X-ray Pulses Yield Protein And Metal Data

The study of metal-containing proteins, such as the complex that mediates the light reaction in photosynthesis, has a new tool: Femto­second X-ray pulses facilitate simultaneous diffraction and spectroscopy analyses of metalloproteins, enabling study of radiation-sensitive, difficult-to-crystallize systems at room temperature (Science, DOI: 10.1126/science.1234273). The approach will promote time-resolved chemical dynamics studies of metalloproteins, say research team leaders Vittal K. Yachandra and Junko Yano of Lawrence Berkeley National Laboratory and Uwe Bergmann of SLAC National Accelerator Laboratory. The technique builds on femtosecond X-ray diffraction—in which ultrashort X-ray pulses are trained on a stream of protein microcrystals—by adding an X-ray emission detector to get information on metal atom oxidation and spin states. The short pulses enable data collection before radiation damages the protein. The group studied photosystem II, a membrane-bound protein found in plants. Photosystem II uses light to convert H₂O to O₂ through redox cycling of a Mn₄CaO₅ cluster. The researchers were able to obtain diffraction and spectral data on two states of the protein, “dark” and “illuminated.” They generated the illuminated state by irradiating the crystals with a laser just before the X-ray pulses.