Effect of copper(II) ternary complex formation on the co-ordination behaviour of dipeptides in aqueous solution

Abstract

By evaluating the multiple equilibria involved in the copper(II)–glycylglycine (GlyGly) or glycinamide (Glam)(A)–DL-2-aminobutyric acid (2-aba), DL-3-aminobutyric acid (3-aba), DL-2,3-diaminopropionic acid (dapa), DL-2,4-diaminobutyric acid (daba), and DL-ornithine (Orn)(B), and also copper(II)–GlyGly (A)–Glam (B) ternary systems under biologically important conditions by potentiometry using advanced computer techniques, the tendency to complexation of dipeptides in the ternary complex species of the type CuABH, CuAB, CuABH_–1, and CuABH_–2 is discussed. It appears that the extra proton in the CuABH complexes in copper(II)–GlyGly or Glam (A)–dapa, daba, and Orn (B) systems resides with the ligand B. In the CuABH and CuAB types of complexes, co-ordination of the dipeptides (A) gives a five-membered chelate ring involving a terminal amino-moiety and an oxygen of the neighbouring amide group. The present investigation shows that the amide-deprotonated dipeptides (AH_–1) are bidentate in the CuABH_–1 and CuABH_–2 ternary complexes when the ligand B is bidentate or tridentate.