Neurotoxic implications of the interactions between Cu(I/II) and amyloid-beta (A beta) indicate a connection between amyloid cascade hypothesis and metal ion hypothesis with respect to the neurodegeneration associated with Alzheimer's disease (AD). Herein, we report a mechanistic strategy for modifying the first coordination sphere of Cu(II) bound to A beta utilizing a rationally designed peptide modifier, L1. Upon reacting with L1, a metal-binding histidine (His) residue, His14, in Cu(II)-A beta was modified through either covalent adduct formation, oxidation, or both. Consequently, the reactivity of L1 with Cu(II)-A beta was able to disrupt binding of Cu(II) to A beta and result in chemically modified A beta with altered aggregation and toxicity profiles. Our molecular-level mechanistic studies revealed that such L1-mediated modifications toward Cu(II)-A beta could stem from the molecule's ability to 1) interact with Cu(II)A beta and 2) foster copper-O-2 chemistry. Collectively, our work demonstrates the development of an effective approach to modify Cu(II)-A beta at a metal-binding amino acid residue and consequently alter A beta's coordination to copper, aggregation, and toxicity, supplemented with an in-depth mechanistic perspective regarding such reactivity.