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

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 CuHxL 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 (H(2)dtbc) by dioxygen in 86 wt% methanol - water, providing the first example that copper( II) - peptide complexes are able to mimic copper containing oxidases.
Keyword: HISTIDINE,PEPTIDES,TERNARY COMPLEXES,BINDING-SITES,AQUEOUS-SOLUTION,NICKEL(II),PRION PROTEIN,EQUILIBRIUM,CRYSTAL-STRUCTURE,METAL-ION