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Stanford Synchrotron Radiation Laboratory, SLAC, Stanford University, Stanford, CA 94305 Polarized XAS of specifically oriented crystals offers a wealth of molecular and electronic structure information unavailable in un-oriented/isotropic systems. By alignment of molecular vectors to the e-vector of the x-ray beam, specific electronic transitions or EXAFS interactions can be emphasized or minimized. The angle-dependence of the polarized XAS signals allow direct electronic-molecular structure correlations and the precision of XAS distance determinations provide more detailed structural information for metallo-protein active sites than can be provided by crystallography alone.
Development of single crystal XAS instrumentation, which allows nearly
simultaneous XAS and crystallographic data collection, has been carried out at
SSRL BL9-3, the 16-pole 2T wiggler beam line 9 side station dedicated to
general user biological XAS. The implementation includes a Huber kappa
goniometer, Canberra 30-element Ge detector for XAS data collection, open-flow
He and N2 coolers, microscope for crystal alignment in the beam, and a MAR CCD
or MAR 345 crystallography detector. The kappa goniometer allows a large
accessible angle range with an open geometry affording access to detectors and
open stream coolers. Applicable standard hardware on SSRL crystallography beam
lines has been incorporated with crystallographic data collection controlled
via the Blu-Ice software developed by the SSRL SMB macromolecular
crystallography group. XAS data collection is handled through the SSRL
standard XAS-Collect software. Initial diffraction and XAS data from single
crystals using an open flow cryostat will be presented. The instrument will be
available to general users after the SPEAR3 upgrade in early 2004.
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