Aqueous chemistry of nalidixic acid and its complexes with biological relevant cations: A combination of potentiometric, UV spectrophotometric, MS and MS/MS study

Nalidixic acid (NAL) is a broad-spectrum antimicrobial widely used for urinary tract infections. As demonstrated, complexation of NAL with Zn2+, Mn2+ and Cu2+ was often used to get new formulations with an enhanced efficiency and potency. Therefore, the elucidation of behavior of NAL in solution and of its interaction with metal cations are crucial to better understand the influence of complexation on NAL efficiency and to find the optimal conditions to propose novel formulations. As a preliminary study, spectrophotometric titrations were carried out on NAL to determine the values of the protonation constants and to define its acid-base behavior. Then, the interaction with the three metal cations Zn2+, Mn2+ and Cu2+ was investigated by potentiometric and spectrophotometric titrations, varying the conditions of temperature, ionic strength and metal-ligand ratio, thus allowing to get the most robust speciation model and to determine the formation constants with Zn2+, Mn2+, and Cu2+ under different conditions, the sequestering ability of NAL towards metal cations, the formation enthalpic and entropic changes. A simulation under serum conditions was reported to show the relevance of the investigated species. Finally, LD-MS (laser desorption ionization mass spectrometry) and MS/MS analyses highlighted for all systems the formation of the complex species between Zn2+, Mn2+ and Cu2+ with NAL. MS/MS investigations assigned the sites of coordination of the ligand with the metal cation. More precisely, deprotonated NAL coordinates the metal cation via the oxygens of the carboxylate and the carbonyl groups.

Automated summary

Nalidixic acid (NAL) is a broad-spectrum antimicrobial commonly used for urinary tract infections. Complexation of NAL with metal cations like Zn2+, Mn2+, and Cu2+ can enhance its efficiency and potency. This study investigates the behavior of NAL in solution and its interaction with metal cations under different conditions to better understand the influence of complexation on NAL and propose novel formulations. Spectrophotometric titrations were conducted to determine the protonation constants and acid-base behavior of NAL. Potentiometric and spectrophotometric titrations were then carried out to study the interaction between NAL and Zn2+, Mn2+, and Cu2+, allowing the determination of formation constants and sequestering ability. Laser desorption ionization mass spectrometry and MS/MS analyses confirmed the formation of complex species between NAL and the metal cations.