Circular Dichroism Spectroscopic Studies on Solution Chemistry of M(II)-Monensinates in Their Competition Reactions

The chirality of the polyether ionophore monensic acid A can be successfully used to study its coordination ability in solution. A complementary approach to gain new insights into the complexation chemistry of the antibiotic (studied previously by circular dichroism (CD) spectroscopy in the ultraviolet range (UV-CD)) is presented. (1) Methods: The CD spectroscopy in the visible (VIS-CD) and near-infrared (NIR-CD) range is applied to evaluate the affinity of deprotonated monensic acid A (monensinate A) towards Ni(II) or Co(II) cations in methanolic solution. Competition experiments between a variety of colorless divalent metal ions for binding the ligand anion were also performed. (2) Results: The stability constants of the species observed in binary Ni(II)/Co(II)-monensinate systems and their distribution were reevaluated with the VIS- and NIR-CD techniques. The data confirmed the formation of mono and bis complexes depending on the metal-to-ligand molar ratio. The studies on the systems containing two competing divalent metal cations exclude the formation of ternary complex species but provide an opportunity to also calculate the stability constants of Zn(II), Mg(II), and Ca(II) monensinates. (3) Conclusions: The advantages of CD spectroscopy in the VIS-NIR range ("invisible" ligand and metal salts, "visible" chiral complex species) simplify the experimental dataset evaluation and increase the reliability of computed results.