Fluorescence studies of gold(III)-norfloxacin complexes in aqueous solutions

Formation of gold(III) complexes with the synthetic antibiotic norfloxacin (NF) was investigated in aqueous solution at pH 4.0, 7.5 and 10.6, with the ligand in cationic, zwitterionic and anionic forms, respectively. UV-Visible spectroscopy, steady state and time-resolved fluorometry were used to characterize the complexes. Binding sites, association constants and fluorescence lifetimes of the complexes were obtained. Au(3+) binding to zwitterionic NF produced a fluorescence decrease and a small red shift. Fluorescence changes as a function of Au(3+) concentration were fitted using a one-site binding model and the association constant was obtained, K(bzw) = 1.7 X 10⁵ M⁻¹. The association of Au(3+) with cationic NF was much weaker, the obtained binding constant being K(bcat) = 2.4 X 10³ M⁻¹. The Au(3+) binding site for these species involves the carboxyl group, in agreement with a much stronger association of the cation with the carboxylate anion than with the neutral acid. Association of Au(3+) with nonfluorescent anionic NF presented a clear evidence of two binding sites. The highest affinity site is the unprotonated piperazinyl group with K(bpip) ≥ 5 X 10⁷ M⁻¹ , and the low affinity site includes the carboxylate anion. The results point out to important pH dependent differences in complex formation between transition metal ions and fluoroquinolones, leading to different binding sites and association constants that change by several orders of magnitude.