(a, b) 1H-detected 13C/1H correlation 2D SSNMR spectra of (a) in vitro-prepared and (b) brain-derived Aβ42 fibrils that were uniformly 13C- and 15N-labeled at Phe-20, Ala-21, Val-24, Gly-25, and Leu-34. The signal assignments are shown in the spectra. SSNMR spectra were recorded at a MAS rate of 90 kHz on an 800 MHz Bruker Avance III NMR spectrometer equipped with a JEOL 0.75-mm 1H/13C/15N/2H quad-resonance probe (see Materials and Methods for details). The pulse sequence in Figure S1 was used to collect the data. The experimental time was (a) 33 sec and (b) 8.7 min. (c, d) Transmission electron micrograph (TEM) images of the (c) synthetic and (d) brain-derived Aβ42 fibril samples.