Figure 1.

Spin-labeled amyloid fibrils show characteristic single-line EPR spectra. (A) A stick model of the spin label R1 in a parallel in-register β-sheet structure, commonly found in the core of amyloid fibrils formed by various proteins, including the yeast prion protein Ure2. This side-chain packing of the spin label R1 is based on the crystal packing of the spin-labeling reagent MTSSL. (B) Simulated EPR spectra with varying strengths of spin–exchange interactions. Note that increasing spin–exchange interactions leads to the collapse of the high-field and low-field resonance lines toward the center line and results in a single-line feature when the exchange frequency is above 100 MHz. When the exchange frequency is above 200 MHz, the bumpy features at both low-field and high-field lines are smoothed out and the EPR spectrum becomes a complete single-line spectrum.