To identify the modifications of Ab in both the absence and presence of Cu(II) upon treatment of 7, we carried out the studies employing electrospray ionization mass spectrometry (ESI-MS). In the sample of metal-free Ab42 with 7, the addition of +16 Da to the +3 charged-Ab42 monomer, assigned to be the oxidized monomeric Ab42, was detected (Fig. ). To determine the oxidized amino acid residues in Ab42, the b fragments generated by collision-induced dissociation were analyzed by tandem MS (ESI-MS 2 ). As illustrated in Figs. 7b and S12a, the Met35 residue was measured to be the oxidation site in the singly oxidized Ab42. The previous studies suggested that the His, Tyr, and Met residues in Ab are susceptible to be oxidized and such oxidation could impact the aggregation profile of Ab. In particular, the oxidation of the Met residue to Met sulfoxide or sulfone can increase its polarity leading to the destabilization of b-strand structures and the decrease in hydrophobic contacts onto the C-terminal region that are associated with amyloid aggregation. Similar to metal-free Ab42, the oxidation of Ab40 at the Met35 residue was monitored by incubation with 7 (Figs. and and). The oxidative modifications of Cu(II)-Ab by addition of 7 were detected to be different from that of metal-free Ab. As shown in Figs. and, the intensity of the peaks assigned to Cu(II)-Ab42 and Cu(II)-Ab40 was suppressed. When collisional energy was selectively applied to the singly oxidized Ab produced by reacting Cu(II)-Ab with 7, the analysis of b fragments indicated the His13, His14, and Met35 residues as plausible oxidation sites, as illustrated in Figs. 7d, S12b, S13d, and S14b. In the case of His residues, their imidazole ring can involve in several intermolecular interactions, such as hydrogen bonding and p-p stacking, as the hydrogen-bond donor/acceptor and an aromatic p-motif. In addition, depending on pH, both e-and d-nitrogen protonated His residues can mediate in the cation-p interactions with aromatic amino acid residues as well as the formation of an intramolecular salt bridge between the carboxylate group of Glu22 and the protonated His13 or His14. Furthermore, His residues (i.e., His6, His13, and His14) are included in metal coordination; 5,68 thus, the transformation of His to 2-oxo-His could disrupt intermolecular and intramolecular interactions within Ab and its metal binding. It should be noted that Cu(II) chelation by 7 can impact metal-binding properties of Ab. Therefore, oxidative modifications onto His13, His14, and Met35 by 7 could modify the aggregation of Cu(II)-Ab. Together, our overall biophysical results and observations demonstrate the direct interactions of 7 with metal-free and metal-treated Ab species, with the consequent modulatory effects on their aggregation.