Small angle X-ray scattering

SAXS data were collected on the SAXS/WAXS beamline at the Australian Synchrotron⁶⁵. Approximately 60 μL of α^43a and α^43_UTI89, at 1.2 mg mL⁻¹, in 25 mM HEPES, 150 mM NaCl, pH 7.4 were loaded into a 1 mm quartz capillary. Samples were flowed during data collection to reduce radiation damage. Data reduction was carried out using the ScatterBrain software (v.2.71) and the data were corrected for solvent scattering and sample transmission, then radially averaged to produce I(q) as a function of q, where q = (4πsin θ)/λ, θ is half the scattering angle, and λ is the X-ray wavelength. For α^43a (SASBDB ID: SASDKQ3) and α^43_UTI89 (SASBDB ID: SASDKP3), data were fit as a linear combination of model curves to quantify the nature and proportions of different oligomeric states in solution⁶⁶. Model scattering curves were calculated using CRYSOL (v.2.8.3)⁶⁷, using monomer and dimer structures taken from PDB: 4KH3 for α^43a and PDB: 7KO9 for α^43_UTI89.