This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.Radiation damage is a serious concern for Anomalous Dispersion phasing experiments because of the increased absorption by the sample at the typical wavelength of interest for MAD or SAD experiments. It has been found that data collection at the absorption edge inflection point and the remote wavelength in 10 degrees wedges allowed structure solution in the case of vinculin, where conventional three-wavelength MAD experiments and SAD at the maximum f energy failed. Subsequent experiments on different proteins hinted that two-wavelength MAD (using small oscillation wedges can indeed be the optimal strategy for phasing structures the novo. To confirm this result with one more case, an experiment was carried out on crystals of chicken Vinculin (tail fragment): Successive MAD and SAD data collections were carried from several crystals until substantial diffraction damage (substantial decrease of diffraction spots at high resolution was observed). The quality of the phases decreased, and the Selenium and Sulphur site decay increased as the total dose absorbed by the crystals increased. Two-wavelength MAD maps were of slightly better quality than the SAD counterparts, as found for other cases.
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