A NANOSECOND FLUORESCENCE STUDY OF THE SIMULTANEOUS INFLUX OF CA2+ AND CD2+ INTO LIPOSOMES

Nanosecond fluorescence decay characteristics of the calcium-binding p robe Quin2 and two of its cation complexes were examined by time-resol ved fluorescence spectroscopy. Binding of Ca2+ and Cd2+ resulted in, f luorescence lifetime enhancements as compared to that of free Quin2 ([ tau] = 0.9 ns). The Quin2-Ca2+ complex displays a monoexponential deca y of tau = 7.4 ns, while the cadmium complex gives an average decay ti me of ca. 4 ns. Lifetime measurements made on heterogeneous cationic s olutions demonstrate that decay times for individual complexes can be retrieved. Time-resolved measurements were used to monitor the kinetic s of ionomycin-mediated calcium and cadmium transport across artificia l membranes. Fluorescence decays, collected on the time-scale of secon ds, were sufficient to measure individual ion fluxes or those of mixtu res into liposomes. The combination of steady-state and time-resolved fluorescence techniques offers the unique advantage of simultaneously detecting other cations in the presence of calcium. (C) 1998 Elsevier Science B.V.