Small-angle Scattering

The scattering of X-rays, or neutrons, from a homogeneous solution of monodisperse particles (such as proteins or protein complexes) is sensitive to the size, shape and internal electron density distribution of the scattering molecule. Although data is measured only to ~ 20 Å resolution, indirectly providing low-resolution molecular envelope information, there are certain features such as the maximum linear dimension and radius of gyration, which are directly determined from the scattering data with up to sub-Angstrom precision. Scattering is, therefore, an effective method for probing conformational changes and/or flexibility, as well as molecular associations in solution, which can be key to understanding biomolecular mechanisms. In the case of neutron scattering, deuterium labeling with contrast variation is a powerful technique for obtaining structural data on individual components within the macromolecular complexes. Sample concentrations need only be in the 10 - 100 micromolar range and the detectable size range is anywhere from 10 to 1000 Å, which for a globular protein or protein complex would correspond to a molecular weight range of ~ 8 - 800 kDa, making it quite useful for studying a wide range of biological problems. In addition, the different neutron scattering properties of the isotopes of hydrogen, combined with the ability to uniformly label biological macromolecules with deuterium, allow one to characterize the conformations and relative dispositions of the individual components in an assembly of biomolecules using methods of "contrast variation".

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