The phenomenon that gives rise to SAS is the same that produces diffraction from a crystal. The key difference between SAS and crystallography is the nature of the sample. In bio-molecular SAS, the sample usually consists of a macromolecule dissolved in an aqueous buffer, whereas crystallography relies on the molecules being aligned in three dimensions within the crystal. During the SAS experiment, the macromolecule is able to sample all possible orientations, and consequently the data represent a rotational average. This averaging results in a loss of information relative to diffraction data. The attraction of SAS is that, compared with crystallography, the experiments are quicker; the samples are in solution and may be measured over a range of conditions (pH, ionic strength, temperature, etc.); and there is no requirement for crystallinity, thereby reducing sample preparation time, and expanding the range of samples and conditions that may be amenable to structural characterisation.
However, a key yet often underappreciated difference between biological SAS and crystallographic samples is the importance of characterising sample quality. A ‘poor quality’ protein crystal yields no measureable diffraction. At this point data interpretation ceases - there are no data to process, so it is not possible to refine a model. In the event that a crystal does diffract, the quality of the data can be estimated from the resolution to which statistically significant diffraction can be observed and from the self-consistency of the data, as indicated by the averaging of equivalent reflections. In other words, crystallography has natural quality control checkpoints, as well as established reporting requirements and as a result, the coordinate files produced from a diffraction experiment carry a certain authority. In the case of a ‘poor quality’ SAS sample, data are still observed and can be measured so long as there is a macromolecule present that has a different scattering density to its supporting solvent. A poor quality sample would be one that fails the tests of containg a mono-disperse solution of non-interacting particles; a stringent requirement for accurate structural interpretation. The scattering data by themselves do not provide all the necessary evidence for sample quality. Independent characterization of sample properties are required; e.g.
purity checks, concentration determination, and comparison with standards [4
]. Without a set of adequate quality control checkpoints, SAS data can be processed and interpreted and incorrect models proposed. Consequently, coordinate files produced from SAS carry very little authority on their own. Without a community agreed reporting framework that requires the reporting of the quality control measures and the necessary information for independent evaluation, the correct structural data and models will have less impact than they deserve based on the very well-understood theory and principles of SAS.
As mentioned above, crystal structures are treated in the wider biological community as carrying an implicit correctness - though this is not strictly true. Atomic coordinates themselves are meaningless without the reporting of the appropriate data processing and refinement statistics. The convention of reporting these statistics in ‘Table 1’ of any crystallographic publication arose from the need of reviewers and the wider readership to be able to independently assess the conclusions that the authors draw from a given structure. This convention in crystallography was established through the intervention of the IUCr.
Due to the importance of demonstrating sample and data quality, the publication of SAS experiments for structural biology purposes requires a similar rigorous reporting framework. In this period wherein the application of SAS in structural biology has ‘blossomed’ it perhaps has been too easy to report SAS results with insufficient rigor. SAS, as an allied technique to crystallography, is benefitting from the experience and authority of the IUCr. At its 2011 congress, the IUCr’s Journals Commission adopted a set of guidelines for the publication of biological SAS data that had been prepared and agreed by the IUCr Small-Angle Scattering Commission. These guidelines are available at http://journals.iucr.org/services/sas/
, and have been described in detail [5
It should be stressed that the IUCr’s guidelines aim to establish a convention for those experiments that report structures in the form of atomic or bead coordinates. While the guidelines do not explicitly mention other types of experiment that may be performed by SAS (e.g.
determining oligomeric equilibria, measuring natively unstructured proteins, etc.) [6
] many of the recommended quality control measures will still be applicable (such as establishing the absence of non-specific aggregation, and the calculation of molecular mass for the scattering particles). It also should be stressed that the aim of these guidelines is not to define a level of quality that needs to be achieved in a SAS experiment, but rather to establish what information needs to be reported so that readers (including reviewers) are able to independently assess the interpretation and conclusions drawn from the data by the authors.