Bead-model calculation of scattering diagrams: Brownian dynamics study of flexibility in immunoglobulin IgG1.

We consider the calculation of the angular dependence of the scattering intensities for models composed of spherical elements of arbitrary size, in which some of the spheres may have a size close to that of the whole particle. This minimizes the number of spheres and makes it possible to use the same bead models for the prediction of scattering diagrams and hydrodynamics properties. A simple expression is employed for the scattering intensity. Particularly, we discuss the validity of some versions of the Debye scattering equation for bead models. The method could be used for any macromolecule either rigid or with conformational variability. The application of the method to a simple model for IgG1 shows that influence of flexibility in the scattering curve is stronger by the end of the first decade of decay and also shows that the range of linearity in the Guinier region is not a good method to characterize flexibility between arms, because it is very likely that experimental errors will hide the small differences between the extreme cases appreciated in our calculations.