Resolution of localized small molecule-Aβ interactions by deep-ultraviolet resonance Raman spectroscopy

The mechanism by which flavonoids prevent formation of amyloid-β (Aβ) fibrils, as well as how they associate with non-fibrillar Aβ is still unclear. Fresh, un-oxidized myricetin exhibited excitation and emission fluorescence maxima at 481 and 531 nm, respectively. Introduction of either Aβ(1-42) or Aβ(25-40) resulted in a fluorescence decrease, when measured at 481 nm, suggesting formation of a myricetin-Aβ complex. Circular dichroism (CD) and ultraviolet resonance Raman (UVRR) studies indicate that the association of myricetin with the Aβ peptide or its hydrophobic fragment, Aβ(25-40), leads to subtle changes in each peptide's conformation. Aβ(25-40) formed amyloid fibrils at a similar rate, when compared to the full-length peptide, Aβ(1-42), using thioflavin T (ThT) fluorescence. Studies also indicated that myricetin was equally effective at preventing the formation of both Aβ(1-42) and Aβ(25-40) fibrils. Although ThT assays indicated that Aβ(1-16) did not form amyloid fibrils, CD studies of the hydrophilic fragment, Aβ(1-16), suggest possible interactions between myricetin and aromatic side chains. UVRR studies of the full-length peptide and Aβ(1-16) showed increases in the intensity of the aromatic modes upon introduction of myricetin. Our findings suggest that myricetin interacts with soluble Aβ via two mechanisms, association with the hydrophobic C-terminal region and interactions with the aromatic side chains.