Carbon‐13 Nuclear‐Magnetic‐Resonance and Infrared‐Absorption Spectroscopy of Valinomycin and Its Alkali‐Ion Complexes

Abstract

The 13 C nuclear magnetic resonance spectra of the membrane‐active antibiotic valinomycin and its alkali‐ion complexes are interpreted. From the 13 C chemical shifts and infrared absorptions of the highly stable K + , Rb + and Cs + complexes in methanol it is concluded that the ligand conformations of these complexes must be similar. The essential features are the coordination of all ester carbonyl groups with the cation in addition to the formation of intramolecular hydrogen bonds between the amide groups. In contrast to these results the weaker Na + complex of the antibiotic exhibits different spectral properties which are more similar to those of the uncomplexed antibiotic in polar solvents thus indicating the existence of a considerably different ligand conformation in this complex. The structure of the Na + complex can be characterized by a reduced number of coordinated ester carbonyl groups and intramolecular hydrogen bonds between the amide groups as compared to the complexes with larger cations. The conformational rearrangements occurring during the complexation of cations are compared to those observed for the uncomplexed valinomycin in solvent systems of different polarities. Evidence is presented that the interaction of the uncomplexed antibiotic with polar solvent molecules leads to conformational rearrangements which can be characterized by a step‐wise opening of a closed structure stabilized by intramolecular hydrogen bond formation.