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 seleno-methionine 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.