Mixed-ligand complex formation by copper(II) with imidazole derivatives and dipeptides in aqueous solution

Abstract

The equilibria involved in 12 new mixed-ligand systems, Cu^II-A-B (A=histamine or L-histidine; B=glycylglycine, glycyl-L-alanine, L-alanylglycine, glycyl-L-leucine, L-leucylglycine, or glycinamide), have been investigated in an aqueous perchlorate medium by pH titrimetry at 37°C and I=0.15 mol dm⁻³(Na[ClO₄]). In addition to the species HA, H₂A, CuAH, CuA, CuA₂H, CuA₂, HB, H₂B, CuB, CuBH₋₁, and CuB₂H₋₁(also H₃A and CuA₂H₂ for A=L-histidine), three complexes of stoicheimoetry CuABH, CuAB, and CuABH₋₁ have been detected in these systems. The site of protonation in the CuABH species is the primary amino-group of the histamine or L-histidine (A) ligand. The stability-constant data obtained indicate the stability-enhancing effect of the ligands (A) on copper(II) mixed-ligand complex formation with dipeptides (B). The influence of the alkyl substituents in the ligands (B) on the stabilities of CuAB species follows the trends observed previously for CuB dipeptide binary complexes. In the CuABH₋₁ species, the amide-deprotonated dipeptides are bidentate.