However, a coherence length estimate of ∼ 5 strands may be too small to be physically reasonable for amyloids, raising doubts about whether the ∼ 15 -35 cm -1 band originates from the β-sheet accordion mode. An alternative explanation is that the band corresponds to the so-called "boson peak," observed in inelastic neutron scattering spectra of proteins, including amyloid fibrils, as well as in amorphous and disordered materials like glasses. While the origins of the boson peak are still debated, 115 its strong intensity in neutron scattering spectra suggests it results in part from large-amplitude diffusive motions of hydrogen atoms in side chains. We are unable to determine which hypothesis our data aligns with. As a result, we leave this region unassigned. Regardless of its origins, the ∼ 15 -35 cm -1 may report on bulk mechanical properties of fibrils. Sokolov and colleagues suggest that the frequency of the boson peak in globular proteins depends on their structural rigidity. This is consistent with eq. 3, where the frequency of the accordion mode depends on the Young's modulus (E) of the fibril β-sheet segments. The key difference between the two models is that they predict different frequency trends. In the elastic continuum model, the frequency of the band decreases as the β-sheet length increases, similar to what is predicted for globular proteins based on hydration radius. However, studies suggest this trend does not hold, at least for globular proteins, whereas Sokolov and colleagues suggest that the boson peak frequency increases with rigidity and not necessarily the size of the protein.