Solvent exposure of Tyr10 as a probe of structural differences between monomeric and aggregated forms of the amyloid-β peptide

Aggregation of amyloid-β (Aβ) peptides is a characteristic pathological feature of Alzheimer's disease. We have exploited the relationship between solvent exposure and intrinsic fluorescence of a single tyrosine residue, Tyr(10), in the Aβ sequence to probe structural features of the monomeric, oligomeric and fibrillar forms of the 42-residue Aβ(1-42). By monitoring the quenching of Tyr(10) fluorescence upon addition of water-soluble acrylamide, we show that in Aβ(1-42) oligomers this residue is solvent-exposed to a similar extent to that found in the unfolded monomer. By contrast, Tyr(10) is significantly shielded from acrylamide quenching in Aβ(1-42) fibrils, consistent with its proximity to the fibrillar cross-β core. Furthermore, circular dichroism measurements reveal that Aβ(1-42) oligomers have a considerably lower β-sheet content than the Aβ(1-42) fibrils, indicative of a less ordered molecular arrangement in the former. Taken together these findings suggest significant differences in the structural assembly of oligomers and fibrils that are consistent with differences in their biological effects.