Electron paramagnetic resonance spectroscopy measures the distance between the external β-strands of folded α-synuclein in amyloid fibrils.

The misfolding of α-synuclein (αS) to a cross-β-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 β-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.