Probing the Secondary Structure of Individual Aβ₄₀ Amorphous Aggregates and Fibrils by AFM-IR Spectroscopy

Structural characterization of aggregates and fibrils of the Aβ protein is pivotal to the molecular-level elucidation of Alzheimer's disease (AD). AFM-IR spectroscopy provides nanoscale resolution, and thus allows the interrogation of individual aggregates and fibrils. During aggregation of Aβ, we observed mainly disordered Aβ at t=15 min, but substantial structural diversity including the co-existence of parallel and antiparallel β-sheets within a large amorphous aggregate at t=2 hours, while fibrils exhibited the expected signature of parallel β-sheets at t=1 week. The resonance observed for parallel β-sheets at t=2 hours coincides with that observed for fibrils (at 1634 cm^-1 ), thus indicating that fibril-like species exist within the large aggregates. Therefore, nucleation might occur within such species, in analogy to current theories of protein crystallization in which nucleation occurs within large protein clusters. Cu²⁺ perturbs Aβ aggregation, catalysing rapid formation of amorphous aggregates with diverse secondary structure, but inhibiting fibril growth.