Fluctuation X-Ray Scattering

Jeffrey J. Donatelli, Peter H. Zwart, and James A. Sethian

Although X-ray crystallography has been successful at determining several thousands of macromolecular structures, it is limited to studying objects which can be formed into large crystals. Furthermore the structure of many biological nanomachines have a different conformation in their natural environment than what is determined through crystallography. Therefore, as an alternative and complementary technique, structural biologists often gather diffraction patterns from particles in solution. However, in these so called small- and wide-angle X-ray scattering experiments, the particles

rotate during the imaging process, which yields angularly isotropic data and results in a loss of information that often ultimately leads to poorly determined structural hypotheses. One solution to overcome the limitations of SAXS and WAXS is to perform the imaging process below rotational diffusion times, so that the particles are frozen in position and orientation during imaging. From these so-called fluctuation X-ray scattering (FXS) experiments, speckle patterns emerge which have angularly varying intensities. By computing angular correlations of these speckle patterns, one can extract information several orders of magnitude larger than traditional SAXS and WAXS, which allows one to obtain medium-to-high resolution three-dimensional reconstructions of macromolecular structure.