New insights into coordination chemistry of Monensin A towards divalent metal ions

Abstract

Complexation reactions of polyether ionophorous antibiotic Monensin A with divalent metal ions were studied in methanolic solutions by means of synchrotron radiation circular dichroism (SRCD) spectroscopy. Data showed that in the presence of organic base (Et4NOH) Monensin A forms two types of metal complexes depending on metal-to-ligand molar ratio. Complexes of composition [M(Mon)2(H2O)2] (Mon– = Monensinate A anion; M = Co2+, Mn2+, Mg2+, Ca2+, Ni2+, Zn2+, Cd2+) exist mainly in the presence of ligand excess (M2+:Mon– ~ 1:10–1:2), while the increase of metal(II) ion concentration results in the formation of new [M(Mon)(H2O)]+ species (M2+:Mon– ~ 1:1–6:1), not detected up to now with these metal ions. Experimental data showed that coordination of divalent metal ions to Monensinate A anion significantly influences the SRCD spectral pattern in different manner. Based on sets of SRCD experiments, species distribution, individual spectra of [M(Mon)2(H2O)2] and [M(Mon)(H2O)]+ as well as their conditional stability constants were calculated. The relative affinity of Monensin A to bind divalent metal ions demonstrates the metal ion selectivity of the ligand.