Copper(II) complexes with peptide fragments encompassing the sequence 122–130 of human doppel protein

Copper(II) complexes of the peptide fragment (Dpl122–130) encompassing the sequence 122–130 of human doppel protein were characterized by potentiometric, UV–Visible, CD and EPR spectroscopic methods. An analogous peptide, in which the aspartate residue was substituted by an asparagine amino acid, was synthesized in order to provide evidence on the possible role of carboxylate group in copper(II) coordination. It was found that the carboxylic group is directly involved in copper(II) coordination at acidic pH, forming the CuLH2 species with Dpl122–130. This copper(II) complex displayed EPR parameters very similar to those of the analogous complex with the whole doppel protein. At pH higher than 7, the complexes showed magnetic parameters similar to those of the major species of protein formed in the pH range 7–8, with the metal coordination environment consisting of one imidazole and three amide nitrogen atoms. The comparison of Cu-Dpl122–130 binding constant values with those of the prion peptide fragments (PrP106–114), showed that doppel peptide had a higher metal binding affinity at acidic pH whereas the prion peptide fragment binds the metal tightly at physiological pH.