C-Terminal Radical Oxidation Inhibits α-Synuclein Aggregation and Cytotoxicity via an Oxidative Oligomer-Disrupting Pathway

α-Synuclein (α-Syn) aggregation is a hallmark of Parkinson’s disease and other neurodegenerative disorders. This study investigates the impact of controlled radical oxidation on α-Syn aggregation and associated cytotoxicity. Using a microscale low-temperature plasma device for submillisecond radical oxidation, combined with native ion mobility-mass spectrometry and liquid chromatography-tandem mass spectrometry, we demonstrate radical-directed preferential oxidation of the α-Syn C-terminal region. This targeted oxidation leads to the inhibition of protein aggregation and reduced cytotoxicity in SH-SY5Y cells. Mechanistic analysis reveals that ultrafast C-terminal radical oxidation impairs α-Syn oligomerization propensity, likely by preventing conformational transitions critical for forming stable amorphous deposits and well-ordered fibers. Notably, this inhibitory effect is specific to monomer oxidation prior to aggregation rather than oxidation of preformed fibers. Our findings unveil a novel oxidative oligomerization-disrupting pathway that modulates α-Syn fibrillization behavior, offering new insights into the complex interplay between oxidative stress and protein aggregation in neurodegenerative diseases. This study challenges conventional views of the detrimental role of oxidative stress in α-Syn pathology and suggests potential neuroprotective strategies based on targeted oxidative modifications.