Electron Paramagnetic Resonance Spectroscopy Measures the Distance between the External β-Strands of Folded α-Synuclein in Amyloid Fibrils

The misfolding of alpha-synuclein (alpha S) to a cross-beta-sheet amyloid structure is associated with pathological conditions in Parkinson's and other neurodegenerative diseases. Using pulse electron paramagnetic resonance spectroscopy combined with a cross-labeling strategy involving four double mutants, we were able to determine the intramolecular distance between the extremal beta-strands. The distance of 4.5 +/- 0.5 nm is in good agreement with the dimensions of a protofilament reported by other low-resolution techniques, such as x-ray scattering and atomic force microscopy.