Copper (II) binding properties of an octapeptide fragment from the R3 region of tau protein: A combined potentiometric, spectroscopic and mass spectrometric study

The copper(II) complexes of a peptide fragment of the R3 domain of tau protein (tau(326-333) Ac-GNIHHKPG-NH2) and its mutants (Ac-GNGHHKPG-NH2, Ac-GNIHHKAG-NH2, Ac-GNGAHKPG-NH2 and Ac-GNGHAKPG-NH2) have been studied by potentiometric and spectroscopic (UV-Vis, CD) methods. ESR spectroscopy and mass spectrometry were also used to prove the coordination mode of the mononuclear complexes and the formation of dinuclear species, respectively. It has been demonstrated that the (326-333) fragment of tau protein is a versatile and effective ligand for copper(II) coordination. The versatility of copper(II) binding is related to the presence of two adjacent histidyl residues in the sequence, which results in the coexistence of mononuclear, bis(ligand) and dinuclear complexes at different metal to ligand ratios. The 1:1 mononuclear complexes are, however, the dominant species with all peptides and the imidazole-N and one to three deprotonated amide nitrogen atoms towards the N-terminal side of the histidyl residue have been suggested as metal binding sites. This binding mode allows the formation of coordination isomers because any of the two histidine moieties can be the primary anchoring site. It is evident from the CD spectroscopic measurements that the isomers are present in almost equal concentration. The copper(II) binding affinity of the native fragment of tau protein is comparable to that of a similar 2-histidine fragment of amyloid-β mutant, Ac-SGAEGHHQK-NH2 but the comparison with an independent histidyl residue (H32) from the N-terminal region of the protein reveals the predominance of H32 over the histidines in the R3 domain.