To investigate the molecular-level (i.e. secondary) structures of the fibrils, we measured their Raman spectra in the extended Amide III region between 1200-1350 cm -1 (Figure ). Asher and coworkers, have assigned this region in detail, identifying three sub-bands called the Amide III 1 , Amide III 2 , and Amide III 3 , that derive from vibrations containing significant contributions of CN stretching, NH bending, and C α H bending motions. Among these bands, the Amide III 3 is the most conformationally sensitive, as its frequency can be correlated to the Ramachandran ψ-dihedral angles of peptide bonds. The Amide III 3 peaks for Aβ 25-35 (Figure ) and amylin 20-29 (Figure ) fibrils occur at 1225 cm -1 , indicating that they are comprised of parallel β-sheet structures, as discussed by Harper et al. The features of the Aβ 25-35 and amylin 20-29 spectra are relatively well-resolved with spectral linewidths close to that of peptide crystals. This indicates that Aβ 25-35 and amylin 20-29 fibrils exhibit highly ordered cross-β structures. In contrast, the bands observed in the spectra of full-length wild-type Aβ 1-40 (Figure ) and amylin (Figure ,f) fibrils are broader, indicating greater structural heterogeneity. The Amide III 3 peaks for Aβ 1-40 (Figure ) occur at 1234 cm -1 and 1238 cm -1 for samples 1 and 2, respectively, while those for amylin (Figure ) are at 1233 cm -1 (sample 1) and 1235 cm -1 (sample 2). To our knowledge, all reported ssNMR and cryo-EM structures of wildtype, full-length Aβ 1-40 and amylin fibrils exhibit parallel β-sheet architectures. The Amide III 3 frequencies that we observe for our Aβ 1-40 and amylin fibril samples are in the range consistent with parallel β-sheet structures.