Significance of Small Angle Scattering/Diffraction in Structural Biology

Small-angle scattering and diffraction (SAXS/D) techniques are used to study largely non-crystalline biological macromolecular systems, including proteins in solution and biological fibers. The information content in SAXS/D is of relatively low-resolution because biological systems of interest are randomly oriented in solution or only partially ordered. It should be realized, however, that the SAXS/D techniques can often study specimens under conditions that are similar to the physiological environment. The ease of controlling sample conditions is also an advantage of SAXS/D techniques in physico-chemical characterization and for time-resolved studies which follow the response of a biological system to a perturbation in the physical or chemical environment, e.g. rapid dilution and pH-jump.

Another merit of SAXS/D techniques is the “complementarity” to other techniques in structural biology. For instance, a solution scattering curve can be readily calculated using a set of atomic coordinates derived from macromolecular crystallography, providing a means of comparing solution structures with crystal structures. Structural changes induced by ligand binding or changes in other chemical or physical environment can be monitored using solution scattering and interpreted, based on high-resolution structures derived from other techniques. Solution scattering can help model a large molecular complex whose over-all structure is unknown but structures of individual components are available.

We have developed an instrument which makes it practical to obtain single crystal diffraction data in the low resolution range of 5-600 Å, depending on the crystal to detector distance - far beyond the reach of diffraction instruments optimized for high-resolution macromolecular crystallography and in a region where the contribution of the molecular envelope is much greater than at higher resolution. The diffraction data in this resolution range complements high-resolution crystallographic studies and are critical in phase extension and density averaging. Low resolution single crystal diffraction data have the potential of visualizing parts of the structure that are otherwise averaged out due to very weak density in higher resolution data.

Examples of Text Resources:

Koch, M.H.J., Vachette, P., Svergun, D.I. (2003) "Small-angle scattering: a view on the properties, structures and structural changes of biological macromolecules in solution" Quarterly Reviews of Biophysics 36(2) 147-227.

Vachette, P., Koch, M.H.J., Svergun, D.I. (2003) "Looking behind the beamstop: X-ray solution scattering studies of structure and conformational changes of biological macromolecules" Methods Enzymol. 374, 584-615.

Doniach, S. (2001) "Changes in biomolecular conformation seen by small angle X-ray scattering" Chem. Rev. 101, 1763-78.

Tsuruta, H., and Johnson, J., J.E. (2001). "Small Angle X-ray Scattering" in the International Tables for Crystallography, Volume F: Macromolecular Crystallography (eds, M.G. Rossmann, E. Arnold), Kluwer Academic Publisher, Chapter 19.3, pp 428-437.

Trewhella, J. (1997) "Insights into biomolecular function from small-angle scattering" Curr. Opin. Struct. Biol. 7, 702-8.

Koch, M.H.J. (1991). "Scattering from non-crystalline systems" in Handbook on Synchrotron Radiation, Vol 4, Chap. 6, Elsevier Science Publisher.

Feigin, L.A. and Svergun, D.I. (1987) "Structure Analysis by Small Angle X-ray and Neutron Scattering" Plenum press, New York.

Glatter, O. and Kratky, O. (1982) "Small Angle X-ray Scattering" Academic Press, New York.