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

Abstract

The acidity and stability constants of M(Trp)1 M: Cu2+, Cu(Bpy2)2+, and Cu(Phen3)2+ complexes, were determined by potentiometric pH titration. It is shown that the stability of the binary Cu(Trp) complexes 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(Har4)2+ + Cu(Trp) Cu(Har)(Trp) + Cu2+, 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 Cu2+, Cu(Bpy)2+ or Cu(Phen)2+ and Trp2−, were determined by potentiometric pH titration in aqueous solution (I = 0.1 M, NaNO3, 25℃). The order of the stability constants was reported.