# Scott Classen: April 2013 Archives

Rob Rambo and John Tainer describe new SAS metrics in a paper titled “Accurate assessment of mass, models and resolution by small-angle scattering.” The paper has been published in the journal Nature.

“In SAS imaging, beams of X-rays or neutrons sent through a sample produce tiny collisions between the X-rays or neutrons and nano- or subnano-sized particles within the sample. How these collisions scatter are unique for each particle and can be measured to determine the particle’s shape and size. The analytic metrics developed by Rambo and Tainer are predicated on the discovery by Rambo of an SAS invariant, meaning its value does not change no matter how or where the measurement was performed. This invariant has been dubbed the “volume-of-correlation” and its value is derived from the scattered intensities of X-rays or neutrons that are specific to the structural states of particles, yet are independent of their concentrations and compositions.”

MW, molecular mass. V_{c} and R_{g} were determined from theoretical atomic X-ray scattering profiles for 9,446 protein PDB structures. For each profile, SAXS data were simulated to a maximum q = 0.5 Å^{−1} (~13 Å). Various ratios of V_{c} and R_{g} against protein mass were examined in a log-log plot. The linear relationship observed for the ratio V_{c}^{2}R_{g}^{−1} (black) suggests that a power-law relationship exists between the ratio and particle mass of the form ratio = c(mass)^{k}. The ratio, V_{c}^{2}R_{g}^{−1}, is defined by units of Å^{3} with mass in Daltons. Additional ratios examined (green, cyan, grey and red) displayed asymmetric nonlinear relationships.