Valinomycin is a naturally occurring cyclic dodecadepsipeptide with the formula cyclo-[D-HiVA→L-Val →L-LA→L-Val]3 (D-HiVA is D-α-hydroxyisovaleic acid, Val is valine and LA is lactic acid), which binds a K(+) ion with high selectively. In the past, several cation-binding modes have been revealed by X-ray crystallography. In the K(+), Rb(+) and Cs(+) complexes, the ester O atoms coordinate the cation with a trigonal antiprismatic geometry, while the six amide groups form intramolecular hydrogen bonds and the network that is formed has a bracelet-like conformation (Type 1 binding). Type 2 binding is seen with the Na(+) cation, in which the valinomycin molecule retains the bracelet conformation but the cations are coordinated by only three ester carbonyl groups and are not centrally located. In addition, a picrate counter-ion and a water molecule is found at the center of the valinomycin bracelet. Type 3 binding is observed with divalent Ba(2+), in which two cations are incorporated, bridged by two anions, and coordinated by amide carbonyl groups, and there are no intramolecular amide hydrogen bonds. In this paper, we present a new Type 4 cation-binding mode, observed in valinomycin hexaaquamagnesium bis(trifluoromethanesulfonate) trihydrate, C54H90N6O18·[Mg(H2O)6](CF3SO3)2·3H2O, in which the valinomycin molecule incorporates a whole hexaaquamagnesium ion, [Mg(H2O)6](2+), via hydrogen bonding between the amide carbonyl groups and the hydrate water H atoms. In this complex, valinomycin retains the threefold symmetry observed in Type 1 binding, but the amide hydrogen-bond network is lost; the hexaaquamagnesium cation is hydrogen bonded by six amide carbonyl groups. (1)H NMR titration data is consistent with the 1:1 binding stoichiometry in acetonitrile solution. This new cation-binding mode of binding a whole hexaaquamagnesium ion by a cyclic polypeptide is likely to have important implications for the study of metal binding with biological models under physiological conditions.
Keywords: NMR titration analysis; conformation; crystal structure; cyclic dodecadepsipeptide; magnesium; polypeptide; valinomycin.