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

Crystalline calcium(II) L‐aspartate dihydrate, Ca(LAsp) · 2H₂O (2), has been obtained in a three‐step procedure from CaCO₃ and L‐aspartic acid L‐AspH₂ via calcium bis(L‐hydrogenaspartate) Ca(L‐AspH)₂ and calcium hydrogen L‐aspartate chloride dihydrate Ca(L‐AspH)Cl · 2H₂O. The crystals of 2 are orthorhombic, space group P2₁2₁2₁. The structure is a coordination polymer with the metal atoms arranged in double chains interconnected by aspartate dianions. Each Asp²⁻ 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 C2) Ca(L‐Asp) · 4H₂O (1) has been prepared from L‐AspH₂ 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‐Asp²⁻ 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.