Glycine and L-Tryptophan, a Comparative Investigation on Interactions in Cu(II) Binary and Ternary Complexes in Aqueous Solution

The acidity and stability constants of M(Trp) M: Cu, Cu(Bpy)2+, and Cu(Phen)2+ complexes, were determined by potentiometric pH titration. It is shown that the stability of the binary Cu(Trp) complex is determined by the basicity of the carboxylate group on one side and amino group on the other side. It is demonstrated that the equilibrium, Cu(Har)2+ + Cu(Trp) Cu(Har)(Trp) + Cu, is displacement due to the well known experience that mixed ligand complexes formed by a divalent 3d ion, a heteroaromatic N base and an O donor ligand possess increased stability. The other part of this displacement, which amount on average to an increased stability of the mixed ligand Cu(Bpy)(Trp) and Cu(Phen)(Trp) complexes of about 0.97 or 1.31 log unit. The stability constants of the 1:1 complexes formed between Cu, Cu(Bpy) or Cu(Phen) and Trp, were determined by potentiometric pH titration in aqueous solution (I = 0.1 M, NaNO3, 25°C). The order of the stability constants was reported. The results show following order for Trp, Cu(Trp) < Cu(Bpy)(Trp) < Cu(Phen)(Trp), and Gly, Cu(Gly) > Cu(Bpy)(Gly) ≤ Cu(Phen)(Gly). A comparative investigation between ternary complexes of Trp and Gly is made. The comparison of stability constants of these ternary complexes show that Cu(Har)(Gly) exist in open form but Cu(Har)(Trp) is found near 100% in closed form. The differences between the above mentioned stability constants based on stacked form of Cu(Har)(Trp). The stacked form provides for increased stability.