Preparation, characterization and biological relevance of calcium(II)/diethylmalonate(-1,-2) complexes

Reactions of Ca(NO3)2·4H2O and diethylmalonic acid (Et2malH2, its anions represent functional side-chain analogs of γ-carboxyglutamic acid residues which are implicated as essential Ca2+-binding ligands in a variety of proteins) in aqueous media have afforded compounds [Ca(Et2malH)2(H2O)3]n (1) and [Ca(Et2mal)(H2O)]n (2) at pH 4 and 8, respectively. The structure of 1 was determined by single-crystal, X-ray crystallography, which revealed an 1D coordination polymer. The diethylmalonate ligands exist in their monoanionic form and present two different coordination modes. The CaII ion is 7-coordinate with a pentagonal bipyramidal geometry. IR data are discussed in terms of the known (1) and proposed (2) structures of the complexes. The role of the carboxylate binding modes in determining the affinity of Ca2+ for the various metal binding sites in proteins containing the γ-carboxyglutamate residue is discussed in the light of our and previous results.