Mixed-ligand complex formation by Cu(II) with imidazole and dipeptides

Abstract

The species distribution and stability constants were evaluated for six mixed imidazole (A)-dipeptide (B) complexes of Cu(II) in aqueous perchlorate medium at I = 0.15 M (NaClO₄) and 37 °C from pH titration data. The dipeptides used are glycylglycine, glycyl-L-alanine, L-alanylglycine, glycyl-L-leucine, L-leucylglycine and glycinamide. In addition to the binary complex species CuA, CuA₂, CuA₃, CuA₄, CuB, CuBH_-1, CuB₂H_-1, three mixed complexes CuA₂B, CuA₂BH_-1 and CuABH_-1 were detected in each system. However in the glycinamide secondary ligand (B) system, the mixed species CuAB formed instead of CuABH_-1. The stability order found for these complexes is discussed in terms of the steric effects due to side chain substituents in the dipeptide (B) ligands. The results suggest that mixed-coordination enhances the stability of the complexes. The amide deprotonated dipeptides (B) appear to be tridentate in the CuA₂BH_-1 and CuABH_-1 complexes.