Metal ion binding by amino acids. Preparation and crystal structures of magnesium, strontium, and barium L‐glutamate hydrates

Crystalline magnesium, strontium, and barium L‐glutamate complexes containing the dianion L‐Glu²⁻ have been prepared as the tetrahydrate (Mg) and hexahydrates (Sr, Ba), respectively. Their aqueous solutions have pH values of 10.45, 11.05, and 10.93, respectively. The solid state structures of the Mg and Sr complexes have been determined by single crystal X‐ray methods, and the structure of the Ba analogue inferred through its isomorphism with the Sr analogue. The crystals are orthorhombic, space group P2₁2₁2₁. The magnesium atoms were found to be hexacoordinated with the L‐glutamate functioning as a chelating N,O_α‐donor. Four water molecules complete the first coordination sphere. The γ‐carboxylate group is only engaged in hydrogen bonding. Strontium and barium are nine‐coordinated. The α‐carboxylate group of the L‐Glu²⁻ ligand is O,O‐chelating to one metal and forming oxygen bridges to two further metal atoms. The γ‐carboxylate group is also chelating a neighbouring strontium atom, but the nitrogen function is not engaged in metal coordination. By MOM bridging double strings of metal‐oxygen chains are formed, which are interlinked by the amino acids to give a layered structure. Three water molecules are also part of the first coordination sphere of each metal, while three others are interlayer hydrogen‐bonded components. The amino groups are also involved in this hydrogen bonding.