The most intense features in the spectra shown in Figure occur in the Amide I region. The Amide I vibration consists mainly of amide C--O stretching. . Its structural sensitivity derives from transition dipole coupling between neighboring C--O oscillators that produce a delocalized Amide I normal mode. Coupling also results in characteristic "excitonic splitting" patterns in the Amide I band that are diagnostic of different protein secondary structure elements. For example, the Amide I band for canonical (infinitely long) parallel β-sheets is predicted to split into two sub-bands, a high frequency A(0, 0) mode and a low frequency B(π, 0) mode that are both Raman and IR-active. In contrast, the Amide I band is predicted to split into four sub-bands for canonical (infinitely long) antiparallel β-sheet structures: the A(0, 0) mode, which is Raman-active and forbidden in the IR; the B 1 (0, π) mode, which is relatively weak in both Raman and IR; the B 2 (π, 0) mode, which is very strong in IR spectra; and the B 3 (π, π), which is essentially forbidden in IR and Raman spectra. In practice, however, it can be difficult to differentiate between parallel and antiparallel β-sheet structures using the Amide I band alone. This is because the Amide I band depends on additional factors such as hydrogen bonding patterns of the peptide bond C--O groups, the registry of β -strands, and the twist of the β -sheets. The intense band at ca. 1670 cm -1 can be assigned to the Amide I A(0, 0) for all polymorphs. The B(π, 0) mode occurs near 1630 cm -1 for both amylin 20-29 polymorphs and is downshifted to ca. 1620 cm -1 in Aβ 25-35 fibrils. We do not observe Raman bands in any of the fibril spectra that can be assigned to the B 1 (0, π) mode characteristic of canonical antiparallel β-sheet structures. However, for amylin 20-29 polymorph 1, we observe a band at 1695 cm -1 in the IR spectrum (Figure ), which we assign to the B 1 (0, π) mode. The presence of this band suggests that these fibrils are composed of antiparallel β-sheet structures (vide infra). We attribute the absence of the B 1 (0, π) mode in the Raman spectrum to the fact that there is likely some local disordering of the β-strand registries in the amylin 20-29 polymorph 1 fibrils. The amylin 20-29 fibrils contain additional bands in the ca. 1600 -1700 cm -1 region. Both polymorphs exhibit bands at ca. 1605 cm -1 , while polymorph 2 exhibits an additional band at 1586 cm -1 . These bands can be assigned to the in-plane ring stretching modes of phenylalanine. Amylin 20-29 fibrils are also expected to contain spectral contributions from asparagine's side chain C--O stretching mode. We see no evidence that the asparagine residues of amylin 20-29 fibrils contribute significant spectral contributions to the 1600 -1700 cm -1 region. However, to confirm that our assignment of 1671 cm -1 bands in amylin 20-29 fibrils is correct, we measured the Raman spectrum of a lyophilized powder of asparagine. The Raman spectrum of asparagine (Figure ) exhibits a strong band at 1640 cm -1 , which can be assigned to the C--O stretching mode of asparagine side chains. This suggests that the spectral contributions of asparagine side chains is negligible for the 1671 cm -1 bands that we assign to the Amide I A(0, 0) of the peptide backbone amide groups.