Equilibrium studies on the metal-peptide system II polarographic and spectral behavior of Ni(II) complexes with di- and tri-glycine

Abstract

Nature of bonding and electrode reduction behavior of Ni(II) di- and tri-glycine complexes have been studied polarographically and spectrophotometrically. From suitable analysis of the polarograms, the complex species (formed at different pH) undergoing reduction over the electrode surface has been ascertained. At relatively lower pH, diglycine (GG) binds Ni(II) through N(amino) and N(protonated peptide), while, at high pH ~12.2, GG forms 1:2 complex in which coordination occurs through N(amino), N(deprotonated peptide) and carboxylate groupings. Triglycine (GGG) at pH ~ 7.5, forms 1:2 octahedral complex in which N(amino) and two N(protonated peptide) are involved in bond formation, however, at higher pH ~10, 1:1 square planar species is formed where, in addition to amino and two (deprotonated peptide), carboxylate group also interacts with the metal ion. From the irreversible reduction waves, rate constants for electrode reduction of the different complex species have been evaluated and probable reduction mechanism has been discussed.