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Review
. 2012 Oct;22(5):602-12.
doi: 10.1016/j.sbi.2012.09.001. Epub 2012 Sep 26.

Micro-crystallography comes of age

Janet L Smith  1 , Robert F Fischetti, Masaki Yamamoto
Affiliations
Review

Micro-crystallography comes of age

Janet L Smith et al. Curr Opin Struct Biol. 2012 Oct.

Abstract

The latest revolution in macromolecular crystallography was incited by the development of dedicated, user friendly, micro-crystallography beam lines. Brilliant X-ray beams of diameter 20 μm or less, now available at most synchrotron sources, enable structure determination from samples that previously were inaccessible. Relative to traditional crystallography, crystals with one or more small dimensions have diffraction patterns with vastly improved signal-to-noise when recorded with an appropriately matched beam size. Structures can be solved from isolated, well diffracting regions within inhomogeneous samples. This review summarizes the technological requirements and approaches to producing micro-beams and how they continue to change the practice of crystallography.

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Figures

Figure 1
Figure 1. Approaches to making a small beam for micro-crystallography
(a) Aperture approach. The full central cone from the undulator source is focused by the mirrors and the beam size is defined with an aperture near the sample, typically 30 mm upstream. (b) Direct focus approach. Short mirrors near the sample capture a portion of the central cone from the undulator source and focus the beam directly to the sample position. (c) Divergence-limited source approach. A portion of the undulator central cone is selected by slits, which are imaged at the sample position by micro-focusing optics near the sample. (d) Secondary source approach. Upstream mirrors focus the full central cone of the undulator onto a set of slits. A second set of optics near the sample images the slits to the sample position. The beam direction is right to left. Focusing optics are depicted as blue ellipses, the source and focal points as red dots, and apertures and slits as black bars.
Figure 2
Figure 2. Micro-crystallography endstation at APS beamline 23ID-B
(a) Sample environment. The sample is mounted on an air-bearing goniometer with 1-μm peak-to-peak SOC [34]. The mini-beam quad collimator is positioned downstream of the high-resolution microscope that views the sample along the beam axis. The active beam stop is mounted from the right. A fluorescence detector is mounted in the upper left. The sample cryo-cooler, usually mounted in the upper right, is removed in this photograph. (b) Mini-beam quad collimator. In this schematic, the beam vector passes through the scatter guard aperture. Apertures in the other beam paths define 5-, 10- and 20-μm diameter beams. (c) Active beamstop. The beamstop is shown before (left) and after (right) assembly. X-rays impinge on the beamstop and eject photoelectrons, which yield a current that is linearly proportional to the incident intensity [34].
Figure 3
Figure 3. SPring-8 BL32XU micro-focus beamline
(a) Focused beam profile at the sample position. The horizontal (left) and the vertical (right) beam profiles are from a wire scan. The FWHM of the focused beam is 0.9 μm in both directions. (b) High-precision air-bearing Kohzu QKSU-1 goniometer, shown in a schematic drawing (top) and photograph (bottom). The QKSU-0 goniometer is installed on beamlines BL17 and BL01 at the Photon Factory, and beamlines BL41XU and BL32XU at SPring-8. (c) Plot of the QKSU-1 SOC through 360° rotation. The displacement of a precise ball on the horizontally mounted goniometer was measured with a capacitive sensor. (Drawing and inspection data courtesy of Kohzu Precision Co., Ltd.)
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