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.The significantly improved beam characteristics provided by the SPEAR3 upgrade, in particular improved beam brightness and stability, provide an excellent opportunity for revisiting some of the highly challenging x-ray scattering studies in structural biology. Recent advances in the use of artificially introduced anomalous scatterers, namely SeMet and Au-clusters primarily developed for single crystal diffraction studies, have brought renewed interests in anomalous solution scattering studies, which has had limited success in the past using naturally occurring anomalous scattering elements. As the first feasibility study at SSRL in nearly two decades, we conducted anomalous protein solution scattering experiments on horse spleen ferritin at the Fe K absorption edge (7113 eV). We successfully detected the anomalous scattering effect, reproducing the principal results reported by Stuhrmann (Acta Cryst A36, 996-1001, 1980). Briefly described, we recorded clear deflection in the radius of gyration (Rg) to 37.6 at the K edge from 36 , consistent with the higher contrast of Fe species which are localized in the periphery of the molecule. This also demonstrates that the energy resolution of our monochromator and the energy stability of ca 0.6 eV over several hours are adequate for anomalous scattering studies. Our result is consistent with the published values of 44 and 42 . We interpret the difference in Rg values due to a higher level of interparticle interference caused by the higher protein concentration in our experiments. We are encouraged by this result, and plan on studying more challenging protein systems at more dilute metal concentrations. We are in the process of incorporating the Radiant Detector Technology s Vortex-EX energy resolving detector (purchased at the end of the last grant year) for absorption edge measurements in fluorescence mode.
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