The behavior of the fluorescent probe diS-C3-(5) in membrane and aqueous media.

Based on an analysis of back fluorometric titration data a partition coefficient, Kp = (5.70 +/- 0.95) x 10(4), and partition constant, K = (2.37 +/- 0.43) x 10(6), were found for the probe diS-C3-(5) in egg lecithin vesicle suspension. The relative probe quantity in an aqueous medium and in liposomes was calculated using these parameters. The number of chromophore states in this system was computer-analysed and it was shown that the probe fluorescence could be described by two fluorescing dye forms, aqueous and membrane monomers. The dependence of fluorescence intensity on the probe concentration was studied in various salt media, and a dimerization (association) constant Ka = 5 x 10(4) mol -1 . l in the buffer, and Ka = (8.1 +/- 1.5) x 10(4) mol-1 . l in 0.1 or 0.2 mol/l salt medium (KCl or NaCl) was found. From the fluorescence and absorption data critical concentrations of the onset of large probe aggregate formation were calculated for various aqueous media. The concentration dependence of the probe fluorescence in the membrane phase was calculated. The critical concentration of interaction characterizing the efficiency of the fluorescence concentration quenching processes (CCI) was found to be approx. 5-6 mol probe per 1000 mol lipid. The top probe concentration in a membrane (the "saturation" concentration) was estimated from the slope of the initial linear parts of the back fluorometric titration curves, and was found to be equal to (59 +/- 13) mol probe per 1000 mol lipid.