USE OF HARKER SECTIONS IN A DENSITY MODIFICATION METHOD FOR DIRECT SOLUTIONS OF CRYSTAL STRUCTURES. By M.Shiono, Department of Physics, Kyushu University, Higashi-ku, Fukuoka, Japan

A real space phase refinement procedure has been developed, which is based on elimination of negative density (Shiono et al. (1992) Acta Cryst. A48, Refaat et al. (1993) Acta Cryst. A49). This method can produce ab initio solutions of crystal structures by employing multi-solution procedure. In order to give more power to this method, modified Patterson-Harker Sections are used as a mask of electron density.

Harker sections include self-interatomic vectors of component atoms and this fact gives useful informations to us for solving crystal structures. However, if the structure is complex, it is very difficult to interpret Harker sections since self-interatomic vectors seriously overlap each other and, in addition, non-self-interatomic vectors may appear in the Harker sections. For above reason, instead of searching self-interatomic vectors, we find regions in which electron density is 0 or very small. In order to use this information as a constraint of density modification, Harker sections are modified as described in following example.

The method has been applied to ab initio phase determination of a protein, Ribonuclease Apl (Space group P2_l). For this space group, the Harker section appears at y=1/2 and self-interatomic vectors are (2x,1/2,2z). The 2-dimensional mask is constructed as follows. Firstly, Harker section is calculated from all observed E's. Secondly, the Harker section is shrunk to 1/4 in area and then 4 pieces of them are placed in the original size of plane. We thus get a section which contains 4 shrunk Harker sections. Finally, negative density in the section is set to 0. This protein has been solved by iterative use of this section as a mask of density.