Styryl Dyes di-4-ANEPPS and RH-421 as Sensors of the Protons on the Surface of Lipid Membranes

It has been shown that electrochromic styryl dyes can be used as probes to study proton binding on the membrane surface. These dyes are adsorbed on the membrane as neutral molecules with a significant dipole moment, due to which a dipole potential jump occurs at the boundary of the membrane with the solution. A decrease in pH led to the disappearance of both this potential jump and fluorescence of the membrane-bound molecules of dyes di-4-ANEPPS and RN-421. This can be explained by the inability of protonated dye molecules formed at low pH to adsorb on lipid membranes. The dependence of the dipole potential jump on pH was measured in the experiment to estimate pK of the dyes, which was estimated to be about 4.5. Desorption of the dye molecules from the surface of the lipid membrane occurred not only under the solution pH decrease, but also upon photoactivated release of protons on the membrane surface caused by photolysis of the caged-H⁺ molecules of sodium 2-methoxy-5-nitrophenyl sulfate. This indicated a significant increase in the concentration of protons on the surface of the lipid membrane caused by this process.