Distinct reactivities of 7 towards metal-free Ab and metal-Ab were supported by various biophysical approaches. As displayed in Fig. , the chemical transformation of 7 triggered by metal-free Ab40 or metal-Ab40 was detected by electronic absorption spectroscopy. In the absence of Ab, an increase or decrease in the absorption of the peaks at ca. 265, 300, 340, and 410 nm was monitored for 24 h, which possibly resulted from the oxidation of 7. These spectral changes were accelerated upon incubation with Ab, indicative of its impact on the oxidative modification of 7. Such transformation of 7 was also indicated in the presence of Cu(II) or Zn(II) with and without Ab. To probe the interaction of 7 with Ab, their binding properties were analyzed by isothermal titration calorimetry (ITC). For the association between 7 and Ab, we recorded the heat change upon constant titration of the compound into the solution of Ab, where the dilution heat was also measured to avoid the heat change induced by its addition into the buffered solution. The complex of 7 and Ab42 or Ab40 was formed spontaneously and exothermically [for Ab42, DG = -6.1 (± 0.1) kcal/mol; DH = -0.5 (± 0.1) kcal/mol; TDS = 5.6 (± 0.1) kcal/mol; for Ab40, DG = -6.0 (± 0.5) kcal/mol; DH = -0.6 (± 0.5) kcal/mol; TDS = 5.4 (± 0.7) kcal/mol], which was driven by both negative enthalpy and positive entropy changes, as summarized in Figs. and. Binding affinities of 7 towards Ab42 and Ab40 were determined to be ca. 19.5 (± 0.7) and 20.5 (± 16.0) µM, respectively. Thus, 7