Fourier Transform Infrared (FTIR) Spectroscopy

αS fibrils were recovered from completed spontaneous and seeded aggregation assays following plateau of the ThT fluorescence. Fibrils were centrifuged at 21 300 rcf for 10 min before resuspension in water. Samples were dehydrated under light flow of dry air. FTIR spectra were recorded on a Bruker Vertex 70 FTIR spectrometer (Billerica, U.S.) on the diamond ATR, with 4 cm resolution and a data range of 800–4000 cm^–1; the data in the amide peak 1 (1580–1720 cm^–1) were analyzed. A rubber band baseline correction was applied to the data, before fitting to a Gaussian equation with 4–7 peaks. The area under each peak was integrated to obtain relative compositions of the secondary structure using the following classifications: peaks under 1640 cm^–1 were assigned to β-sheet structures, peaks from 1640 to 1660 cm^–1 were assigned to disordered random coils/α-helices, and peaks above 1660 and 1685 cm^–1 were also assigned to β-sheet structures.