Iron coordination compounds with glycine, glycylglycine and diglycylglycine

Abstract

The investigation of biometals complex-forming processes with aminoacids and peptides is of interest as the model of metal–protein interactions. In this work, the composition of coordination compounds at T = 308 K and pH interval from 0 to 11 units was established on the basis of the obtained partial dependences of oxidizing potential value ( φ) on iron(III), iron(II), glycine (Gly), glycylglycine (Gly) 2, and diglycylglycine (Gly) 3 concentrations. The fall of the oxidizing potential value with pH of the solution (Fig. 1) in pH sphere < pK 1 is accounted for by the stepped iron complex-forming processes in aminoacid and peptide aqueous solutions. From the oxidizing potential dependence on (C L)-glycine and glycylglycine and diglycylglycine ligand concentrations obtained at different pH values, it has been found that in pH sphere = 2.0 + 5.0 iron(III) complex compounds are formed where only one acidoligand is formed for the complex-forming ion and two acidoligands are formed as the aminoacid and peptide concentrations increase. These results confirm the competition for complex-formation between Fe(III) and Fe(II) in this pH sphere. The idea that protolytic processes proceed in aminoacid ▪ and peptide aqueous solutions made it possible to establish the coordination form of the ligands in the complexes. Graphical analysis of the experimental oxidizing potential dependences on the concentration of redox iron forms obtained at different fixed pH values made it possible to establish the formation of polynuclear acidohydroxyl iron(III) complex compounds and iron mononuclear complexes in neutral and alkaline solutions. The formation of heterovalent compounds may be suggested as well. The catalytic activity of iron coordination compounds with glycine, glycylglycine and diglycylglycine in the process of L-cysteine (Cys) liquid-phase oxidation by molecular oxygen in aqueous solutions at pH = 7.0 (Fig. 2) has been determined. ▪ The increase in the iron coordination compounds' catalytic activity by the ligand's nature change in the series: aminoacid < dipeptide < tripeptide, may be accounted for by the increase in the strength of mixed iron(III) polynuclear complexes and mononuclear iron(II) complexes in the above-cited sequence.