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

Structural characterization of aggregates and fibrils of the A beta 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 A beta, we observed mainly disordered A beta at t=15 min, but substantial structural diversity including the co-existence of parallel and antiparallel beta-sheets within a large amorphous aggregate at t=2 hours, while fibrils exhibited the expected signature of parallel beta-sheets at t=1 week. The resonance observed for parallel beta-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 A beta aggregation, catalysing rapid formation of amorphous aggregates with diverse secondary structure, but inhibiting fibril growth.