Spontaneous assembly of monomeric α-synuclein into protofibrils was monitored using the ThT assay. ThT binds to beta-sheet contents α-synuclein protofils and increasing ThT flourescence (FU, in arbitrary fluorescence units) reflects the presence of higher contents of α-synuclein protofibrils. We note that ThT fluorescence is not a direct measure of fibril content. However, since β-sheet formation correlates with fibril formation, ThT fluorescence is a useful surrogate marker [32]. (A, B, C) Monomeric α-synuclein was incubated for 0–94 hours at 37°C in the presence of vehicle (○), or individual phenolic acids at a low dose (LD, equal molar concentration of phenolic acid relative to α-synuclein) (●), or a high dose (HD, 4-fold higher molar concentration of phenolic acid relative to α-synuclein) (■). Phenolic acids tested were 3-HBA (A), 3,4-diHBA (B) or 3-HPPA (C).