Interaction and reactivity of synthetic aminoisoflavones with metal-free and metalassociated amyloid-b

Disturbed metal ion homeostasis in conjunction with amyloid-β (Aβ) aggregation in the brain has been implicated in Alzheimer’s disease (AD) pathogenesis. To uncover the interplay between metal ions and Aβ peptides, multifunctional small molecules have been employed to monitor Aβ aggregation and toxicity in vitro. Naturally occurring flavonoids have emerged as a valuable class of compounds for this purpose due to their ability to modulate both metal-free and metal-induced Aβ aggregation leading to non-toxic amorphous peptide products; however, the structural moieties of flavonoids responsible for such reactivity have not been fully identified. Herein, we designed, synthesized, and characterized a set of isoflavone derivatives, aminoisoflavones (1-4), that interact with metal ions and Aβ species, modulate aggregation of Aβ and metal–Aβ in vitro, and regulate toxicity induced by metal-free Aβ/metal–Aβ. Furthermore, NMR studies revealed a specific binding site for aminoisoflavones between the N-terminal loop and central helix on prefibrillar Aβ different from the non-specific binding observed for other flavonoids. The absence or presence of the catechol group differentiated the binding affinities and enthalpy/entropy balance between aminoisoflavones and Aβ. Furthermore, having a catechol group influenced the binding mode with fibrillar Aβ and was essential to mitigate toxicity from both Aβ and metal–Aβ compared to that of the compound lacking this group. Inclusion of additional substituents moderately tuned the impact of aminoisoflavones on Aβ aggregation and toxicity. Overall, through these studies we obtained valuable insights on the requirements for parity among metal chelation, intermolecular interactions, and substituent variation for Aβ interactions using the aminoisoflavones, which offer a basic fundamental understanding of a structure-interaction-reactivity relationship within the flavonoid family for metal-free Aβ and metal–Aβ species.