This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Although there is evidence that the energy chosen for data collection does not have a great deal of effect in the global quality of the data [Shimizu, Hirata, Hasegawa, Ueno &Yamamoto (2007) J. Synchrotron Radiat. 14 4-10], the effects of local radiation damage at different energies have not been sufficiently studied. Experiments were undertaken to study the reduction rate of disulfide bridges at 9 and 14.5 keV on HEWL lysozyme. The damage to disulfide bridges happens relatively quickly and it is easy to observe and quantify, and the radiation damage mechanism is thought to be the same as for other elements. Measurements from different crystals indicate that the rate of reduction per dose unit of the disulfide bridges is site depending, with more exposed sites showing an increased rate of decay. Careful analysis of the data suggest that the site decay per dose unit is overall slightly faster at the higher energy. The experiment was repeated for a Se-MET substituted nucleotide binding protein from methylobacillus Flagellatus. The decay for the Se-MET sites also appears faster at higher energy, but in this case the trend was not so clear and could be attributed to noise in the data. These results point to a flaw in the way dose is calculated for crystal diffraction experiments and provide the path for future developments.
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