SLAC National Accelerator Laboratory
BL4-2 Biological Small Angle Scattering/Diffraction

Data Interpretation Basics for Solution Scattering

1. Guinier Plot

    One can learn the overall size of a protein by making a Guinier plot which gives an estimate of the radius of gyration and the forward scattered intensity.  The latter is proportional to the square of the molecular weight for a given number concentration.  These two quantities are useful in making sure that proteins are behaving well under the x-ray beam.  The plot is very simple: ln I(Q) vs. Q2.  It is critical that the plot be linear in the Q range of Q less than Qmax=M/Rg, where M is typically 1 to 2 and Rg is an estimate of the radius of gyration.  If a straight line is not obtained, there are a few possibile explanations, including high polydispersity of the sample and a possible elongated shape of the protein.

2. Electron pair distribution function P

    The electron pair distribution function is usually obtained by taking an indirect Fourier transform of the scattering curve.  It is important to have a good representation in the  low angle region as well as at higher angles in order to reduce termination error in the Fourier transform.  The useful parameter immediately obtained from the pair distribution  function is the maximum dimension of the praticle in solution.  P(r) drops to zero on the r axis where the maximum dimension is.