SASREF performs quaternary structure modeling of a complex formed by subunits with known atomic structure against the SAXS data set. Further, it can simultaneously fit multiple SAXS data sets from the subcomplexes, if available, and account for the particle symmetry.
A simulated annealing protocol is employed to construct an interconnected ensemble of subunits without steric clashes, while minimizing the discrepancy between the experimental scattering data and the curves calculated from the appropriate subunits assemblies.
The theoretical scattering patterns I(s) are expressed in terms of spherical harmonics from the partial scattering amplitudes of the subunits Alm(s) in their given positions and orientations. The subunit's amplitudes in arbitrary arrangement depend on its scattering amplitudes in the reference position and on three rotational and three translational parameters. The reference partial scattering amplitudes of the subunits have to be precomputed by the programCRYSOL (recommended values are lm=15, ns=51).
Eventual symmetry (must be the same for all subcomplexes) can be taken into account, whereby SASREF searches for the subunits arrangement inside the asymmetic part and the rest is generated according to symmetry rules.
Please refer to the paper cited above for further details about the implemented algorithm.