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International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 9.1, p. 194
Section 9.1.14. The importance of low-resolution data
a
National 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 |
The low-resolution terms define the overall shape of the object irradiated in the diffraction experiment. Omission of the low-resolution reflections, especially those with high amplitude, considerably degrades the contrast between the major features of the object and its surroundings. For a macromolecule, this means that the contrast between it and the envelope of the disordered aqueous solvent is diminished and, furthermore, the continuity of structural features along the polymeric chain may be lost. Refinement and analysis of macromolecules at all resolutions, be it high or low, involves the inspection of electron-density syntheses. These can be interpreted visually, on a graphics station, or interpreted automatically with a variety of software. In all of these, at all resolutions, the importance of the low-resolution terms is crucial. A special problem is in the interpretation of the partially ordered solvent interface. The biological activity of most enzymes and ligand-binding proteins is located precisely at this interface, and for a true structural understanding of how they function this region should be optimally defined. This is seriously impaired by the absence of the strong, low-resolution terms. The problems become more severe as the upper resolution limit of the analysis becomes poorer. Thus at 1 Å resolution, the omission of the 7 Å data shell will have less effect compared with a 3 Å analysis – but remember that ideally, no low-resolution data should be omitted!
In some phasing procedures, the presence of complete, especially high-intensity, low-resolution, data is even more crucial. The big, low-resolution amplitudes dominate the Patterson function and methods based on the Patterson function are therefore especially sensitive. This encompasses one of the major techniques of phase determination for macromolecules: molecular replacement. Direct methods of phase determination utilize normalized structure factors and predominantly exploit those of high amplitude. The relations between the phases of those reflections with high amplitudes, such as the classical triple-product relationship, are strongest and most abundant for reflections with low Miller indices, hence at low resolution.
The importance of the low-resolution reflections in terms of geometric and qualitative context cannot be overemphasized.
