Crystallography data quality

Despite these limitations, the accuracy with which EXAFS can determine M-L bond distances for metalloproteins is greater than in the case of protein X-ray crystallography. High quality EXAFS data can typically be obtained in about 12 hours of scanning using solutions of 2-3 mM in the absorber atom. This compares very favorably with the days to weeks of data collection necessary... 

In HREM images of inorganic crystals, phase information of structure factors is preserved. However, because of the effects of the contrast transfer function (CTF), the quality of the amplitudes is not very high and the resolution is relatively low. Electron diffraction is not affected by the CTF and extends to much higher resolution (often better than lA), but on the other hand no phase information is available. Thus, the best way of determining structures by electron crystallography is to combine HREM images with electron diffraction data. This was applied by Unwin and Henderson (1975) to determine and then compensate for the CTF in the study of the purple membrane. 

A fully automated protein crystallography beamline at a third-generation synchrotron source can screen several hundred crystals daily. Automatic evaluation of the diffraction images to ascertain crystal quality is, therefore, a critical step for high-throughput data acquisition. Evaluation of each image requires software that mimics the traditional visual assessment of crystal quality. 

In crystallography, unlike microscopy, the term resolution simply refers to the amount of data ultimately phased and used in the structure determination. In contrast, the precision of atom positions depends in part upon the resolution limits of the data, but also depends critically upon the quality of the data, as reflected by the R-factor. Good data can yield atom positions that are precise to within one-fifth to one-tenth of the stated resolution. 

The Crystallographic Information Format (CIF) developed by the International Union of Crystallography (IUCr) (Hall et al. 1991) is the best applied standard within chemistry and is an exemplar of the difficulties of standardization. The CIF is well accepted and understood by the community, is supported by publishers, and easy-to-use validation tools exist. However, ensuring that the resulting CIFs are valid and accurate requires additional quality assurance publishers of CIFs may carry this out (as, for example, the RSC does). So the development and adoption of standards is not sufficient to ensure that accurate and unambiguous data will be presented within the standards. 

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