Ternary co-ordination complexes of copper(II) with L-histidine and some selected amino-acids

Abstract

The multiple equilibria involved in some ternary systems of copper(II) containing L-histidine (A) and DL-2-aminobutyric acid, DL-3-aminobutyric acid, 4-aminobutyric acid, DL-4-amino-3-hydroxybutyric acid, DL-2,3-diaminopropionic acid, DL-2,4-diaminobutyric acid, or DL-ornithine (B) have been investigated by pH titrimetry in aqueous perchlorate media at 37 °C and l= 0.15 mol dm^–3(Na[ClO₄]). Ternary complex species of the type [CuAB], [CuABH], and [CuABH₂] were detected. In every system studied, greater stability than would be expected statistically is observed. L-Histidine (A) binds in a tridentate manner in all the systems. Among the potentially tridentate secondary ligands (B) used, DL-2,3-diaminopropionic acid, DL-2,4-diaminobutyric acid, and DL-ornithine bind in a tridentate manner, while DL-4-amino-3-hydroxybutyric acid appears to be bidentate. The probable sites of protonation in the [CuABH] and [CuABH₂] complexes are discussed in terms of their stability-constant data.