[31] Treatment of multiwavelength anomalous diffraction data as a special case of multiple isomorphous replacement

This chapter discusses the treatment of multiwavelength anomalous diffraction (MAD) data as a special case of multiple isomorphous replacement (MIR). In macromolecular crystallography, the chief method for the determination of phases for a completely unknown structure is MIR; this method involves the collection of data from the crystals of a protein alone and crystals soaked in various heavy-atom compounds. If heavy atoms bind specifically to the protein, their locations can be identified, and the phase problem can be solved from the differences in the structure factors between the protein and its heavy-atom derivatives. These differences are referred to as “isomorphous differences.” The significant imaginary component of the atomic scattering factors of most heavy atoms leads to anomalous scattering in which Friedel pairs are nonequivalent. The measurement of the differences between these, known as “the Bijvoet differences,” leads to an additional source of phase information from heavy atoms. Thus, modern isomorphous replacement often involves the use of both isomorphous and Bijvoet differences from a heavy-atom derivative. The chapter presents a method in which MAD data is converted to a form that simulates MIR with isomorphous and anomalous differences at a single reference wavelength; this method also allows standard heavy-atom refinement programs to be used.