Raman optical activity of human α -synuclein in intrinsically disordered, micelle-bound α -helical, molten globule and oligomeric β -sheet state

Alpha-synuclein (α-syn) is a 140 residue protein that plays a central role in Parkinson's disease and other neurological disorders. The precise function and pathological properties of α-syn remain however poorly understood. While α-syn is considered to be a flexible and disordered protein under native conditions, its ability to adopt a variety of conformational ensembles depending on the environment is considered to be related to its pathology. Raman optical activity (ROA) is a chiroptical spectroscopic technique that is uniquely sensitive to the secondary structure of proteins in solution and was used here for the first time to study the different conformational ensembles of α-syn. In this paper, the Raman and ROA spectral characteristics of these different conformations of α-syn are investigated. We show that Raman and ROA spectroscopy are sensitive enough not only to detect transitions from a disordered to an α-helical or a β-sheet rich ensemble but also to differentiate between the α-helical forms of wild-type and C-terminal truncated α-syn 107. Using increasing concentrations of fluorinated alcohols, we induce the aggregation pathway of α-syn and identify a molten globule intermediate structure and β-sheet rich oligomers. Taken together, these results demonstrate the power of Raman and ROA spectroscopies for the structural elucidation of proteins that are challenging to characterise.