The cellular form of prion protein is a precursor of the infectious isoform, which causes fatal neurodegenerative diseases through intermolecular association. One of the characteristics of the prion protein is a high affinity for Cu(II) ions. The site of Cu(II) binding is considered to be the N-terminal region, where the octapeptide sequence PHGGGWGQ repeats 4 times in tandem. We have examined the Cu(II) binding mode of the octapeptide motif and its pH dependence by Raman and absorption spectroscopy. At neutral and basic pH, the single octapeptide PHGGGWGQ forms a 1:1 complex with Cu(II) by coordinating via the imidazole N pi atom of histidine together with two deprotonated main-chain amide nitrogens in the triglycine segment. A similar 1:1 complex is formed by each octapeptide unit in (PHGGGWGQ)2 and (PHGGGWGQ)4. Under weakly acidic conditions (pH approximately 6), however, the Cu(II)-amide- linkages are broken and the metal binding site of histidine switches from N pi to N tau to share a Cu(II) ion between two histidine residues of different peptide chains. The drastic change of the Cu(II) binding mode on going from neutral to weakly acidic conditions suggests that the micro-environmental pH in the brain cell regulates the Cu(II) affinity of the prion protein, which is supposed to undergo pH changes in the pathway from the cell surface to endosomes. The intermolecular His(N tau)-Cu(II)-His(N tau) bridge may be related to the aggregation of prion protein in the pathogenic form.