Comparison of vector search and feedback methods for finding heavy-atom sites in isomorphous derivatives

Heavy-atom sites in an isomorphous replacement derivative are usually found by inspection of a difference Patterson map. A systematic search procedure is necessary in the presence of high non-crystallographic symmetry as in a crystalline virus. A reciprocal-space equivalent of the Patterson search procedure has been developed. Furthermore, it is shown that the Patterson search is closely analogous to the usual 'feedback' tests applied in checking a proposed site. The separation of self and cross vectors in the Patterson map is shown to have its equivalent in reciprocal space. The procedure was used to identify the heavy-atom sites and the particle position in crystals of a human common cold virus.