Brain metal dyshomeostasis and altered structural dynamics of the presynaptic protein alpha-synudein (alpha S) are both implicated in the pathology of Parkinson's disease (PD), yet a mechanistic understanding of disease progression in the context of alpha S structure and metal interactions remains elusive. In this Communication, we detail the influence of iron, a prevalent redox-active brain biometal, on the aggregation propensity and secondary structure of N-terminally acetylated alpha S (NAccrs), the physiologically relevant form in humans. We demonstrate that under aerobic conditions, Fe(II) commits (NAc)alpha S to a PD-relevant oligomeric assembly, verified by the oligomerselective All antibody, that does not have any parallel beta-sheet character but contains a substantial right-twisted antiparallel beta-sheet component based on CD analyses and descriptive deconvolution of the secondary structure. This (NAc)alpha S-Fe-II oligomer does not develop into the beta-sheet fibrils that have become hallmarks of PD, even after extended incubation, as verified by TEM imaging and the fibril-specific OC antibody. Thioflavin T (ThT), a fluorescent probe for beta-sheet fibril formation, also lacks coordination to this antiparallel conformer. We further show that this oligomeric state is not observed when O-2 is excluded, indicating a role for iron(II)-mediated O-2 chemistry in locking this dynamic protein into a conformation that may have physiological or pathological implications.