The model resolution function - a technique for estimating the quality of approximation of particles by models in small-angle X-ray or neutron scattering

Although the quality of a structure model obtained from small-angle X-ray or neutron scattering curves for polymers can be determined qualitatively by comparing the isotropic scattering curve calculated for the model with the experimental scattering data for a solution of polymer molecules, other methods are needed for a more precise evaluation. A model resolution function has been defined to permit quantitative comparisons. With this function, the quality of the approximation can be assessed, and the structure resolution can be determined. An overinterpretation of scattering curves by use of complex but uniform-density models can thus be avoided. Furthermore, the value of the Porod volume calculated from the scattering data has been found to depend strongly on the interval in which the scattering data are recorded or selected for evaluation. The calculations with the atomic model curves showed that it is impossible to compute physically meaningful values of the hydration of the molecules from the Porod volume and the dry volume by use of extrapolated scattering curves with an insufficient resolution. The theory of the model resolution function and the interpretation of the Porod volume have been verified and tested with experimental scattering curves from solutions of RNA molecules.