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.
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