Alzheimer's disease (AD) is the most common cause of dementia worldwide. AD brains display deposits of insoluble amyloid plaques consisting mainly of aggregated amyloid-beta (A beta) peptides, and A beta oligomers are likely a toxic species in AD pathology. AD patients display altered metal homeostasis, and AD plaques show elevated concentrations of metals such as Cu, Fe, and Zn. Yet, the metal chemistry in AD pathology remains unclear. Ni(II) ions are known to interact with A beta peptides, but the nature and effects of such interactions are unknown. Here, we use numerous biophysical methods-mainly spectroscopy and imaging techniques-to characterize A beta/Ni(II) interactions in vitro, for different A beta variants: A beta(1-40), A beta(1-40)(H6A, H13A, H14A), A beta(4-40), and A beta(1-42). We show for the first time that Ni(II) ions display specific binding to the N-terminal segment of full-length A beta monomers. Equimolar amounts of Ni(II) ions retard A beta aggregation and direct it towards non-structured aggregates. The His6, His13, and His14 residues are implicated as binding ligands, and the Ni(II)center dot A beta binding affinity is in the low mu M range. The redox-active Ni(II) ions induce formation of dityrosine cross-links via redox chemistry, thereby creating covalent A beta dimers. In aqueous buffer Ni(II) ions promote formation of beta sheet structure in A beta monomers, while in a membrane-mimicking environment (SDS micelles) coil-coil helix interactions appear to be induced. For SDS-stabilized A beta oligomers, Ni(II) ions direct the oligomers towards larger sizes and more diverse (heterogeneous) populations. All of these structural rearrangements may be relevant for the A beta aggregation processes that are involved in AD brain pathology.