1070 <strong>SDS</strong>-induced Fibril Growth <strong>of</strong> a-Synucleingiven concentration <strong>of</strong> α-synuclein in the same buffer at30 °C. In a control experiment (blank titration), thecorresponding <strong>SDS</strong> solution was injected into the bufferwithout the protein.bis-ANS bindingThe sample <strong>of</strong> α-synuclein (1 mg/ml or 0.22 mg/ml) in20 mM Hepes–NaOH buffer (pH 7.0) containing 5 μM bis-ANS was titrated with increasing concentrations <strong>of</strong> <strong>SDS</strong>.The fluorescence spectrum <strong>of</strong> bis-ANS was recorded usinga Hitachi F-4000 fluorescence spectrophotometer with theexcitation wavelength set at 390 nm. The excitation <strong>and</strong>emission b<strong>and</strong> passes were set at 5 nm. All the spectrawere recorded in the corrected spectrum mode.Time-resolved fluorescence measurementsFluorescence lifetime measurements were performedusing the TCSPC Picosecond Lifetime measurementsystem, model 5000 U (Horiba Jobin Yvon, France) intime-correlated single photon counting (TCSPC) mode.The excitation source was a 337 nm wavelength NanoLEDpulsed laser with a repetition rate <strong>of</strong> 250 kHz. Theinstrument response function (prompt) was obtained at337 nm using Ludox suspension. The sample <strong>of</strong> α-synuclein (0.22 mg/ml) in 20 mM Hepes–NaOH buffer(pH 7.0) containing 5 μM bis-ANS was titrated withincreasing concentrations <strong>of</strong> <strong>SDS</strong>. The emission decay (at485 nm) data were analyzed using the DAS6s<strong>of</strong>twareprovided with the instrument. The analysis used astatistical method <strong>of</strong> iterative non-linear, least-squaresreconvolution, assuming the three-exponential decayfunction:ðA þ B 1 exp½ i=s 1 ŠþB 2 exp½ i=s 2 ŠþB 3 exp½ i=s 3 ŠÞThe best fit was assessed based on χ2, which was closeto 1 for all the samples. The goodness <strong>of</strong> the fit is evidentalso from the distribution <strong>of</strong> weighted residuals along thezero line. In another experiment, a sample <strong>of</strong> <strong>SDS</strong> micelle(20 mM) in the same buffer containing 5 μM bis-ANS wasused. The decay data could be fit best with a twoexponentialdecay model.Circular dichroism spectroscopyFar-UV CD spectra (200–250 nm) <strong>of</strong> α-synuclein (eitherat 1 mg/ml or 0.22 mg/ml) in 20 mM Hepes–NaOH buffer(pH 7.0) in the absence or in the presence <strong>of</strong> variousconcentrations <strong>of</strong> <strong>SDS</strong> were recorded at room temperatureusing a Jasco J-715 spectropolarimeter. The spectra shownare the average <strong>of</strong> four scans.<strong>SDS</strong> concentration-dependent dissociation <strong>of</strong>α-synuclein fibrilsFibrils <strong>of</strong> α-synuclein (1 mg/ml with respect to theconcentration <strong>of</strong> monomeric protein) were prepared asdescribed earlier. The fibrils were resuspended at a finalconcentration <strong>of</strong> 0.2 mg/ml in 20 mM Hepes buffer (pH7.0) in the presence <strong>of</strong> various concentrations <strong>of</strong> <strong>SDS</strong> <strong>and</strong>the samples were incubated at 37 °C without agitation. A(5 μl) portion was withdrawn from the samples atdifferent time-points <strong>and</strong> added to 1 ml <strong>of</strong> 10 μM ThT in50 mM glycine–NaOH buffer (pH 8). The fluorescence <strong>of</strong>fibril-bound ThT, which is a measure <strong>of</strong> amount <strong>of</strong> fibrilspresent in the sample at the indicated time-point, wasmeasured as described earlier.Far-UV CD spectral changes upon fibril dissociationwere studied by incubating the α-synuclein fibrils (1 mg/ml) in 20 mM Hepes (pH 7.0) in the presence <strong>of</strong> either0.5 mM or 4 mM <strong>SDS</strong> at 37 °C. The CD spectra wererecorded at the indicated incubation time-point using aJasco J-715 spectropolarimeter. Spectra shown are theaverage <strong>of</strong> four scans.Light-scattering <strong>of</strong> α-synuclein upon treating with<strong>SDS</strong>To test whether treating α-synuclein with <strong>SDS</strong> promotesamorphous aggregation, a sample <strong>of</strong> 0.3 mg/ml <strong>of</strong> α-synuclein in 20 mM Hepes–NaOH buffer (pH 7.0) wasequilibrated at 37 °C in the cuvette holder <strong>of</strong> a Hitachi F-4000 fluorescence spectrophotometer <strong>and</strong> the sample wastreated with increasing concentration <strong>of</strong> <strong>SDS</strong>. Lightscatteringwas measured by setting both the excitation<strong>and</strong> emission monochromator at 465 nm. Similarly, asample <strong>of</strong> 0.3 mg/ml <strong>of</strong> human recombinant αB-crystallinin the same buffer was treated with <strong>SDS</strong> <strong>and</strong> the lightscatteringwas measured.Measurement <strong>of</strong> critical micellar concentration (cmc)<strong>of</strong> <strong>SDS</strong>The cmc <strong>of</strong> <strong>SDS</strong> in 20 mM Hepes–NaOH buffer (pH 7.0)was measured using the fluorescent probe diphenylhexatriene.50 The fluorescence intensity <strong>of</strong> the probe(10 μM) at 430 nm was measured in buffer alone or in thepresence <strong>of</strong> 0.3 mg/ml <strong>of</strong> α-synuclein with differentconcentrations <strong>of</strong> <strong>SDS</strong> using a Hitachi F-4000 fluorescencespectrophotometer. The probe was excited at 358 nm. Theexcitation <strong>and</strong> emission b<strong>and</strong> passes were set at 1.5 nm <strong>and</strong>10 nm, respectively.AcknowledgementsWe thank Pr<strong>of</strong>essor Antony L. 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