Zn2+'s ability to alter the distribution of Cu2+ among the available binding sites of Aß(1-16)-polyethylenglycol-ylated peptide: implications in Alzheimer's disease.

The formation of mixed copper(II) and zinc(II) complexes with Aß(1-16)-PEG has been investigated. The peptide fragment forms stable mixed metal complexes at physiological pH in which the His13/His14 dyad is the zinc(II)'s preferred binding site, while copper(II) coordination occurs at the N-terminus also involving the His6 imidazole. Copper(II) is prevented by zinc(II) excess from the binding to the two His residues, His13 and His14. As the latter binding mode has been recently invoked to explain the redox activity of the copper-Aß complex, the formation of ternary metal complexes may justify the recently proposed protective role of zinc(II) in Alzheimer's disease. Therefore, the reported results suggest that zinc(II) competes with copper for Aß binding and inhibits copper-mediated Aß redox chemistry.

Nickel(II) and mixed metal complexes of amyloid-beta N-terminus.

Nickel(II) complexes of Abeta(1-16)Y10A and its smaller fragments including Abeta(1-4), Abeta(1-6), Ac-Abeta(1-6) and Ac-Abeta(8-16)Y10A have been studied by potentiometric, UV-Vis and circular dichroism spectroscopic measurements. The formation of mixed metal complexes and the distribution of metal ions among the possible coordination sites in the Cu(II)-Ni(II)-Abeta(1-16)Y10A and Cu(II)-Ni(II)-Zn(ii)-Abeta(1-16)Y10A systems have also been evaluated. It was found that the hexadecapeptide and its fragments are effective nickel(II) binding ligands and complex formation processes of nickel(II) ions are quite similar to those of copper(II). Formation of mono- and di-nuclear complexes was detected in the nickel(II)-Abeta(1-16)Y10A system suggesting the existence of two separated metal binding motifs: the N-terminus and internal histidyl residues. The preference for the coordination at the N-terminus was supported by the spectroscopic measurements but in equilibrium with the metal binding at the internal histidyl sites. Neither zinc(II) nor nickel(II) can, however, substitute copper(II) in the mixed metal complexes of Abeta(1-16)Y10A, but both metal ions are able to alter the distribution of copper(II) ions among the various binding sites. Both N-terminus (amino and His6) and internal histidyl residues (His13 and His14) can work as dinuclear binding motifs, preferably accommodating copper(II) and zinc(II), respectively, while nickel(II) can occupy the remaining free coordination sites.

Metal loading capacity of Abeta N-terminus: a combined potentiometric and spectroscopic study of zinc(II) complexes with Abeta(1-16), its short or mutated peptide fragments and its polyethylene glycol-ylated analogue.

Aggregation of the amyloid beta-peptide (Abeta) into insoluble fibrils is a key pathological event in Alzheimer's Disease (AD). There is now compelling evidence that metal binding to Abeta is involved in AD pathogenesis. The amino acid region 1-16 is widely considered as the metal binding domain of Abeta. In this work, we used a combined potentiometric, NMR, and electrospray ionization mass spectrometry (ESI-MS) approach to study the zinc(II) binding to a new polyethylene glycol (PEG)-conjugated peptide fragment encompassing the 1-16 amino acid sequence of Abeta (Abeta(1-16)PEG). Our results demonstrate for the first time that the Abeta(1-16) is able to coordinate up to three zinc ions, all the histidyl residues acting as independent anchor sites. The study was complemented by systematically investigating the zinc(II) complexes of a series of shorter peptide fragments related to the Abeta(1-16) sequence, namely, Abeta(1-4), Abeta(1-6), AcAbeta(1-6), AcAbeta(8-16)Y10A. The comparison of the whole results allowed the identification of the zinc(II) preferred binding sites within the longer Abeta(1-16) amino acid sequence. Unlike copper(II) that prefers the N-terminal amino group as the main binding site, the zinc(II) is preferentially placed in the 8-16 amino acidic region of Abeta(1-16).

Copper(II) binding to two novel histidine-containing model hexapeptides: evidence for a metal ion driven turn conformation.

The solution conformation and the copper(II) binding properties have comparatively been investigated for the two novel hexapeptides Ac-HPSGHA-NH(2) (P2) and Ac-HGSPHA-NH(2) (P4). The study has been carried out by means of CD, NMR, EPR and UV-Vis spectroscopic techniques in addition to potentiometric measurements to determine the stability constants of the different copper(II) complex species formed in the pH range 3-11. The peptides contain two histidine residues as anchor sites for the metal ion and differ only for the exchanged position of the proline residue with glycine. CD and NMR results for the uncomplexed peptide ligands suggest a predominantly unstructured peptide chain in aqueous solution. Potentiometric and spectroscopic data (UV-Vis, CD and EPR) show that both peptides strongly interact with copper(II) ions by forming complexes with identical stoichiometries but different structures. Furthermore, Far-UV CD experiments indicate that the conformation of the peptides is dramatically affected following copper(II) complexation with the P4 peptide adopting a beta-turn-like conformation.

The metal loading ability of beta-amyloid N-terminus: a combined potentiometric and spectroscopic study of copper(II) complexes with beta-amyloid(1-16), its short or mutated peptide fragments, and its polyethylene glycol (PEG)-ylated analogue.

Alzheimer's disease (AD) is becoming a rapidly growing health problem, as it is one of the main causes of dementia in the elderly. Interestingly, copper(II) (together with zinc and iron) ions are accumulated in amyloid deposits, suggesting that metal binding to Abeta could be involved in AD pathogenesis. In Abeta, the metal binding is believed to occur within the N-terminal region encompassing the amino acid residues 1-16. In this work, potentiometric, spectroscopic (UV-vis, circular dichroism, and electron paramagnetic resonance), and electrospray ionization mass spectrometry (ESI-MS) approaches were used to investigate the copper(II) coordination features of a new polyethylene glycol (PEG)-conjugated Abeta peptide fragment encompassing the 1-16 amino acid residues of the N-terminal region (Abeta(1-16)PEG). The high water solubility of the resulting metal complexes allowed us to obtain a complete complex speciation at different metal-to-ligand ratios ranging from 1:1 to 4:1. Potentiometric and ESI-MS data indicate that Abeta(1-16)PEG is able to bind up to four copper(II) ions. Furthermore, in order to establish the coordination environment at each metal binding site, a series of shorter peptide fragments of Abeta, namely, Abeta(1-4), Abeta(1-6), AcAbeta(1-6), and AcAbeta(8-16)Y10A, were synthesized, each encompassing a potential copper(II) binding site. The complexation properties of these shorter peptides were also comparatively investigated by using the same experimental approach.