We can use eq. 2 to estimate the average force constant per hydrogen bond for the β-sheets of Aβ 25-35 and amylin 20-29 fibrils. We calculate this value to be ∼ 0.56 mdyne Å -1 (see Supporting Information for details), roughly 3-4 times larger than the force constants determined for β-poly-gly and β-polyala. This indicates that the hydrogen bonds in Aβ 25-35 and amylin 20-29 fibrils are significantly stronger, consistent with the prevailing belief that β-sheets in amyloid fibrils are generally more highly ordered, rigid, and stable than those found in globular proteins. F. The ∼ 15 -40 cm -1 spectral region