title: Mechanism of fluorescent response of the probe diS-C3-(5) to transmembrane potential changes in a lecithin vesicle suspension.
abstract: The dependence of both the magnitude and the sign of fluorescent responses of the probe diS-C3-(5) in egg lecithin vesicle suspensions on the magnitude of the inside-negative transmembrane potential and on the total probe concentration in the sample volume has been studied. Results were compared with theoretical calculations made on the basis of the equilibrium thermodynamic model suggested earlier as well as on the observed concentration dependence of the probe fluorescence in aqueous media and membranes. It was shown that transmembrane potential results in a redistribution of the probe between the aqueous and the membrane phases and in the membrane interior. The model calculations showed that the probe concentrations in the external aqueous medium and in the outer lipid monolayer of a vesicle significantly decrease. At the same time, the dye concentration in the inner membrane monolayer increases significantly, which should lead to a marked quenching of the dye fluorescence. The changes in the fluorescence and absorption spectra are well explained in terms of the proposed mechanism. The highest responses of diS-C3-(5) to changes of the transmembrane potential were observed in a shorter wavelength region of the fluorescence spectrum at the probe to lipid ratios in membrane of 15-20 moles of probe per 1000 moles of lipid. In the longwave region, the increase in fluorescence is not an obligatory indication of a decrease in the transmembrane potential and, under certain conditions, this process can take place when the transmembrane potential increases. The generation of a quasi-equilibrium diffusion transmembrane potential results in an increase in the average probe concentration in membranes if the signs of the probe charge and the potential inside the vesicles are opposite. Thus, the "on-off" mechanism, working under conditions of steady-state processes, is not valid under equilibrium conditions.
mesh_terms: D052160:Benzothiazoles; D002232:Carbocyanines; D005456:Fluorescent Dyes; D008081:Liposomes; D008564:Membrane Potentials; D010713:Phosphatidylcholines; D011804:Quinolines; D013050:Spectrometry, Fluorescence
chemical_list: D052160:Benzothiazoles; D002232:Carbocyanines; D005456:Fluorescent Dyes; D008081:Liposomes; D010713:Phosphatidylcholines; D011804:Quinolines; C012944:3,3'-dipropyl-2,2'-thiadicarbocyanine