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.

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).

The formation of mixed copper(II) and zinc(II) complexes with Aβ(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-Aβ 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 Aβ binding and inhibits copper-mediated Aβ redox chemistry.

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