the light was switched on, the potential increased rapidly, and when the light was switched off, the potential slowly returned to its initial value. The rate of rapid potential increase depended on the lipid composition of BLM, buffer concentration, and pH of the medium. The dependence of this rate on pH was different for BLMs formed from phosphatidylcholine and its mixture with phosphatidylserine. With increasing buffer concentration, the rate decreased tens of times. The results obtained indicate that the reaction of proton release formed during the excitation of MNPS molecules occurs both on the membrane surface and in the water near it. The main contribution to the change in the electrostatic potential at the membrane boundary is given by protons bound at its surface from the reaction in water.
V. Yu. Tashkin (Wed,) studied this question.