The most structurally-sensitive bands in the Raman spectra shown in Figure occur between ca. 1200 -1350 cm -1 in the Amide III region. The canonical Amide III band, as characterized in N-methylacetamide, occurs at ca. 1315 cm -1 and originates from a vibration consisting of an in-phase combination of NH in-plane bending and CN stretching. In peptides and proteins, however, the Amide III region is considerably more complex, consisting of several bands that derive from vibrations containing significant contributions of CN stretching, NH bending, and/or C α H bending motions. Asher and coworkers, have assigned the Amide III region of peptides and proteins in detail, identifying three sub-bands called the Amide III 1 , Amide III 2 , and Amide III 3 . They have shown that the Amide III 3 is the most conformationally sensitive, as its frequency can be correlated to the Ramachandran ψdihedral angles of peptide bonds. For β-sheet structures, the Amide III 3 band occurs between ca. 1220 -1240 cm -1 , and it is easily identified due to its relatively strong intensity (even with visible Raman excitation) compared to other bands in the region. Based on peak intensities from our spectral deconvolution analysis (Figures and), we assign the 1236 cm -1 band for amylin 20-29 polymorph 1, the 1224 cm -1 band for amylin 20-29 polymorph 2, and the 1225 cm -1 band for Aβ 25-35 fibrils to Amide III 3 modes that are diagnostic of amyloid fibril β-sheet structures. We capitalized on the structural sensitivity of the Amide III 3 band to determine the distribution of Ramachandran ψangles for the amylin 20-29 and Aβ 25-35 fibril peptide bonds. To do this, we utilized the methodology of Asher and coworkers, which correlates the frequencies of the Amide III 3 band envelope to different ψ-angles (see SI for details). As shown in Figure , the ψ-angles for all the fibril polymorphs occur between ca. 120 -160°. The distribution for amylin 20-29 polymorph 1 (Figure ) is centered at 151°, well within the range of canonical antiparallel β-sheet structures. In contrast, the distributions for the other polymorphs are centered at ca. 140°(Figure ), indicating that they are composed of parallel β-sheet structures. We observe no ψ-angles in the fibrils that suggest there are β-turn structures. This indicates that all three polymorphs consist of extended β-strands that assemble into β-sheets. All spectra were smoothed with a Savitzky-Golay Filter 66 using a 4 th order polynomial over an 11-point window for visual clarity. The original spectra can be found in Figure .