Metal ion binding by amino acids. Preparation and crystal structures of two calcium L ‐asparatate hydrates

Crystalline calcium(II) L ‐aspartate dihydrate, Ca( L Asp) · 2H 2 O ( 2 ), has been obtained in a three‐step procedure from CaCO 3 and L ‐aspartic acid L ‐AspH 2 via calcium bis( L ‐hydrogenaspartate) Ca( L ‐AspH) 2 and calcium hydrogen L ‐aspartate chloride dihydrate Ca( L ‐AspH)Cl · 2H 2 O. The crystals of 2 are orthorhombic, space group P 2 1 2 1 2 1 . The structure is a coordination polymer with the metal atoms arranged in double chains interconnected by aspartate dianions. Each Asp 2− ligand is coordinated to the metal ion as a N,O α ,O β tripod. In addition, two water molecules and three β‐carboxylate oxygen atoms from neighbouring units are also coordinated to each metal atom, giving a coordination number of eight for each calcium. The β‐carboxylate oxygen atoms form also a bridge between metals of a parallel chain of metal atoms. – A crystalline tetrahydrate (monoclinic, space group C 2) Ca( L ‐Asp) · 4H 2 O ( 1 ) has been prepared from L ‐AspH 2 and Ca(OH) 2 . The structure contains two molecules in the asymmetric unit. Each calcium atom is seven‐coordinate and surrounded by an N , O α ‐chelating L ‐Asp 2− ligand, four water molecules, and one oxygen atom of the β‐carboxylate group of a neighbouring molecule. Due to this interaction, the monomeric units are linked to give a one‐dimensional coordination polymer chain. – In both structures the double stands and chains, respectively, are crosslinked by a network of hydrogen bonds.