Rapid Transfer of Photo-Released Protons from Water to Lipid Membrane

Abstract

The transport of protons between the membrane boundary and water can be hindered by the presence of a high potential barrier, which affects their transport across the membrane performed by membrane proteins. To estimate the rate of proton transport across this barrier, photoactivatable compounds whose molecules can adsorb on the membrane boundary and release protons upon excitation are used. We studied such a compound, sodium 2-methoxy-5-nitrophenyl sulfate (MNPS). Its molecule is able to adsorb on a lipid bilayer membrane (BLM) as an 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. Slow changes in potential were measured using the intramembrane field compensation method, while fast changes were measured using an electrometric amplifier. When the light was switched on, the potential increased rapidly, and when it was switched off, it 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 by a factor of ten. These results indicate that the reaction of proton release during the excitation of MNPS molecules occurs both on the membrane surface and in the water near it. The binding at the membrane of protons released by the reaction of MNPS in water provides the major contribution to the change in electrostatic potential at the membrane boundary, considerably exceeding the contribution of MNPS anions released by the reaction at the membrane.