Copper(II) complexes with chicken prion repeats: influence of proline and tyrosine residues on the coordination features

The prion protein (PrPc) is a copper-binding glycoprotein that can misfold into a beta-sheet-rich and pathogenic isoform (PrPSc) leading to prion diseases. The first non-mammalian PrPc was identified in chicken and it was found to keep many structural motifs present in mammalian PrPc, despite the low sequence identity (approximately 40%) between the two primary structures. The present paper describes the synthesis and the coordination properties of some hexapeptide fragments (namely, PHNPGY, HNPGYP and NPGYPH) as well as a bishexapeptide (PHNPGYPHNPGY), which encompasses two hexarepeats. The copper(II) complexes were characterized by means of potentiometric, UV-vis, circular dichroism and electron paramagnetic resonance techniques. We also report the synthesis of three hexapeptides (PHNPGF, HNPGFP and NPGFPH), in which one tyrosine was replaced by phenylalanine as well as two bishexapeptides in which either one (PHNPGFPHNPGY and PHNPGYPHNPGF), or two tyrosines were replaced by phenylalanine, in order to check whether tyrosine was involved in copper(II) binding. Overall, the results indicate that the major copper(II) species formed by the chicken PrP dodecapeptides are stabler than the analogous species reported for the peptide fragments containing two octarepeat peptides from the mammalian prion protein. It is concluded that the presence of four prolyl residues, that are break points in copper coordination, induces the metal-assisted formation of macrochelates as well as the formation of binuclear species. Furthermore, it has been shown that the phenolic group is directly involved in the formation of copper binuclear species.