Electrostatic Potentials during Adsorption and Photochemical Reactions of Pyranine on Bilayer Lipid Membranes

Abstract

Adsorption and photochemical reactions of pyranine on a bilayer lipid membrane (BLM) have been studied by measuring electrostatic potentials at the membrane–water interface. The dependence of the electrostatic potentials due to the adsorption of pyranine on its concentration in solution is described by the Gouy–Chapman theory assuming that anions with three charged groups are adsorbed on the membrane. No significant changes in the boundary potential were found when BLM with pyranine adsorbed on it was illuminated. Significant changes in the potential were observed if molecules of styryl dyes di-4-ANEPPS or RH-421 were adsorbed on BLM in addition to pyranine. The sign and magnitude of these changes correspond to the disappearance of the dipole potential created by styryl dye molecules on the BLM. The rate of potential disappearance was proportional to pyranine concentration and illumination intensity. The disappearance of the potential can be caused either by the binding of protons released from the pyranine molecule to the dye mo-lecules with their subsequent desorption from the BLM or by their destruction. Pyranine and styryl dye molecules can form complexes at the BLM boundary. This is evidenced by experiments in which the sum of the potential changes caused by their adsorption separately differed significantly from the change in the boundary potential during their simultaneous adsorption. The kinetics of the disappearance of the dipole potential of BLM with styryl dyes upon excitation of pyranine turned out to be similar to that observed earlier with another compound, 2-methoxy-5-nitrophenyl sodium sulfate, which releases protons at the membrane boundary upon illumination (Konstantinova et al., 2021. Biochem. (Mosc.), Suppl. Series A: Membr. Cell Biol. 15 (2), 142–146). This suggests that it is associated with the desorption of dye molecules from the membrane, due to the binding of protons released from excited pyranin molecules to them.