The relation of alpha-synuclein (alphaS) aggregation to Parkinson's disease has long been recognized, but the pathogenic species and its molecular properties have yet to be identified. To obtain insight into the properties of alphaS in an aggregation-prone state, we studied the structural properties of alphaS at acidic pH using NMR spectroscopy and computation. NMR demonstrated that alphaS remains natively unfolded at lower pH, but secondary structure propensities were changed in proximity to acidic residues. The ensemble of conformations of alphaS at acidic pH is characterized by a rigidification and compaction of the Asp and Glu-rich C-terminal region, an increased probability for proximity between the NAC-region and the C-terminal region and a lower probability for interactions between the N- and C-terminal regions.