The acidity and stability constants of M(Gly)¹, M(Ttr)¹, and 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(L), (L: Gly, Ttr, and 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(Ha⁴)²⁺ + Cu(L) Cu(Har)(L) + 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 stability constants of the 1:1 complexes formed between Cu²⁺, Cu(Bpy)²⁺ or Cu(Phen)²⁺ and L^2-, were determined by potentiometric pH titration in aqueous solution (I = 0.1 M, NaNO₃, 25?C). The order of the stability constants was reported. A comparative investigation between ternary complexes of Trp, Ttr, and Gly is made. The comparison of stability constants of these ternary complexes show that Cu(Har)(Trp) is found near 100% in closed form but Cu(Har)(Gly) exists in open form. The differences between the above mentioned stability constants are based on stacked form of Cu(Har)(Trp). The last provides increased stability.