[Congress Report]Macromolecular phasing

Macromolecular crystallographers are united by problems that plague the molecular replacement technique (multiple copies of a molecule, high crystallographic or non-crystallographic symmetry, and low homology). J. Navaza discussed combating the problem of high non-crystallographic symmetry, by exploiting the prior information available from self-rotation and Patterson functions. In complementary work, R. Read showed that likelihood functions give a much clearer indication of the correct answer than conventional scores, particularly for poor or incomplete models. Two talks described phase information that can be gained by averaging multiple images of a structure, either through noncrystallographic symmetry or multiple crystals. K. Cowtan described combining data from a crystal at room temperature and the same crystal at cryogenic temperatures, showing how changes in cell dimensions can give useful phase information. J. Wang presented an example where 14-fold non-crystallographic symmetry was used to generate an excellent map from nearly random phases. Finally, Q. Shen gave a lucid explanation of 3-beam X-ray diffraction and the use of a reference beam geometry to collect 3-beam data. In favourable cases, the resulting profiles give clear indications of triplet phases, which can be used in conjunction with direct methods programs to give clear electron density maps.

Randy J. Read