Analysis of Radiation Damage in Lysozyme Crystals with High Resolution Triple Axis X-Ray Diffraction
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ANALYSIS OF RADIATION DAMAGE IN LYSOZYME CRYSTALS WITH HIGH RESOLUTION TRIPLE AXIS X-RAY DIFFRACTION Richard J. Matyi1 and Heather M. Volz2 Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899 2 Materials Science Program, University of Wisconsin, Madison, WI 53706 (current address: IBM Microelectronics, Essex Junction, VT 05452) 1
ABSRACT High resolution triple axis X-ray diffraction has been used to monitor the effects of X-ray radiation damage in hen egg white lysozyme crystals. At long irradiation the expected decrease in peak intensity and increase in the angular extent of the peak breadth was seen. In contrast, both the intesities and peak breadths exhibited more complex behavior during the initial stages of irradiation. Possible reasons for these observations are discussed. INTRODUCTION Much of our knowledge of the structure of protein and related macromolecules has come from X-ray diffraction analyses. Typically a protein is crystallized; the crystal is irradiated, and the angular positions and intensities of the diffracted beams are recorded. By measuring the intensities of a large number of reflections it is possible to describe in detail the atomic structures of protein and other biologically important molecules [1]. Unfortunately, protein crystals are notorious for their high densities of structural defects. The resultant crystalline imperfection has a very serious effect on subsequent analyses of molecular structure, since the displacements caused by defects reduce the intensities of the diffracted beams generated by the crystal. As fewer reflections are measured, the accuracy in determining atomic positions within the protein molecule is degraded. X-ray scattering methods with high angular resolution have a successful history for the analysis of structural defects in both nearly perfect and highly defective single crystals. In the case of protein crystals, both double axis rocking curves [2-6] and X-ray topography [7-10] have been used for defect characterization. Recently, we have reported studies of protein crystals using high resolution triple axis X-ray diffraction (HRTXD) [11-13]. As shown in Figure 1, crystal defects will alter the characteristics of a reciprocal lattice vector Hhkl (and hence the intensity distribution about a reciprocal space point) in two principal ways. First, strains and/or compositional variations will change the lattice parameter of the sample that will be manifested in a change in the length of the reciprocal lattice vector (equal to 1/dhkl). This will result in a redistribution of the intensity away from the exact Bragg condition in the θ/2θ direction (parallel to the direction of the reciprocal lattice vector in the analyzer monochromator detector
Hhkl So/λ X-ray source
sample
Hhkl
Hhkl
S/λ
+δd
-δd
Figure 1. The configuration of a high resolution triple axis X-ray diffraction experiment (left) and the effects of strains (middle) and misorientations (right) on a reciprocal lattice point. FF2.4.1
symmetric geometry shown in
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