SAS, refined

Small-angle scattering (SAS) is a technique that provides thermodynamic and structural information about macromolecules in solution, such as particle size and mass distribution. Notwithstanding the one-dimensional nature of its data, SAS is sensitive to perturbations in experimental conditions, and this hampers extraction of information from the SAS profile. With the lack of widely accepted standards for the assessment of SAS data analysis and over-fitting, SAS has remained a complementary technique with limited utility in cases where the macromolecule is flexible, for which key parameters often cannot be determined. Now Rambo and Tainer describe new approaches to address these fundamental SAS limitations. They discovered that an empirically determined scattering ratio, termed the volume of correlation (V_c), greatly facilitates accurate measurement of particle mass, even in cases of conformational flexibility. The V_c parameter can be used to calculate a new residual, R_SAS, which describes the extent of agreement between a structural model and experimental data. Tapping principles from information theory, the authors also developed an approach to calculate a cross-validated statistical indicator of over-fitting, χ²_free, analogous to the R_free statistic in crystallography. Models with minimal R_SAS values constrained by a fixed χ²_free limit are unlikely to be over-fitted in high-noise datasets, and the χ²_free cutoff provides a meaningful, objective definition of SAS resolution limit. These developments extend the power of SAS to a wider range of macromolecules than was previously possible and add much-needed rigor to the analysis and interpretation of SAS data. (Nature 496, 477–481, 2013)