A Reinvestigation of Chlortetracycline Fluorescence: Effect of pH, Metal Ions, and Environment

The pH dependence of chlortetracycline fluorescence has been reinvestigated. Below pH 7.5, excitation at 400 nm results in an emission band at 530 nm Above pH 75, in addition to the above band, a strong fluorescence is observed at 430 nm on excitation at 345 nm. ${pK}_{\alpha}$ values of 3.5 and 7.7 have been determined for the 530-and 430 nm bands, permitting their assignment to the ring A and ring BCD cbromophores, respectively. Both bands are shown to be sensitive to divalent metal binding. Excitation energy transfer from the 430- band to the 530-nm hand is observed in metal complexes, with transfer efficiency being greater for the ${Mg}^{2+}$ chelate than the ${Ca}^{2+}$ chelate. The effect of solvent and lipid on band intensities and transfer efficiencies is reported. The interaction of chlortetracycline with BSA in the presence of ${Mg}^{2+}$ leads to the observation of energy transfer from protein tryptophan residues to the 520-nm band of the complex via the intermediacy of the 430-nm band