Some proteins or their complexes can be characterized using small angle X-ray (SAXS) experiments. SAXS differs from X-ray crystallography in that it is applied to proteins in solution rather than crystals; thus, it can be applied to a much wider range of proteins in states more closely resembling their functional forms than X-ray crystallography. Because the sample is in solution, the SAXS information is rotationally averaged and so the resulting SAXS profile gives less structural information than X-ray crystallography; however, this is often sufficient to determine problems with the structure or to distinguish possible conformations.

IMP includes tools for fitting both single-state and multi-state models against SAXS data, both of which are introduced here:

• FoXS: fitting single structures against SAXS data
• MultiFoXS: determination of multi-state models