Fluorescence Studies of gold(III)-Norfloxacin Complexes in Aqueous Solutions

Abstract

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.

Norfloxacin La(III)-based complex: synthesis, characterization, and DNA-binding studies

“…When the pH decreased, the fluorescence increased, and at pH 7.4, the maximum intensity at 412 nm was registered. Such dependence of fluorescence with the change in pH values agrees with the reported work of NOR [2]. This transition between pH 11 and 7 is due to the protonation of the distal amine of the piperazine group.…”

“…Fluorescent quenching can occur in two different mechanisms, static quenching and dynamic quenching. For dynamic quenching, the mechanism can be described by the Stern-Volmer equation (2). In order to confirm the quenching mechanism, the fluorescence quenching was first assumed to be a dynamic quenching process.…”

“…Usually, the binding sites of metal ions with quinolones involve the 4-oxo and 3-carboxyl oxygens, but Ag + , Au 3+ , and Pt 2+ ions coordinate with the norfloxacin ligand through the N atom of the piperazinyl ring [6]. Recently, Luiz et al [2] studied the interaction of norfloxacin with gold(III) and observed that Au 3+ produces pH-dependent spectral modifications, attributed to different interactions with the distinct ionic species. Very recently, Zn(II) complexes with NOR were synthesized and the biological activity of these complexes was directly related to the acid ionization constant (pK) [4].…”

Stable lead-halide perovskite quantum dots as efficient visible light photocatalysts for organic transformations

“…4a based on electrochemical data. The hole accepting tendency of the substrate is further confirmed by Stern–Volmer studies 64 (Fig. S21 † ).…”

Metal Complexes of Quinolone Antibiotics and Their Applications: An Update

“…Apart from the main analytical applications in determination of quinolones in pharmaceutical forms and in biological samples, the fluorescent complexes were used also for other purposes. In this regard, fluorescence studies of Au(III)-norfloxacin system were carried out in order to study the association of Au 3+ ions with cationic, zwitterionic and anionic forms of the drug [211]. Cu(II)-ofloxacin interaction, studied by means of ofloxacin fluorescence quenching experiments in the presence of Cu(II), was evaluated for its environmental impact [212].…”