The methyl and methylene torsional and umbrella modes are useful probes for monitoring the packing of aliphatic side chains in amyloid fibrils. Although the torsional motion of symmetric side chains groups in polymers and proteins, such as methyls, are theoretically Raman and IR-active, their low-frequency bands are often difficult to observe due to the small dipole moments and polarizability changes that are induced. 101,106 However, these bands are relatively strong and well-resolved in the fibril spectra (see insets of Figure ), particularly for Aβ 25-35 and amylin 20-29 , suggesting that the aliphatic side chain methyl groups are highly ordered. This likely results from steric zipper interfaces that are formed between neighboring β-sheets in the fibrils, which locks aliphatic side chains into rigid conformations. Interestingly, although amylin sample 1 and 2 have similar morphologies (Figures and) and mid-frequency spectra (Figure ,f), their low-frequency spectra (Figure ) are noticeably very different. The region between ∼ 200 -400 cm -1 have different frequencies for the aliphatic groups (Table I) and seem to be more resolved for amylin sample 2. This indicates variations in the packing of aliphatic side chain groups for the two fibril samples in their respective β-sheet ).