What is Synchroton SAXS?

Small-Angle X-ray Scattering Explained

Scientists have been using X-rays to probe the structure of matter since the early 20^th century. The first technique to be developed, X-ray crystallography, provides detailed information about the atomic makeup of materials – but as the name suggests, it only works with crystalline solids. This can be a problem, since many substances, notably proteins and other biological materials, are reluctant to form crystals. For these difficult cases, a technique known as small angle X-ray scattering (SAXS) is more appropriate.

SAXS is a fundamental method for structure analysis of condensed matter. The applications cover various fields, from metal alloys to synthetic polymers in solution and in bulk, biological macromolecules in solution, emulsions, porous materials, nanoparticles, etc. First X-ray applications date back to the late nineteen thirties when the main principles of SAXS were developed in the seminal work of A. Guinier following his studies of metallic alloys. The scattering of X-rays at small angles (close to the primary beam) was found to provide structural information on inhomogeneities of the electron density with characteristic dimensions between one and a few hundred nm. Already in the first monograph on SAXS by Guinier and Fournet (1955) it was demonstrated that the method yields not just information on the sizes and shapes of particles but also on the internal structure of disordered and partially ordered systems.

Compared to X-ray diffraction, SAXS has a modest resolution (1-3 nm) that is not sufficient to reveal the atomic structure of materials. SAXS can, however, measure the shapes and sizes of nanoparticles and large molecules. Used in "scanning" mode it can also show the large-scale organisation of solid materials. SAXS is therefore a valuable tool in fields such as biology and materials science. SAXS works in liquids or solids and has short response times, so it can be used to follow biological processes in real time. This makes SAXS an ideal complement to time-consuming analytical techniques such as electron microscopy and X-ray diffraction.

Conceptually, a SAXS experiment is simple: a sample is illuminated by X-rays and the scatteried radiation is registered by a detector. Until the 1970s, SAXS experiments were done on instruments equipped by laboratory X-ray tubes. Also neutrons can be employed, and neutron scattering may usefully complement the X-ray studies. As the SAXS measurements are done very close to the primary beam ("small-angles"), the technique profits immensely from the brilliance of X-ray photon beams provided by particle accelerators known as synchrotrons. Europe has nearly 20 of these huge machines, including new "third-generation" or "high brilliance" designs that produce especially intense, narrow X-ray beams.

The synchrotrons used as high-intensity X-ray sources for SAXS are ring-shaped machines that are often very large. The ESRF in Grenoble, France, is around 1 km in circumference. The SAXS beamlines are also usually rather long. Below is an example of a synchrotron SAXS beamline (X33 at DORIS ring, Hamburg). The sample-to-detector distance is about 3 meters, the first useful measured data point on the detector is less than 1 cm away from the primary beam.

Using the precise scattering patterns recorded at synchrotrons, SAXS can reveal the shapes of protein molecules and nanoparticles. The X-ray scattering curve on the left (intensity versus scattering angle) was used to create a low-resolution model of a protein, shown in grey. Superimposed on this (in colour) are atomic models of separate domains obtained from crystallography and positioned to fit the SAXS data. The protein molecule is about 10 nm across. This example illustrates how SAXS can help to assemble together high resolution models of individual domains provided by protein crystallography into the model of the entire macromolecular complex.