Small Angle X-Ray and Neutron Scattering (SAXS/SANS)

Equipment, Experiments and Modelling

Scattering experiments study the overall structure of macromolecules in random orientations in solution1. This is a diffraction experiment akin to crystallography, but in a scattering experiment, the crystal is replaced by molecules free in solution. The intensities of the scattering curve are measured as a function of scattering angle. From this, the radius of gyration Rg (degree of elongation) and molecular mass are determined, together with dimensional information on macromolecular shape and length L. Thus scattering provides a multi-parameter description of a protein structure in near-physiological conditions, but at low structural resolution (2–4 nm). This method is especially useful for macromolecules that have not been crystallised.

We perform our scattering work on the X-ray instrument ID02 at the European Synchrotron Radiation Facility and on the neutron instruments SANS2D at the ISIS neutron spallation source and D22 at the ILL neutron reactor.

Constrained modelling based on known crystal structures is able to extract further structural information at details of 0.5–1.0 nm. This modelling is based on randomizing arrangements of the macromolecular subunits (e.g. defined by small crystal structures), then comparing these arrangements with the scattering data to identify the best fits by trial-and-error. Even though unique structure determinations are not possible owing to random molecular orientations, modelling is able to rule out structures that are incompatible with the scattering curves. The best-fit scattering structures are deposited in the Protein Data Bank.

Publications. Perkins, S. J. (2009). Unravelling antibody and complement structures in immunity using TS-2 neutrons at ISIS: Neutron scattering. The Biochemist. 31 (4), 32-35.

Perkins, S. J., Okemefuna, A. I., Nan, R., Li, K. & Bonner, A. (2009). Constrained solution scattering modelling of human antibodies and complement proteins reveals novel biological insights. J. Roy. Soc. Interface. 6, S679-S696.