Effect of lipid composition changes on carbocyanine dye fluorescent response

Egg yolk phosphatidyl choline liposomes containing variable amounts of phosphatidyl ethanolamine, phosphatidyl inositol or phosphatidyl serine demonstrated important variations in the fluorescence of 3.3' dipropylthiodicarbocyanine. When the membrane contained no cholesterol, fluorescence was not correlated with membrane fluidity as measured by diphenyl hexatriene polarization. Increasing cholesterol concentration in valinomycin containing liposome membranes decreased the potassium induced apparent membrane potential and prevented sorption of dye to the membrane. Discontinuity in the apparent potential occurred at 30 mol% cholesterol but could not be correlated with changes in microviscosity. These results indicate that great care should be taken when correlating rapid variations of fluorescence to changes in membrane potential. We propose that changes in phospholipid metabolism could well explain fluorescent changes when monitoring the fluorescence of cyanine dye molecules sorbed to biological membranes.