Metal Bonding by Amino Acids: Preparation and Crystal Structures of Calcium Bis-L-pyroglutamate and Lithium L-Pyroglutamate
Calcium bis-L-pyroglutamate and lithium L-pyroglutamate are prepared by reaction of the metal hydroxides with L-pyroglutamic acid in aqueous solution.In the solid state calcium bis-L-pyroglutamate adopts a layer structure, built up by sixteen-membered rings consisting of four L-pyroglutamate ligands bridging four calcium centers by their carboxylate groups.All three oxygen donors of the L-pyroglutamate ligands are attached to metal centers.The calcium atom lies at the center of a slightly distorted octahedron formed by four carboxylate oxygens and two amide oxygen atoms.Lithium L-pyroglutamate also features a layer structure.All three oxygen donors of the L-pyroglutamate ligand have contacts with metal atoms.The coordination sphere of the lithium atom is a slightly distorted tetrahedron consisting of three carboxylate oxygen atoms and one amide oxygen atom.The investigation of aqueous solutions of the compounds by multinuclear NMR (1H, 13C, 17O) indicates extensive electrolytic dissociation in dilute solutions.Key Words: Calcium bis-L-pyroglutamate / Lithium L-pyroglutamate / Pyroglutamate coordination and conformation
