New biomimetic chemistry in water: the anomalous ‘salting-in’ of neutral compounds of natural amino acids with transition metals

The addition of simple salts [such as CaCl2, MgCl2 and AgNO3] greatly enhances the solubility in water of many stable non-electrolytes [M(α)n] where M is a transition metal ion, α represents the conjugate base of a natural amino acid Hα (operating as a bidentate chelating ligand) and n is the oxidation state of the metal ion. The complex compounds are ‘salted-in’. Both kinetically labile (Cu2+) and inert (Co3+, Cr3+) ions show the effects. Some are due to local changes in the thermodynamic activity of water. The concept of a ‘spectator’ ion is questioned and ‘cybotaxis’ revived. Novel equilibria have been detected in a number of cases [with Cu(α)2] and several solid adducts are described. The novel adducts trans-[Cu(glyO−)2]·2AgNO3 (1) and trans,trans-[Cu(glyO−)2Cl2][Mg(H2O)6]·H2O (2), have been isolated from aqueous silver nitrate and magnesium chloride solutions respectively. Structural features of both are reported and in each case the glycinate ligands coordinate to the copper atoms in a trans arrangement. Structural features of other adducts are briefly reported.