Umbrella-like helical structure of alpha-synuclein at the air-water interface observed with experimental and theoretical sum frequency generation spectroscopy

The misfolding of ⍺-synuclein (aS) into amyloid aggregates is associated with severe brain disorders. Aggregat-ed copies of aS are found in the amyloid aggregates observed in brain tissues from Parkinson’s patients. Surfaces are known to catalyze the formation of amyloid aS aggregates. Despite the importance of the role of inter-faces and several decades of structural studies, the 3D structure of aS when bound to interfaces is still not completely clear. Hydrophobic interfaces are particularly important here. We report interface-specific sum-frequency generation (SFG) experiments to determine how monomeric aS binds to the air-water interface, a model system for hydrophobic surfaces in general. We model the SFG data by combining the experimental data directly to theoretical spectra calculations from molecular dynamics simulations. We find that aS, which is an intrinsically disordered protein in solution, folds into a defined, mostly helical, secondary structure at the air-water interface. The binding pose is reminiscent of an umbrella-shape, where the C-terminus represents the ‘pole’ and protrudes into the water phase, while the N-terminus and the NAC region span the canopy at the interface. In this binding pose, aS is prone to aggregate, which could explain the catalytic effect of hydrophobic interfaces and air bubbles on aS fibrillation.