Anion Binding and Aggregation of N‑Terminal α‑Synuclein Peptides

α-Synuclein (α-Syn) is linked to the pathogenesis of Parkinson's disease by its misfolding, aggregation, and accumulation in Lewy bodies, the characteristic amyloids of Parkinson's. N-terminal binding to phospholipid membranes and the resulting random-coil to helical transition are key to the aggregation of α-Syn. However, despite the recognized affinity for the N-terminal domain for phospholipids, the anion affinity for this region has not been comprehensively examined. To probe the effects of monovalent anion binding to the N-terminus, we report here on studies with the 15-mer N-terminal peptide of α-Syn and two mutants in which all three lysines of the wild-type sequence are replaced with either arginine or histidine (₁MDVFMXGLSXAXEGV₁₅; X = K, R, or H). Our studies reveal that charge-diffuse anions have a measurable affinity, binding weakly to the midsection of the sequences. However, binding does not induce significant long-range ordering. Nevertheless, MD simulations do reveal a compaction of the peptides in the presence of ClO₄ ^-, supporting the conclusion that anion binding screens the positively charged residues, reducing the effective net positive charge of the peptide and inducing aggregation. Aggregation studies revealed that this reverse Hofmeister effect correlates with anion affinity and that at intermediate salt concentrations or low pH, aggregation follows the Finke-Watzky model. Our findings suggest that changes in simple salt concentrations are unlikely to affect the structure of the N-terminal region of α-Syn and highlight that multipoint interactions between polyanionic phospholipid membranes are a necessary requirement for the random-coil to helical transition observed in the wild type.