Abstract
The intrinsically disordered protein α‐synuclein aggregates into amyloid fibrils, a process known to be implicated in several neurodegenerative states. Partially folded forms of the protein are thought to trigger the aggregation process. Here, α‐synuclein conformers are characterized by analysis of the charge‐state distributions observed in electrospray‐ionization mass spectrometry under negative‐ion mode. It is found that, even at neutral pH, a small fraction of the molecular population is in a compact conformation. Several distinct partially folded forms are then identified under conditions that promote α‐synuclein aggregation, such as solutions of simple and fluorinated alcohols. Specific intermediates accumulate at increasing concentrations of ethanol, hexafluoro‐2‐propanol, and trifluoroethanol. Finally, extensive folding induced by Cu^2+^ binding is revealed by titrations in the presence of Cu^2+^–glycine. The data confirm the existence of a single, high‐affinity binding site for Cu^2+^. Because accumulation of this partially folded form correlates with enhancement of fibrillation kinetics, it is likely to represent an amyloidogenic intermediate in α‐synuclein conformational transitions. Proteins 2011. © 2010 Wiley‐Liss, Inc.