Fujita Megumi, Kazerouni Amaan M, Bacsa John
Department of Chemistry, University of West Georgia, 1601 Maple St., Carrollton, GA 30322, USA.
Department of Chemistry, Emory University, Atlanta, GA 30322, USA.
Acta Crystallogr C Struct Chem. 2016 Aug 1;72(Pt 8):627-33. doi: 10.1107/S2053229616010676. Epub 2016 Jul 6.
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)62·3H2O, in which the valinomycin molecule incorporates a whole hexaaquamagnesium ion, Mg(H2O)6, 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.
缬氨霉素是一种天然存在的环状十二肽缩酚酸肽，分子式为环-[D-α-羟基异缬草酸→L-缬氨酸→L-乳酸→L-缬氨酸]3（D-α-羟基异缬草酸为D-α-羟基异缬草酸，缬氨酸为缬氨酸，乳酸为乳酸），它能高度选择性地结合钾离子（K⁺）。过去，通过X射线晶体学揭示了几种阳离子结合模式。在钾离子（K⁺）、铷离子（Rb⁺）和铯离子（Cs⁺）配合物中，酯氧原子以三角反棱柱几何构型配位阳离子，而六个酰胺基团形成分子内氢键，所形成的网络具有类似手镯的构象（1型结合）。钠离子（Na⁺）呈现2型结合，其中缬氨霉素分子保持手镯构象，但阳离子仅由三个酯羰基配位且不在中心位置。此外，在缬氨霉素手镯的中心发现一个苦味酸盐抗衡离子和一个水分子。钡离子（Ba²⁺）呈现3型结合，其中包含两个阳离子，由两个阴离子桥连，并由酰胺羰基配位，且不存在分子内酰胺氢键。在本文中，我们展示了一种新的4型阳离子结合模式，在缬氨霉素六水合镁双（三氟甲磺酸）三水合物C₅₄H₉₀N₆O₁₈·Mg(H₂O)₆₂·3H₂O中观察到，其中缬氨霉素分子通过酰胺羰基与水合氢离子之间的氢键结合一个完整的六水合镁离子[Mg(H₂O)₆]²⁺。在这种配合物中，缬氨霉素保持了1型结合中观察到的三重对称性，但酰胺氢键网络消失；六水合镁阳离子通过六个酰胺羰基形成氢键。¹H NMR滴定数据与乙腈溶液中的1:1结合化学计量比一致。这种由环状多肽结合完整六水合镁离子的新阳离子结合模式可能对生理条件下生物模型中金属结合的研究具有重要意义。
