Connectivity-based ab initio phasing: from low resolution to a secondary structure.

The connectivity-based phasing method currently allows ab initio determination of phases for several hundred reflections. In the case of large macromolecular crystals, these reflections correspond to a very low resolution and the structural information deduced essentially consists of the molecular packing and an approximate molecular envelope. However, when the unit cell is relatively small, such a phasing procedure can produce phases such that secondary-structure elements can be identified in the corresponding maps. In the case of the pheromone Er-1, all three alpha-helices present are seen in the ab initio phased maps. In the case of protein G, not only the alpha-helix but also some individual beta-strands are distinguishable.