Potential‐sensitive membrane association of a fluorescent dye

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Abstract
Unilamellar phosphatidylcholine/cholesterol (5:1, w/w) vesicles and the fluorescent dye safranine O mixed in appropriate ratios produced a membrane potential‐dependent enhancement of dye fluorescence. The fluorescence enhancement was shown to be dependent on the sign and magnitude of valinomycin‐induced potassium diffusion potentials. The enhancement and a blue‐shifted maximum (both of which also occur in ethanol vs aqueous solution) provided evidence that the enhanced fluorescence arises from an additional population of safranine O molecules which become associated with a hydrophobic region of the vesicular membrane. Consistent with this interpretation, the polarization of safranine O fluorescence was also found to increase in a potential‐dependent manner. A time‐dependent decay of the fluorescence enhancement — presumably due to decay of the membrane potential — was attributed to non‐specific ion leakage at valinomycin concentrations above 3 μM.