FIGURE 4.

The SAXS pattern analysis of MARS in solution suggests striking differences with previous EM three-dimensional reconstructions. A, the experimental SAXS pattern of monodisperse MARS particles (black) diverges significantly, especially at low Q values between 0 and 0.05 Å⁻¹, from the scattering patterns computed from bead models filling the negative stain (green) and cryo (cyan) EM three-dimensional reconstructions filtered to 30 and 36 Å resolution, respectively, and shown in Fig. 5B. The red theoretical SAXS pattern computed from a typical ab initio elongated SAXS bead model generated by DAMMIF, as shown in Fig. 5A, displays a good fit with the experimental data with a χ² of ∼1.5 up to Qmax = 0.25 Å⁻¹. B, shown are dimensionless Kratky plots for MARS SAXS data (black) and the two EM models with the same color code as in A, suggesting an overall well-folded but non-compact conformation of MARS particles in solution (black). C, shown is distance distribution function P(r) derived from the intensity curves I(q) extrapolated to zero concentration with the same color codes. The values of R_g = 139 ± 2 Å and Dmax = 520 ± 25 Å for MARS particles derived from the black curve extend significantly beyond the values obtained from the two EM three-dimensional reconstructions (R_g/Dmax = 67/217 or 76/274 Å).