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The structure of a valinomycin–hexaaquamagnesium trifluoromethanesulfonate compound
Author(s) -
Fujita Megumi,
Kazerouni Amaan M.,
Bacsa John
Publication year - 2016
Publication title -
acta crystallographica section c
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.304
H-Index - 17
ISSN - 2053-2296
DOI - 10.1107/s2053229616010676
Subject(s) - chemistry , valinomycin , intramolecular force , hydrogen bond , amide , molecule , stereochemistry , crystallography , trifluoromethanesulfonate , oxonium ion , hydrate , moiety , ion , organic chemistry , catalysis
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, C 54 H 90 N 6 O 18 ·[Mg(H 2 O) 6 ](CF 3 SO 3 ) 2 ·3H 2 O, in which the valinomycin molecule incorporates a whole hexaaquamagnesium ion, [Mg(H 2 O) 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.