Some Applications of Dummy Point Scatterers for Phasing in Macromolecular X-Ray Crystallography

The purpose of the X-ray macromolecular crystallography is to determine the electron density distribution ρ(r) of the crystal and interpret it by atoms. ρ(r) may be calculated by a Fourier series with complex coefficients. Their magnitudes are available from X-ray-diffraction experiment, however an accurate calculation of ρ(r) is often impossible due to absence of estimates for corresponding arguments (phases) or their insufficient accuracy. To define or improve the phase estimates a model composed from 'dummy' scatterers may be used. The number and size of these scatterers depend on problem. At a conventional resolution the scatterers similar to carbon atoms are used for phase improvement. When phase information is not available models composed from a small number of large scatterers presenting whole molecular domains may be used. In another extreme case, at a subatomic resolution, scatterers presenting partial atomic charges may be used to model the density deformation.