Two-Dimensional Infrared Spectroscopy and Electron Microscopy of Seeded and Non-Seeded Amyloid β Peptide Fibrils

Abstract

Seeded and non-seeded fibrils from amyloid β 40-residue (Aβ40) peptides have been studied with two-dimensional infrared (2D-IR) spectroscopy and electron microscopy (EM). For the former, 13C=18O isotope labels were placed in various residues to probe residue-specific amide-I' vibrations of Aβ40 peptides. 2D-IR spectroscopy (2D-IR photon echo) of both kinds of fibrils shows distinct linear chain excitons of amide units due to the interamide vibrational coupling. Structural differences between the seeded and the non-seeded fibrils vary across the sequence. However, seeded fibrils tend to have a better aligned tertiary structure than non-seeded fibrils. Negatively stained EM images indicate that non-seeded fibrils had no discernible narrowing or nodes. In comparison, seeded fibrils tend to appear twisted like a ribbon, with periodic narrowing or nodes. The distances between nodes (the “internodal” distances) were relatively homogeneous distributions (∼75 nm) and the same in all isotopically labeled peptides. The apparent width of the fibrils in these images indicate that seeded fibrils are two kinds and that the width of the narrower fibrils is the same as the width of non-seeded fibrils. The mass-per-length evaluated from dark-field EM images indicates that the most prevalent number of filaments in seeded fibrils is 6, which is twice as many as the number in non-seeded fibrils (3 filaments per fibril). Overall, these results demonstrate that seeded and non-seeded fibrils have distinctly different tertiary structures.