Equilibrium study of complex formation among trivalent metals, glycine peptides and phenolates in aqueous solution

The stability of binary and mixed-ligand complexes among trivalent transition metal ions (chromium and iron), glycine peptides (glycylglycine and glycylglycylglycine) and phenolates (ferulic acid and gallic acid) were studied by using pH-potentiometric titration in aqueous solution at 298.15 K and ionic strength of 0.15 mol�dm-3 NaNO3. The complexation model for each system was obtained by processing the potentiometric titration data using the HYPERQUAD2008 program. The stability constant trend of complexes in both systems and the contributions of deprotonated or protonated amide peptides to the stability of the complexes is discussed. The stability of the mixed-ligand complexes relative to their corresponding binary complexes was also investigated by calculating the Dlog10 K parameter of each system. In addition, the Gibbs energies of reaction (DrG) obtained from the Gaussian modeling program with B3LYP/6-31?G(d) basis set were used to verify the contributing binding sites of the ligands and to predict the structures of the M–L complexe