7.4 and 370C for 3 d. The scale bars represent 500 nm, 1000 nm and 500 nm, respectively. (d) CD spectra of the corresponding samples shown in AFM: black squares correspond to Fig. (a); blue triangles correspond to Fig. (b); red circles correspond to Fig. (c). (TIF) Figure S4 Measurements of NHSY5Y cell line viability by WST1 assay in the presence of (a): freshly dissolved Ab1–40 at pH 7.4. Different colors denote different concentrations of Ab1–40. (b): 0.2 mM Ab12–24 fibrils, 0.2 mM Ab1–40 fibrils and 0.2 mM Ab1–40 plaque. Ab1–40 plaques were formed by incubation at pH 3.0 and 370C for 3 d. To avoid pH effect, pre-formed Ab plaques were carefully dialyzed to pH 7.4. Ab12–24 and Ab1–40 fibrils were formed by incubation at pH 7.4 and 370C for over 10 d. (c) The relationship between the kinetics of S100A9 amyloid formation (Relative ThT: left Y axis) at pH 3.0 and the effects of S100A9 (0.02 mM) on NHSY5Y cell viability (WST1 assay: right Y axis). 0 Before mixing with cells, S100A9 was incubated at pH 3.0 and 4 C for up to 70 d. Aliquots were taken for measurements of ThT and cytotoxicity at different time during incubation. The columns in each group correspond to 1 to 3 d of incubation with cells (In day 6 aliquots, S100A9 samples were incubated with cell for up to 2 d). (d) Measurements of NHSY5Y cell line viability by WST1 assay in the presence of S100A9 (0.02 mM) at pH 7.4 and 3.0. Different colors denote 1 d to 3 d of incubation with cells. (TIF) Figure S5 (a) Luminescent cytotoxicity assay of neuron cells in presence of 0.02 mM S100A9 and S100A9/Ab1–40 mixture. S100A9 and the mixture were incubated at 370C for 3, 6, 12 and 24 h before mixing with cells. Ab1–40 plaques were formed by incubation at pH 3.0 and 370C for 3 d. 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(b) Measurements of NHSY5Y cell line viability by WST1 assay in the presence of freshly dissolved (1) 0.2 mM Ab12–24; (2) 0.2 mM Ab12–24 and 0.02 mM S100A9 mixture; (3) 0.2 mM Ab1–16; (4) 0.2 mM Ab1–16 and 0.02 mM S100A9 mixture; (5) 0.2 mM Ab1–42; (6) 0.2 mM Ab1–42 and 0.02 mM S100A9 mixture. (TIF) Figure S6 AFM images of (a) Ab12–24 peptide (0.2 mM) incubated at pH 7.4 and 370C for 3 d; (b) the mixture of freshly dissolved Ab12–24 peptide (0.2 mM) and S100A9 protein (0.02 mM) incubated at pH 7.4 and 370C for 3 d; (c) Ab1–42 peptide (0.2 mM) incubated at pH 7.4 and 370C for 3 d; (d) the mixture of freshly dissolved Ab1–42 peptide (0.2 mM) and S100A9 protein (0.02 mM) incubated at pH 7.4 and 370C for 3 d. In fig. a and b, the scale bars denote 100 nm in the figure, and 1000 nm in the insertion, respectively. The scale bars denote 200 nm in fig. c and 500 nm in fig. d. 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