Hiramatsu and coworkers 66 observe similar spectral fea-tures at 30 cm -1 , 60 cm -1 , 90 cm -1 , and 120 cm -1 in β 2microglobulin fibrils. They claim that the 60 cm -1 and 120 cm -1 bands are associated with antiparallel and parallel β-sheets, respectively, while the 30 cm -1 and 90 cm -1 features appear unrelated to fibril secondary structure. Because of this apparent lack of structural sensitivity, they suggest that the 30 cm -1 and 90 cm -1 bands may arise from longitudinal accordion-like motions that occur along the direction of the peptide chains. Although they do not definitively assign the 60 cm -1 and 120 cm -1 bands, the authors speculate they may originate from vibrations sensitive to hydrophobic side-chain interactions.