Automatic Solution of Heavy-Atom Substructures

The determination of heavy-atom substructures using multiwavelength anomalous dispersion (MAD) or multiple isomorphous replacement (MIR) data is a straightforward, although often lengthy, process. SOLVE is designed to automate fully the analysis of such data. The MAD and MIR approaches to structure solution are conceptually similar and share several important steps. In each method, trial partial structures for heavy or anomalously scattering atoms are often obtained by the inspection of difference-Patterson functions or by semiautomated analysis. These initial structures are refined against the observed data and are used to generate initial phases. Then, additional sites and sites in other derivatives can be found from weighted difference or gradient maps using these phases. SOLVE prepares data for a heavy-atom substructure solution in two steps. First, the data are scaled using the local scaling procedure of Matthews and Czerwinski. Second, MAD data are converted to a pseudo-single isomorphous replacement with anomalous scattering (SIRAS) form that permits more rapid analysis. In many cases, an analysis of heavy-atom sites by the sequential deletion of individual sites or derivatives is also an important criterion of quality.