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.