Laue Diffraction from Protein Crystals: Theoretical Aspects
If a stationary crystal is illuminated by a polychromatic beam of x-rays or neutrons, then a Laue diffraction pattern results. Each Laue reflection arises from a central line (a ray) in reciprocal space, which may contain one or more reciprocal lattice points (RLPs) with associated structure factors. It has been clear since the first protein x-ray Laue patterns were obtained (Moffat et al., 1984, 1986; Bilderback et al., 1984; Helliwell, 1984, 1985; Hedman et al., 1985) that the Laue technique possesses a number of advantages. It makes optimum use of a naturally polychromatic spectrum such as that emitted by a bending magnet, wiggler or undulator on a synchrotron or storage ring source. Very brief exposure times in the millisecond range have been obtained on strongly scattering protein samples (Bilderback et al., 1984; Hajdu et al., 1986; Moffat et al., 1986). A stationary crystal yields integrated intensities directly, which are much less sensitive to transient changes in unit cell dimensions or crystal orientation than the (partial) intensities obtained by diffraction of monochromatic radiation. A typical Laue diffraction pattern contains many more reflections than a typical monochromatic, oscillation pattern. These advantages are particularly appropriate for dynamic experiments, in which the diffraction intensities change rapidly with time in response to a structural perturbation (Moffat et al., 1984, 1986; Helliwell, 1985).
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