Conformation in solution of carboxylic ionophore cationomycin: acid form and Li+, Na+, K+, Rb+, Cs+, NMe4+ complexes

Abstract

NMR study of the acid form and Li⁺, Na⁺, K⁺, Rb ⁺, Cs⁺ and NMe₄⁺ neutral 1:1 complexes of the title compound was carried out in CDCl₃ and CD₃OD. Comparison was made with the corresponding species obtained with monensin a well-known related carboxylic ionophore. Resonance assignments were made by 2D NMR methods and nuclear Overhauser effects were measured by ROESY experiments. In CDCl₃, postulated conformations are very similar for the two ionophores; only local differences were observed between the acid and the complexes. In CD₃OD the monensin free acid presents a more open structure. Very small differences were recorded for cationomycin in the two solvents, this unusual behaviour may be related to the presence of the aromatic ester moiety. The different behaviour observed between the two bacterial ionophores cannot explain by itself the pronounced selectivity in binding for Na⁺(monensin) and K⁺(cationomycin).