Solution chemistry of monensin and its alkali metal ion complexes. Potentiometric and spectroscopic studies

Sodium complex of an antibiotic drug, monensin, was investigated by potentiometric, infrared, and 23Na, 7Li, 13C, and 1H NMR techniques in several nonaqueous solvents. The acidity constant of monensin and the stability constant for the monensin--sodium ion complex were determined in methanol solutions. Complexation titrations with other cations show that the selectivity of monensin varies in the order Ag+ greater than Na+ greater than K+ greater than Rb+ greater than Cs+ greater than Li+ approximately NH4+. Spectroscopic measurements indicate that monensin forms two sodium complexes of different configuration and stability. These measurements also indicate that water molecules can bind to the monensin molecule at two different sites. Addition of the sodium ion results in water exchange between these two sites and the free water in the solvent. Effects of deuterium substitution on NMR and ir spectra are discussed. (U.S.)