Stability of binary and ternary copper (II) complexes of 2((4-methyl-5-nitro-6-(pyrrolidine-1-yl)pyrimidine-2- yl)amino) propionic acid in aqueoussolution

The acidity and stability constants of M (MNPPAP) (MNPPAP: 2((4-methyl- 5-nitro-6-(pyrrolidine-1-yl)pyrimidine-2-yl)amino)propionic acid) M: Cu2+, Cu (Bpy)2+ (Bpy: 2,20-Bipyridine), or Cu(Phen)2+ (Phen: 1,10-Phenanthroline) complexes were determined by potentiometric pH titration. It is shown that the stability of the binary Cu (MNPPAP) complex is determined by the basicity of the carboxylate group on one side and amine group on the other side. It is demonstrated that the equilibrium, Cu(Har)2+ (Har: Heteroaromatic ligand such as Bpy or Phen) + Cu(MNPPAP)  Cu(Har)(MNPPAP) + 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 possessing increased stability. The other part of this displacement amounts, on average, to no increased stability of the mixed ligand Cu (Bpy)(MNPPAP) and Cu(Phen)(MNPPAP) complexes. The stability constants were determined by potentiometric pH titration in aqueous solution. The order of the stability constants was reported. A comparative investigation between ternary complexes of MNPPAP, Trp (Trp: L-Tryptophan) and Gly (Gly: Glycine) is made. The comparison of stability constants of these ternary complexes show that Cu (Har) (MNPPAP) and Cu (Har) (Gly) exist in open form, but Cu (Har)(Trp) is found near 100% in closed form. The dierences between the above mentioned stability constants are based on stacked form of Cu (Har) (Trp), and provide increased stability compared with Cu (Har)(MNPPAP).