Using X-rays to Determine Molecular Structure
A protein crystal is made up of billions of molecules arranged in a regular array. When a crystal is exposed to X-rays, constructive interference between the molecules in the lattice causes emission of intense diffracted X-rays in specific directions. Measurement of the relative intensity of the diffracted X-rays and estimation of phases allows calculation of the electron density and determination of molecular structure.
Small angle X-ray scattering (SAXS) makes use of the same physical process - scattering of X-rays by electrons - but the protein molecues are free in solution. Because the molecules are randomly oriented and moving, the scattering pattern is continuous and radially symmetric. Instead of a diffraction pattern, you get a plot of intensity versus scattering angle (blue and red curves above). The scattering is very weak and is generally measured only at low angles (yellow shaded region); this is why SAXS measurements are usually made at a synchrotron source like the BioCAT beamline at the Advanced Photon Source (right).