Protein structure and coupled solvent dynamics in α-synuclein fibrils under controlled confinement.

The formation of amyloid fibrils by the intrinsically disordered protein α-synuclein in human brain neurons and extracellular tissue is a hallmark of Parkinson disease. Although the stable protein β-structure in α-synuclein fibrils has been determined at high resolution, the status of the dynamically disordered domains and their responses to in vivo confinement and small-molecule interactions remain elusive. We use electron paramagnetic resonance spectroscopy of the spin probe, TEMPOL, colocalized with α-synuclein fibrils, and temperature-controlled, ice boundary confinement to characterize the physical and mechanical properties of the dynamically disordered protein and surrounding coupled solvent regions. The spin probe rotational correlation time at all temperature (T) values is characterized by two motional components, which are assigned to dynamically disordered protein terminal and hydration phases around the central fibril β-structure. The observed transformation from fast to slow motion components with decreasing T and increasing confinement is proposed to proceed through compaction 1 (protein collapse and folding) and compaction 2 (collapse and restriction) processes, intervened by tertiary structure consolidation and stable hydration layer formation. Hysteresis in the return, increasing T direction originates from frustration of cooperative dynamics of the spatially overlapping terminal domains and reveals bistability of compact, low-enthalpy and disordered high-entropy structures. Arrhenius and van't Hoff analyses support the proposed molecular model and indicate that compaction transitions are accessible under physiological conditions. Confinement relief effects of the small molecule dimethylsulfoxide are consistent with the model. The system and results provide a robust platform for assessment of α-synuclein fibril-targeted drugs.