AQUEOUS EQUILIBRIA OF COPPER(II)- AND NICKEL(II)-POLYGLYCINE COMPLEXES

Abstract

Magnetic susceptibility measurements, new potentiometric data, optical spectra, and a new statistical method of calculation are combined in the formulation of an equilibrium scheme defining the dilute solution interactions of nickel(II) and copper(II) ions with diglycine, triglycine, and tetraglycine as a function of pH. At low pH appreciable concentrations of a previously unreported complex, MHL²⁺ (HL =polyglycine ligands) are shown to be present in all nickel(II)-polyglycine systems and in the copper(II)-triglycine system. This new protonated species is assigned a structure in which the metal ion is coordinated to the terminal carboxylate and to the adjacent peptide carbonyl oxygen with the proton residing on the terminal amino group. As the pH is raised in the 1:1 systems, MH₋₁L, MH₋₂L⁻ and MH_{_3}L^2- are formed in succession depending on the number of peptide linkages in the ligands, HL. The concentration of the monodeprotonated intermediate species NiH₋₁L never exceeds 10% of the total metal ion concentration in the triglycine case and is always less than 0.5% when tetraglycine is the ligand. The dideprotonated intermediate NiH₋₂L- reaches a maximum of 38% of the total metal concentration in the 1:1 Ni-tetraglycine system. An explanation is presented for this negative deviation from the predictions based on statistical grounds. Complete species distribution diagrams which include the new protonated complexes are presented and are employed to explain the differences in the interactions of copper(II) and nickel(II) ions with polyglycine ligands. Probable coordinate bonding sites suggested for the complexes formed in solution are inferred on the basis of stoichiometry, relative stabilities, and available microscopic information.