The beta bind: Copper(I) binds to amyloid β-peptide fragments (see structure) as a stable bis(histidine), two-coordinate, near-linear complex, even in the presence of potential additional ligands. As has been proposed or assumed in other studies, the copper(I)–peptide complexes react with dioxygen to form the reactive oxygen species H2O2, without the need for a third histidine ligand to promote the chemistry. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.