Published: May 25, 2011 r2011 American Chemical Society 5342 dx.doi.org/10.1021/ic101537m | Inorg. Chem. 2011, 50, 5342–5350 ARTICLE pubs.acs.org/IC Zn 2þ ’s Ability to Alter the Distribution of Cu 2þ among the Available Binding Sites of Aβ(116)-Polyethylenglycol-ylated Peptide: Implications in Alzheimer’s Disease Chiara A. Damante, † Katalin € Osz, ‡ Zolt  an Nagy, § Giuseppe Grasso, † Giuseppe Pappalardo, || Enrico Rizzarelli,* ,†,|| and Imre S ov  ag o* ,§ † Department of Chemical Sciences, University of Catania, V.le A. Doria 6, 95125 Catania, Italy ‡ Department of Physical Chemistry, University of Debrecen, 4010 Debrecen, Hungary § Department of Inorganic and Analytical Chemistry, University of Debrecen, 4010 Debrecen, Hungary ) CNR Institute of Biostructures and Bioimaging, V.le A. Doria 6, 95125 Catania, Italy ’ INTRODUCTION Amyloid plaque formation in the brain plays a central role in the cognitive dysfunction characteristic of Alzheimer’s disease (AD). 1,2 The plaques are composed primarily of 3943 amino acid amyloid peptides (Aβ) that are derived from enzymatic cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. 3 Transition metals, such as Zn, Fe, and Cu, are present at elevated concentrations in AD brain deposits, suggesting that these ions may act as seeding factors, 4,5 while it has been proposed that Aβ could have a protective role if it forms a Cu(I) complex rather than a Cu(II) complex in vivo. 6 The oligomerization of the Aβ peptides is facilitated by the presence of metal ions under physiological conditions. 710 Re- cently, contrasting results have been reported, as Cu(II) and Zn(II) have been indicated both as risk factors 11 and as protective agents in AD. 12,13 Inhibition of Aβ aggregation 1416 and forma- tion of fibrillar or amorphous aggregates 1619 caused by the peptide interaction with different metal ions have been shown to be metal concentration dependent. 14 TEM images showed that the aggregates are mature fibrils at low concentrations of Cu 2þ ions, whereas granular aggregates have been observed at high Cu 2þ ion concentrations. 20 The metal-loading of the Aβ peptide involves the formation of amorphous aggregates, and a second binding site has been invoked to explain the different morphology of copper(II) complexes. 19 It has also been reported that neither soluble nor fibrillar forms of Aβ(140) with Cu 2þ contain binuclear Cu 2þ sites in which two Cu 2þ ions are bridged by an intervening ligand. 21 Moreover, other results highlight the effect of Zn 2þ and Cu 2þ on pre-existing fibrillar aggregates and are highly significant, as they provide clear evidence that the proper concentrations of different metal ions may even decrease the extent of Aβ fibrillation. 17 Particularly, it has been suggested that (i) the misfolding mechanism is dependent on Cu 2þ ion con- centration and (ii) the different metal complex species can lead to profound changes in Aβ self-assembly kinetics, morphology, and toxicity. 22 Similar results have been reported on the interactions of Zn 2þ with Aβ peptides, showing that zinc ions promote Aβ aggregation. 23 The same study also reports that, under certain conditions, the increase of zinc(II) concentration (i) prevents fibril formation and (ii) decreases the aggregation rate. In addition, it has been found that different oligomeric species are selectively stabilized by the changes of Zn 2þ concentrations and that Zn 2þ ions possibly promote interpeptide aggregation modes. 24 This complexity rendered by zinc(II) coordination has been invoked to explain zinc selectivity in precipitating aggrega- tion intermediates. 23 In summary, different morphologies and related toxicities 22 have been attributed to different coordination modes experienced by copper(II) and zinc(II), 25,26 and many studies have been performed to clarify the major metal binding sites of Aβ peptides. 2729 However, the literature has not achieved consensus as to the stoichiometry and binding affinity Received: May 24, 2010 ABSTRACT: 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.