High-resolution NMR characterization of low abundance oligomers of amyloid-β without purification

Alzheimer's disease is characterized by the misfolding and self-assembly of the amyloidogenic protein amyloid-beta (A beta). The aggregation of A beta leads to diverse oligomeric states, each of which may be potential targets for intervention. Obtaining insight into A beta oligomers at the atomic level has been a major challenge to most techniques. Here, we use magic angle spinning recoupling H-1-H-1 NMR experiments to overcome many of these limitations. Using H-1-H-1 dipolar couplings as a NMR spectral filter to remove both high and low molecular weight species, we provide atomic-level characterization of a non-fibrillar aggregation product of the A beta(1-40) peptide using non-frozen samples without isotopic labeling. Importantly, this spectral filter allows the detection of the specific oligomer signal without a separate purification procedure. In comparison to other solid-state NMR techniques, the experiment is extraordinarily selective and sensitive. A resolved 2D spectra could be acquired of a small population of oligomers (6 micrograms, 7% of the total) amongst a much larger population of monomers and fibers (93% of the total). By coupling real-time H-1-H-1 NMR experiments with other biophysical measurements, we show that a stable, primarily disordered A beta(1-40) oligomer 5-15 nm in diameter can form and coexist in parallel with the well-known cross-beta-sheet fibrils.