Small-angle scattering of biological structures

Small-angle scattering studies on biological structures provide low-resolution models. More detailed models need a more elaborate analysis in order to show their uniqueness. The representation of small-angle scattering of both in Cartesian coordinates and polar coordinates is discussed. The degree of non-uniqueness of structural analysis is best presented in terms of a multipole expansion. Contrast variation leads to the evaluation of the basic scattering functions J_c(κ), J_cs(κ) and J_s(κ), which add useful constraints to a model. The experimental aspects of this method in both X-ray and neutron scattering are discussed. Furthermore, isomorphous (or isotopic) replacement of parts of macromolecular structures have been very useful in structure determination of ribosomes. Kinetic investigations of solutions in the subsecond region have been performed so far only for the investigation of H–D exchange with some proteins. Small-angle scattering from fibrous and lamellar systems received a considerable impact from a combined use of X-ray and neutron scattering, as is shown for collagen. Structural changes during active muscle contraction have been investigated both with classical X-ray equipment and with synchrotron radiation.