Solution chemical properties and catecholase-like activity of the copper(ii)–Ac-His-His-Gly-His-OH system, a relevant functional model for copper containing oxidases

Abstract

The solution chemical properties, superoxide dismutase and catecholase activity of the copper(II)–Ac-His-His-Gly-His-OH (hhgh) complexes were studied to identify functional and structural models of copper-containing oxidases. The solution speciation was determined in the pH range 3–11 by two independent methods (potentiometry and pH-dependent EPR measurements). The results obtained by the two methods agree very well with each other and show the formation of differently protonated CuH_xL complexes (where x = 2 ,1, 0, −1, −2, −3) in aqueous solution. The spectroscopic (UV–Vis, CD, EPR) data indicate that the coordination of the imidazole rings is a determinant factor in all these complexes. Amide coordinated complexes are dominant only above pH 8. This offers excellent possibilities for structural/functional modelling of copper(II) containing metalloenzymes. Indeed, the {3N_im} coordinated CuL species (pH = 6–7) has efficient superoxide dismutase-like activity. The {3N_im,OH⁻} coordinated CuH₋₁L possesses outstanding activity to catalyze the oxidation of 3,5-di-tert-butylcatechol (H₂dtbc) by dioxygen in 86 wt% methanol–water, providing the first example that copper(II)–peptide complexes are able to mimic copper containing oxidases.