Investigation of pH dependent Fe3+ - levofloxacin interactions in water by fluorescence spectroscopy

Abstract

Interactions of antibiotics with biometals can affect their antimicrobial activity by changing their bioavailability, redox properties, and stability toward hydrolysis. [1,2] The pH of biological fluids can significantly affect interactions between biometals and ionizable drugs. In this study, we investigated the interactions of Fe3+ with levofloxacin in water at different pH using fluorescence spectroscopy. The coordination of Fe3+ with levofloxacin is pH dependent due to the ionization of levofloxacin and solubility of Fe3+(aq) ions. The formation of levofloxacin-Fe3+ complex was detected in the pH range 4–5 as a decrease in fluorescence intensity of levofloxacin. The Stern-Volmer diagrams of levofloxacin in the presence of different concentrations of Fe3+ ion at pH 4 and pH 5 correspond to the static fluorescence quenching confirming the coordination of levofloxacin with high-spin Fe3+. At pH < 4 and pH > 5 the addition of FeCl3 as a source of Fe3+ ions results in an increase in fluorescence intensity which can be attributed to the change of the ionic strength of solution and electrostatic interactions between different ionic species instead of coordination of Fe3+ ions with levofloxacin. The coordination of Fe3+ with levofloxacin at pH < 4 is prevented by protonation of its carboxyl and piperazinyl groups (pKa1 = 6.02, pKa2 = 8.15). At pH > 5, despite the presence of zwitterionic and anionic forms of levofloxacin available for coordination, there is no complex formation due to the precipitation of insoluble ferric hydroxide. References 1. Božić B, Korać J, Stanković DM, Stanić M, Romanović M, Pristov JB, Spasić S, Popović-Bijelić A, Spasojević I, Bajčetić M. Free Radical Biol. Med. 2018, 129, 279. 2. Chen J, Wang Y, Qian Y, Huang T. J. Hazard. Mater. 2017, 335, 117.