Single Isomorphous Replacement With Anomalous Scattering Research Articles

30] Multiwavelength anomalous diffraction phasing of macromolecular structures: Analysis of MAD data as single isomorphous replacement with anomalous scattering data using the MADMRG program

Publisher Summary Multiwavelength anomalous diffraction (MAD) technique is a widely used and important tool in macromolecular structure determination. In the MAD technique, intensities of Bijvoet pairs of reflections are measured at several X-ray wavelengths, near to and distant from an absorption edge for an anomalously scattering atom. The anomalous and dispersive differences among measurements of a reflection at several wavelengths, therefore, contain information about the structure of the anomalously scattering atoms, as well as the relation between the anomalously scattering atoms and the remainder of the structure. Several methods for the analysis of MAD data have been developed. An alternative approach is to make use of the similarity between the MAD and isomorphous replacement experiments. MAD data are either treated as if they are multiple isomorphous replacement with anomalous scattering (MIRAS) data or converted into a form that is equivalent to that obtained in a single isomorphous replacement with anomalous scattering (SIRAS) experiment. In either case, the MAD data are then employed for phasing using the robust techniques developed for the method of isomorphous replacement. The chapter discusses an approach for the conversion of MAD data to a form compatible with SIRAS analysis.