A study of metal complexes of a naturally occurring macrocyclic ionophore-monensin

The acid-base and complexing properties of a naturally occurring antibiotic ionophore-monensin (MonH)-were studied in anhydrous methanol solutions primarily by potentiometric measurements. The pK _a of the acid was found to be 10.30±0.05 at 25°C. Complexes of silver, thallium, and alkali ions with the undissociated ligand were also studied, and their stability constants were determined. The acid dissociation constant of MonH as well as the stability constants of Mon^– Na⁺ and MonH·NaClO₄ complexes were determined in the 5–45°C temperature range, and the enthalpy and entropy of the acidity and stability constants were determined from van't Hoff's plots. The formation of MonH and Mon^–Na⁺ species are both enthalpy and entropy stabilized, but the formation of the MonH·NaClO₄ complex is enthalpy stabilized but entropy destabilized.