International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F, ch. 9.1, p. 177   | 1 | 2 |

Section 9.1.1. Introduction

Z. Dautera* and K. S. Wilsonb

aNational Cancer Institute, Brookhaven National Laboratory, NSLS, Building 725A-X9, Upton, NY 11973, USA, and bStructural Biology Laboratory, Department of Chemistry, University of York, York YO10 5DD, England
Correspondence e-mail:  dauter@bnl.gov

9.1.1. Introduction

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X-ray data collection is the central experiment in a crystal structure analysis. For small-molecule structures, the availability of intensity data to atomic resolution, usually around 0.8 Å, means that the phase problem can be solved directly and the atomic positions refined with a full anisotropic model. This results in a truly automatic structure solution for most small molecules.

Macromolecular crystals pose much greater problems with regard to data collection. The first arise from the size of the unit cell, resulting in lower average intensities of individual reflections coupled with a much greater number of reflections (Table 9.1.1.1[link]). Secondly, the crystals usually contain considerable proportions of disordered aqueous solvent, giving further reduction in intensity at high resolution and, in the majority of cases, restricting the resolution to be much less than atomic. Thirdly, again mostly owing to the solvent content, the crystals are sensitive to radiation damage. Such problems have severe implications for all subsequent steps in a structure analysis. Solution of the phase problem is generally not possible through direct methods, except for a small number of exceptionally well diffracting proteins. The refined models require the imposition of stereochemical constraints or restraints to maintain an acceptable geometry. Recent advances, such as the use of synchrotron beamlines, cryogenic cooling and high-efficiency two-dimensional (2D) detectors, have made data collection technically easier, but it remains a fundamental scientific procedure underpinning the whole structural analysis. Therefore, it is essential to take the greatest care over this key step. The aim of this chapter is to indicate procedures for optimizing data acquisition. Overviews on several issues related to this topic have been published recently (Carter & Sweet, 1997[link]; Turkenburg et al., 1999[link]).

Table 9.1.1.1| top | pdf |
Size of the unit cell and number of reflections

CompoundUnit cellReflectionsAverage intensity
Edge (Å)Volume (Å3)
Small organic 10 1000 2000 1
Supramolecule 30 25000 30000 1/25000
Protein 100 1000000 100000 1/1000000
Virus 400 100000000 1000000 1/100000000

References

Carter, C. W. Jr & Sweet, R. M. (1997). Editors. Methods in enzymology, Vol. 276, pp. 183–358. San Diego: Academic Press.
Turkenburg, J., Brady, L., Bailey, S., Ashton, A., Broadhurst, P. & Brown, D. (1999). Editors. Data collection and processing. Proceedings of the CCP4 study weekend. Acta Cryst. D55, 1631–1772.








































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