EPR and potentiometric reinvestigation of copper(II) complexation with simple oligopeptides and related compounds

The coordination modes of Cu(II) to Di-, Tri-, and Tetra-glycine and related ligands were investigated, within the entire measurable pH range and over a wide range of ligand excess, by means of electron paramagnetic resonance, electron spectroscopy, and in some cases, pH potentiometry. The results, besides allowing the identification of the complex species involved and the attribution of distinctive spectral data set to the various structures, provide significant insight in relation to: i) the coordination ability of the zwitterionic AH form of simple oligopeptides to yield COO-- or 2COO--complexes or species with mixed [(NH2, CO)(COO-)] or [(NH2, N-, COO-)(COO-)] coordination; ii) the influence of the size of the (NH2, CO), (NH2, N-), and (N-, COO-) chelated rings on the stability and structure of the complex species; iii) the bis-complex formation processes taking place with Gly-β-ala, β-Alagly, Tri-, and Tetraglycine