Structural Characterization of Individual α-Synuclein Oligomers Formed at Different Stages of Protein Aggregation by Atomic Force Microscopy-Infrared Spectroscopy.

Aberrant α-synuclein aggregation is strongly associated with the onset and development of Parkinson's disease (PD). Therefore, characterizing the structure of toxic intermediate oligomers plays an essential role in better understanding their neurotoxicity. Using atomic force microscopy-infrared spectroscopy (AFM-IR), we were able to reveal the structure of α-synuclein oligomers present at different stages of protein aggregation and establish a relationship between morphology and structure on the single oligomer level. We were also able to probe the secondary structure evolution of individual oligomers. Moreover, the IR spectra of individual oligomers suggest structural rearrangement that is necessary for oligomers with an antiparallel β-sheet to propagate into fibrils that have a parallel-β-sheet secondary structure. Detailed investigation of structural organization of α-synuclein oligomers reported in this study is critically important to understand the toxicity of these protein species. We also anticipate that this work will help developing approaches for oligomer detection and consequently presymptomatic diagnostic of PD.