Changes of the Capacitance and Boundary Potential of a Bilayer Lipid Membrane Associated with a Fast Release of Protons on Its Surface

The kinetics of binding of proton with bilayer lipid membrane (BLM) upon their fast release on the surface of the membrane has been studied by measuring the changes of the capacitance and electrostatic potential of the membrane. The release of proton was a result of excitation of 2-methoxy-5-nitrosulphate (MNPS) molecules, adsorbed on the surface of the membrane, by UV light. The adsorption of anions of MNPS on BLM led to a change of the boundary potential measured either by the inner field compensation method or as a change of ζ-potential of liposomes determined by the dynamic light scattering method. The illumination of BLM with the adsorbed MNPS molecules led to changes of the membrane capacitance and a shift of the boundary potential to positive values. This shift of the boundary potential was due partially to the MNPS decomposition and partially to the binding of protons on the membrane surface. With an increase in the buffer capacity, both the potential jump and the change in capacitance were significantly reduced in magnitude. Restoration of the membrane capacitance and boundary potential after the light flash took about tens of seconds. The changes of the potential and capacitance were observed not only on BLM formed from phospholipids but also on BLM formed from glycerol monooleate. These results are explained assuming that the protons released from MNPS during the flash of light are bound on the surface of the membrane in a layer of oriented water molecules.