Metal ion binding by amino acids: Strontium and barium l‐aspartate trihydrate Sr/Ba(l‐Asp) · 3 H₂O

Strontium and barium l ‐aspartate are obtained by neutralization of aqueous solutions of l ‐aspartic acid with strontium or barium hydroxide, respectively. Slow crystallization from hot water affords the crystalline trihydrates, saturated solutions of which show pH values of 11.0 and 10.8 at ambient temperature. – The crystal structure of the two compounds has been determined by single crystal X‐ray diffraction. The compounds are isomorphous (orthorhombic, space group P 2 1 2 1 2 1 ), and the structural parameters are very similar as expected from the small differences in the ionic radii of the two metals. – The cations are arranged in double strings parallel to the b axis. The l ‐aspartate dianions are bridging these double strings by chelating contacts of the two carboxylate groups with the metal atoms of different strings to give layers. While the α‐carboxylate groups are only bidentate and associated with one metal atom each, the β‐carboxylate oxygen atoms are also each bridging two adjacent metals of the neighbouring string. The three water molecules are all coordinated to the metal atoms, which attain coordination number 9 . The amino groups are not engaged in metal coordination, but are part of a system of hydrogen bonds cross‐linking the layers.