Based on our assignments, the bands between 60 -120 cm -1 may serve as spectroscopic markers for probing the ordering and interaction strength between β-sheet strands in fibrils. The well-defined peaks observed in Aβ 25-35 and amylin 20-29 fibrils, compared to the broad, featureless peaks in Aβ 1-40 and full-length amylin fibrils, suggest their β-sheets are more highly ordered. This is confirmed by our spectra of Aβ 25-35 and amylin 20-29 fibrils in aqueous solution (Figures and), which show relatively little band broadening compared to globular proteins. Detailed normal mode analysis of β-poly-ala by Fanconi indicates that the fibril bands near 140 cm -1 are primarily due to stretching of C--O• • •H-N hydrogen bonds between β-strands. According to Fanconi, the two strands undergo displacements relative to one another in the direction of the hydrogen bonding axis in this vibration. Consequently, the frequency of this vibration can be roughly estimated by assuming a harmonic oscillation of two rigid, rod-like β-strands of uniform mass such that: