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 w here synchrotron time, monochromator tunability or radiation damage is a co ncern 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 revi sit single-wavelength anomalous diffraction (SAD) phasing used in conjuncti on with a phase-ambiguity resolving method such as solvent flattening. The anomalous diffraction from seven different selenomethionine-labelled protei n crystals has been analysed and it is shown that in conjunction with solve nt flattening, diffraction data from the peak anomalous wavelength alone ca n produce interpretable electron-density maps of comparable quality to thos e resulting from full MAD phasing. Single-wavelength anomalous diffraction (SAD) phasing can therefore be a time-efficient alternative to MAD. The dat a also show that radiation damage can have a significant effect on the qual ity of SAD/MAD diffraction data. These results may be useful in the design of optimal strategies for collection of the diffraction data.