Toward an understanding of the fluorescence intensity changes observed on fluorescein 5'-Isothiocyanate-Na(+),K...

The fluorescence emission intensity between the Na(+), and the K(+) complex of Na(+),K(+)-ATPase, labeled with fluorescein 5'-isothiocyanate (FITC), differs by 30 to 40%. Experimental studies are carried out to elucidate the physical reasons which account this intensity difference. The dissociation constant of protolysis of the covalently bound FITC and its fluorescence decay times are determined in media of different ionic compositions and are compared with the corresponding properties of a synthetic model compound. The fluorophore bound to the protein is characterized by two decay times in the nanosecond range; the model compound, by a single one. The static fluorescence intensity changes are discussed on the basis of these results.