Thermodynamic and spectroscopic study of copper( II )–glycyl- L -histidylglycine complexes in aqueous solution

The complexes formed in aqueous solution by copper(II) with glycyl-L-histidylglycine (L–) have been extensively studied, by using potentiometry, calorimetry, visible spectrophotometry and circular dichroism techniques, with the aim of elucidating their type and probable structure. Only monomeric species are formed in the acidic range of pH ([CuL]+, [CuLH–1], [CuL2] and [CuL2H–1]–), while at higher pH (>8) polynuclear [Cu4L4H–6]4– is formed in addition to [CuLH–2]–. The interpretation of all the experimental data is strongly consistent with a structure involving three nitrogen atoms co-ordinated to copper(II) for [CuLH–1] and [CuL]+ and with the participation of a fourth nitrogen atom in [CuLH–2]– and [Cu4L4H–8]4–. In the tetrameric species very probably there is a dissociation of the N(1)-pyrrole hydrogen ion, with consequent formation of imidazolate bridges.