A survey of diamagnetic probes for copper²⁺binding to the prion protein. ¹H NMR solution structure of the palladium²⁺bound single octarepeat

The prion protein (PrP^C) is a copper binding cell surface glycoprotein which when misfolded causes transmissible spongiform encephalopathies. The cooperative binding of Cu²⁺to an unstructured octarepeat sequence within PrP^Ccauses profound folding of this region. The use of NMR to determine the solution structure of the octarepeat region of PrP with Cu²⁺bound has been hampered by the paramagnetic nature of the Cu²⁺ions. Using NMR we have investigated the binding of candidate diamagnetic replacement ions, to the octarepeat region of PrP. We show that Pd²⁺forms diamagnetic complexes with the peptides HGGG, HGGGW and QPHGGGWGQ with 1 : 1 stoichiometry. The ¹H NMR spectra indicate that these peptides are in slow-exchange between free and bound Pd²⁺on the chemical-shift time-scale. We demonstrate that the Pd–peptide complex forms slowly with a time taken to reach half-maximal signal of 3 hours. Other candidate metal ions, Ni²⁺, Pt²⁺and Au³⁺, were investigated but only the Pd²⁺complexes gave resolvable ¹H NMR spectra. We have determined the solution structure of the QPHGGGWGQ–Pd 1 : 1 complex using 71 NOE distance restraints. A backbone RMSD of 0.30 Å was observed over residues 3 to 7 in the final ensemble. The co-ordinating ligands consist of the histidine imidazole side chain Nɛ, the amide N of the second and third glycines with possibly H₂O as the fourth ligand. The co-ordination geometry differs markedly from that of the HGGGW–Cu crystal structure. This survey of potential replacement metal ions to Cu²⁺provides insight into the metal specificity and co-ordination chemistry of the metal bound octarepeats.