Copper complex species within a fragment of the N-terminal repeat region in opossum PrP protein

Abstract

A spectroscopic (UV-Vis, CD and EPR), thermodynamic and voltammetric study of the copper(II) complexes with the Ac-PHPGGSNWGQ-NH₂ polypeptide (L), a fragment of the opossum PrP protein N-terminal four-repeat region, was carried out in aqueous solution. It suggests the formation of a highly distorted [Cu(L)H₋₂] complex species in the neutral region, the stereochemistry of which is ascribable to a square base pyramid and a CuN₃O₂ chromophore, resulting from the coordination of a histidine imidazole and two peptide nitrogen atoms and probably oxygen atoms from water molecules. At basic pH values a [Cu(L)H₋₃]⁻ species with a pseudo-octahedral geometry was also obtained, with four nitrogen donor atoms in its equatorial plane, coming from the histidine residue and from peptidic nitrogen atoms. Interestingly, at pH values relatively higher than the neutrality, the coordination sphere of the copper complex in the [Cu(L)H₋₂] species changes its stereochemistry towards a pseudo-octahedron, as suggested by the change in the parallel copper hyperfine coupling constant of the EPR spectra at low temperature. A slight difference in the redox potentials between this two-faced [Cu(L)H₋₂] complex species seems to confirm this behaviour. Both potentiometric and spectroscopic data were compared with the analogous species obtained with the Ac-PHGGGWGQ-NH₂ peptide, belonging to the octarepeat domain of the human prion protein (hPrP) N-terminal region. The [Cu(L)H₋₂] species formed by the Ac-PHPGGSNWGQ-NH₂ decapeptide, having a slightly lower stability, turned out to be less abundant and to exist within a narrow pH range.