Tip-Enhanced Raman Spectroscopy to Distinguish Toxic Oligomers from Aβ1-42 Fibrils at the Nanometer Scale

For the first time, natural Aβ_1-42 fibrils (WT) implicated in Alzheimer's disease, as well as two synthetic mutants forming less toxic amyloid fibrils (L34T) and highly toxic oligomers (oG37C), are chemically characterized at the scale of a single structure using tip-enhanced Raman spectroscopy (TERS). While the proportion of TERS features associated with amino acid residues is similar for the three peptides, a careful examination of amide I and amide III bands allows us to clearly distinguish WT and L34T fibers organized in parallel β-sheets from the small and more toxic oG37C oligomers organized in anti-parallel β-sheets.

Effect of mid-infrared free-electron laser irradiation on refolding of amyloid-like fibrils of lysozyme into native form

Aggregation of lysozyme in an acidic solution generates inactive amyloid-like fibrils, with a broad infrared peak appearing at 1,610-1,630 cm(-1), characteristic of a β-sheet rich structure. We report here that spontaneous refolding of these fibrils in water could be promoted by mid-infrared free-electron laser (mid-IR FEL) irradiation targeting the amide bands. The Fourier transform infrared spectrum of the fibrils reflected a β-sheet content that was as low as that of the native structure, following FEL irradiation at 1,620 cm(-1) (amide I band); both transmission-electron microscopy imaging and Congo Red assay results also demonstrated a reduced fibril structure, and the enzymatic activity of lysozyme fibrils recovered to 70-90 % of the native form. Both irradiations at 1,535 cm(-1)(amide II band) and 1,240 cm(-1) (amide III band) were also more effective for the refolding of the fibrils than mere heating in the absence of FEL. On the contrary, either irradiation at 1,100 or 2,000 cm(-1) afforded only about 60 % recovery of lysozyme activity. These results indicate that the specific FEL irradiation tuned to amide bands is efficient in refolding of lysozyme fibrils into native form.