CONFORMATION IN SOLUTION OF CARBOXYLIC IONOPHORE CATIONOMYCIN - ACID FORM AND LI+, NA+, K+, RB+, CS+, NME4+ COMPLEXES

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).