Structural Context Modulates the Conformational Ensemble of the Intrinsically Disordered Amino Terminus of α-Synuclein

Regions of intrinsic disorder play crucial roles in biological systems, yet they often elude characterization by conventional biophysical techniques. To capture conformational distributions across different time scales, we employed a freezing approach coupled with solid-state NMR analysis. Using segmentally isotopically labeled alpha-synuclein (alpha-syn), we investigated the conformational ensembles of the six alanines, three glycines, and a single site (L8) in the disordered amino terminus under three distinct conditions: in 8 M urea, as a frozen monomer in buffer, and within the disordered regions flanking the amyloid core. The experimental spectra varied significantly among these conditions and deviated from those of a statistical coil. In 8 M urea, monomeric alpha-syn exhibited the most restricted conformational sampling, rarely accessing chemical shifts characteristic of alpha-helices or beta-strands. In buffer, monomeric alpha-syn showed a broader conformational sampling, favoring alpha-helical conformations and, to a lesser extent, random coil states. Notably, amino acids in the disordered regions flanking the amyloid core demonstrated the most extensive conformational sampling, with broad peaks encompassing the entire range of possible chemical shifts and a marked increase in highly extended beta-strand conformations. Collectively, this work demonstrates that intrinsically disordered regions exhibit distinct conformational ensembles, which are influenced not only by the chemical environment but also by the conformations of adjacent protein sequences. The differences in the conformational ensembles of the disordered amino terminus may explain why the monomer and the amyloid form of alpha-syn interact with different biomolecules inside cells.