Complex Formation Equilibria and Molecular Structure of Divalent Metal Ions–Vitamin B3–Glycine Oligopeptides Systems

Complex formation of divalent transition metal ions (copper(II), cobalt(II) and nickel(II)), vitamin B3 (nicotinic acid) and glycine oligopeptides (glycine, glycylglycine, glycyl-l-phenylalanine, and glycylglycylglycine) were studied at 298 K in aqueous solutions using the pH-potentiometric technique. The copper Cu(II), cobalt Co(II), and nickel Ni(II) complexing capacity of vitamin B3 in the absence and in the presence of glycine peptides and their overall stability constants in aqueous solutions were obtained and explained by the HYPERQUAD 2008 program using the potentiometric data. From the protonation and complex formation constants, representative complex species distribution diagrams were obtained using HYSS 2009 software. The UV–Vis spectroscopic, cyclic voltammeteric and conductometric titration measurements were carried out to give qualitative information about the conformation of the complexes formed in these solutions and their stoichiometric ratios. The Gibbs energies and the molecular structures of the complexes were evaluated and predicted using Gaussian 09 software molecular modeling and density functional theory calculations.