Covalent Metal−Peptide Framework Compounds That Extend in One and Two Dimensions

Peptides are attractive ligands for the design of metal organic frameworks, with the potential to confer chirality and biological activity on these materials. However, very few such materials have been reported. Here we describe the X-ray structures of four extended molecular framework compounds formed by the complexation of di- and tripeptides with cadmium ions. The tripeptide complex of cadmium with glycine, Cd(Gly3)2·H2O, forms a two-dimensional complex in which the carboxylate group of the peptide bridges between Cd ions. In contrast, the tripeptide complex of cadmium with alanine, Cd(l-Ala3)2, forms a one-dimensional extended molecular chain comprised of an infinite series of rings linked together through the Cd ions. The dipeptide complexes Cd(l-Ala2)2 and Cd(l-Ala,l-Thr)2·4H2O form covalently linked two-dimensional square lattices. Hydrogen bonding between peptide amide groups and hydrophobic interactions between side chains are seen to play important roles in defining these extended structures. Most interestingly, substitution of the more hydrophilic threonine side chain in place of alanine introduces a layer of water molecules into the crystal lattice. These results suggest that it may be possible to engineer the properties of these extended networks through judicious choice of amino acid side chains.

肽是制备金属有机框架（metal organic frameworks）的理想配体，可赋予这类材料手性与生物活性，具备广阔应用潜力。然而，目前此类材料的相关报道仍较为匮乏。本文报道了四种由二肽、三肽与镉离子配位复合得到的拓展型分子框架化合物的X射线结构。其中，甘氨酸三肽-镉配合物Cd(Gly₃)₂·H₂O可形成二维配合物，肽链的羧酸根基团可在镉离子之间起到桥联作用。与之形成鲜明对比的是，丙氨酸三肽-镉配合物Cd(L-Ala₃)₂可形成一维拓展分子链，该链由一系列通过镉离子互连的无限环状结构组成。二肽配合物Cd(L-Ala₂)₂与Cd(L-Ala,L-Thr)₂·4H₂O则可形成共价连接的二维方格晶格。研究表明，肽酰胺基团间的氢键作用与侧链间的疏水相互作用，对上述拓展结构的构建起到了关键作用。尤为值得关注的是，将丙氨酸侧链替换为亲水性更强的苏氨酸侧链后，晶体晶格中会引入一层水分子。上述研究结果表明，通过合理选择氨基酸侧链，有望实现对这类拓展型网络材料性能的精准调控。