[37] Patterson superposition and ab initio phasing

This chapter presents the possibility of the ab initio solution of macromolecular structures, given two unusually favorable circumstances: the availability of very high-resolution data and the presence of one or more heavier atoms, such as transition metals or even sulfur. Under ab initio, the solution of a structure from the single-wavelength native data can be understood without the use of a known structure as a search model. Automated heavy-atom location via the Patterson function provides an efficient approach in small-molecule crystallography. For unequal atom structures involving atoms on special positions or pseudosymmetry, Patterson-based methods are often more effective than direct methods. In the case of macromolecules, the Patterson approach can be used to locate the heavier atoms, which then provide slightly better than random phases for partial structure expansion by direct or indirect methods.