This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. In preparation for experiments at the LCLS involving measurement of correlated scattering by proteins, we propose to immobilize protein molecules in a trehalose class and use a pinhole to increase the transverse coherence of the storage ring x-rays. Correlated X-ray Scattering (CXS) involves determination of scattering profiles in which two photons scatter from the same molecule at different times leading to the 4-point correlation function C4(q,q',cos(theta))= - I(q')>from which structural details may be extracted by suitable non-linear fitting. CXS measurements at the LCLS look at scattering from protein-containing droplets in which the scattering occurs in the first 20 fs before Brownian rotations can decorrelate the two-photon events. In preparation for LCLS experiments we here propose to test the ideas by immobilizing a protein in a trehalose solution which forms a glass at room temperature. To measure the correlated x-ray scattering (CXS) one needs an x-ray beam with sufficient transverse coherence that incident photons arriving at different times occupy a small enough solid angle so that the correlations are not washed out by subtraction of the product of the averages x . At a storage ring source the needed transverse coherence is obtained by placing a pinhole in the beam. This cuts down the incident flux, resulting in lowering the rate of generating correlated 2-photon scattering events. Nevertheless we expect to learn a lot about the statistics of CXS measurements and their potential for determining structural changes as conditions such as substrate binding, are altered.
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