A comparison of SAD and two-wavelength MAD phasing for radiation-damaged Se-MET crystals

Although a case has been made that single-wavelength anomalous dispersion (SAD) is the optimal strategy for data collection in the presence of radiation damage, two-wavelength MAD experiments at the inflection and a high-energy remote point of the absorption edge have been shown to be a potentially successful alternative method. In order to further investigate the performance of both data collection strategies, a comparison of SAD and MAD phasing was carried out for increasingly damaged data sets from three different selenomethionine protein samples collected under similar experimental conditions. In all but one example the MAD phases appeared to be less affected than SAD phases with increasing exposure to X-rays, and had a better overall success rate, indicating that this method should be given serious consideration when dealing with radiation-sensitive crystals. Simultaneous data collection in wedges at all wavelengths seems to be a very important factor in the success of MAD experiments; the decreased absorbed dose resulting from eschewing data collection at the maximum f '' wavelength may play a less important role. Specific radiation damage to the selenium atoms is found to be a minor effect compared with the effect on the anomalous dispersion signal, although potentially large enough to be a useful contribution to phasing in both SAD and MAD experiments.