SPECTROSCOPIC PROPERTIES OF THE POTENTIOMETRIC PROBE MEROCYANINE-540 IN SOLUTIONS AND IN LIPOSOMES

Absorption, fluorescence and resonance Raman spectra of the membrane dye merocyanine-540 (MC540) were measured. The aggregation of the dye, its binding to lipid membranes and its response to crossmembrane electric potential differences were studied. The dye was found to aggregate even at micromolar concentrations in water, but not in organic solvents. The dimerization constant was evaluated by spectroscopic techniques. The binding constant to liposomes was estimated by a spectroscopic titration method. Resonance Raman spectra of MC540 were measured for the first time. Distinct changes were observed in the vibrational spectrum upon the generation of a valinomycin-induced K+ diffusion potential (Nernst potential) on liposomes. The ratio of Raman band intensities, which was found to be related to the membrane potential, can be used to evaluate the absolute value of the electric potential.