Patterson-map interpretation with noncrystallographic symmetry

Abstract

The interpretation of difference Patterson maps for heavy-atom positions is an important step in the structure determination of biological molecules. In the absence of a large number of noncrystallographically related subunits, the first step is usually a Harker-vector search to locate single-site solutions, which is followed by a cross-vector search to locate two-atom solutions and subsequent searches to locate more sites. Crystals of biological macromolecules frequently contain assemblies obeying noncrystallographic symmetry. The information about their noncrystallographic symmetry can be incorporated in search procedures to aid the Patterson-map interpretation. In the presence of noncrystallographic symmetry, a self-vector search is useful for locating the positions of heavy atoms relative to the center of the noncrystallographic symmetry point group and a cross-vector search is then used to locate that center in the crystal unit cell. An automated procedure is presented which is a generalized Patterson search method.