SASSIM:: a method for calculating small-angle X-ray and neutron scattering and the associated molecular envelope from explicit-atom models of solvated proteins

A method is presented to calculate efficiently small-angle neutron and X-ray solution scattering intensities from explicit-atom models of macromolecules and the surrounding solvent. The method is based on a multipole expansion of the scattering amplitude. It is particularly appropriate for extensive configurational averaging, as is required for calculations based on computer-simulation results. In test calculations, excellent agreement with experiment is found between neutron and X-ray scattering profiles calculated from a molecular-dynamics simulation of lysozyme in water. The question of definition of the protein surface is also addressed. For comparison with the continuum model, an analytical envelope around the protein is defined in terms of spherical harmonics and is calculated using a Lebedev grid. The analytical surface thus defined is shown to reproduce well the scattering profile calculated from the explicit-atom model of the protein.