Single-wavelength anomalous diffraction phasing revisited

Multiwavelength anomalous diffraction (MAD) phasing has become a routinely used tool for determining new macromolecular structures. The MAD method has stringent data‐­collection requirements, typically necessitating radiation‐resistant crystals and access to a tunable synchrotron beamline. In cases where synchrotron time, monochromator tunability or radiation damage is a concern or where high‐throughput structure determination is desired, phasing methods capable of producing interpretable electron‐density maps from less data become attractive alternatives to MAD. The increasing availability of tunable synchrotron data‐collection facilities prompted the authors to revisit single‐wavelength anomalous diffraction (SAD) phasing used in conjunction with a phase‐ambiguity resolving method such as solvent flattening. The anomalous diffraction from seven different selenomethionine‐labelled protein crystals has been analysed and it is shown that in conjunction with solvent flattening, diffraction data from the peak anomalous wavelength alone can produce interpretable electron‐density maps of comparable quality to those resulting from full MAD phasing. Single‐wavelength anomalous diffraction (SAD) phasing can therefore be a time‐efficient alternative to MAD. The data also show that radiation damage can have a significant effect on the quality of SAD/MAD diffraction data. These results may be useful in the design of optimal strategies for collection of the diffraction data.