Biological small-angle scattering
Biological small-angle scattering is a small-angle scattering method for structure analysis of biological materials. Small-angle scattering is used to study the structure of a variety of objects such as solutions of biological macromolecules, nanocomposites, alloys, and synthetic polymers. Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) are the two complementary techniques known jointly as small-angle scattering (SAS). SAS is an analogous method to X-ray and neutron diffraction, wide angle X-ray scattering, as well as to static light scattering. In contrast to other X-ray and neutron scattering methods, SAS yields information on the sizes and shapes of both crystalline and non-crystalline particles. When used to study biological materials, which are very often in aqueous solution, the scattering pattern is orientation averaged. SAS patterns are collected at small angles of a few degrees. SAS is capable of delivering structural information in the resolution range between 1 and 25 nm, and of repeat distances in partially ordered systems of up to 150 nm in size. Ultra small-angle scattering (USAS) can resolve even larger dimensions. The grazing-incidence small-angle scattering (GISAS) is a powerful technique for studying of biological molecule layers on surfaces. In biological applications SAS is used to determine the structure of a particle in terms of average particle size and shape. One can also get information on the surface-to-volume ratio. Typically, the biological macromolecules are dispersed in a liquid. The method is accurate, mostly non-destructive and usually requires only a minimum of sample preparation. However, biological molecules are always susceptible to radiation damage. In comparison to other structure determination methods, such as solution NMR or X-ray crystallography, SAS allows one to overcome some restraints. For example, solution NMR is limited to protein size, whereas SAS can be used for small molecules as well as for large multi-molecular assemblies. Solid-State NMR is still an indispensable tool for determining atomic level information of macromolecules greater than 40 kDa or non-crystalline samples such as amyloid fibrils. Structure determination by X-ray crystallography may take several weeks or even years, whereas SAS measurements take days. SAS can also be coupled to other analytical techniques like size-exclusion chromatography to study heterogeneous samples. However, with SAS it is not possible to measure the positions of the atoms within the molecule. (Wikipedia).
