Fig. 1.

SAXS intensity for a dilute solution of BSA (5 mg/mL, room temperature) in 150 mM Hepes buffer after subtraction of background contributions. The data (circle) can be fitted with the form factor of an oblate ellipsoid (solid line). The deviation at higher Q is caused from the deviation of the protein shape from an ellipsoid at smaller length scales. The fitting of scattering data from several solutions with protein concentration below 10 mg/mL and varying concentration of Hepes buffer and NaCl is consistent with an oblate ellipsoid with polar semiaxis a ≈ 1.8 ± 0.05 nm and equatorial semiaxes b ≈ 4.6 ± 0.15 nm. This protein model of an oblate ellipsoid (inset lower left corner) is used as input for the further data analysis based on colloid theory.