In crystal structure determination

Fig. 2 Schematic diagram showing the sequence of stages involved in crystal structure determination from powder XRD data...

The labour involved in a three-dimensional synthesis is very great, except for the simplest structures for this reason (and others which will appear later), the two-dimensional synthesis, giving a view of a projection of the structure, is most often used in crystal structure determination. No synthesis can be carried out, however, unless the phase angles of the reflections with respect to a reference point in the structure are known. We now consider under what circumstances it is possible to be sure of the phase angles from the start, or to deduce them from crystallographic evidence. 

Finally one parameter in XPS, which so far has been neglected to a great deal, should be mentioned briefly the angular distribution of the emitted photoelectrons. Studies on sodium chloride single crystals 165 178> showed promising results and might open a new field of application in crystal structure determinations. 

Application of space group symmetry in crystal structure determination... 

This paper was to have been prepared by Walter C. Hamilton. It is a matter of great sadness to us all that he has been taken from us. Aside from our personal loss, his absence has been a substantial scientific loss to this conference. I cannot feel that I am in any real sense a replacement for him. My paper by no means covers all that Walter Hamilton would have included, in view of his unique position at the forefront of many areas of structural crystallography as well as my own relative detachment from direct technical involvement in crystal-structure determination over the past few years. 

Water molecules are an essential aspect of protein structures. Without knowledge of the location of all tightly bound waters, many aspects of the structure, function, and stability of proteins cannot be properly studied. Unfortunately, waters are often abused in crystal structure determination. From the kinds of errors we detect, we assume that some crystallographers, shortly before they manually place waters in the density map, use software that places a water molecule close to each alpha carbon (in a well-determined structure there is about 1 bound water molecule visible per residue). Unfortunately, it regularly occurs that a few waters that are not moved to another position are forgotten, and remain part of the structure. 

The introduction of CCD area detectors into many laboratories engaged in crystal structure determination as replacements for scintillation detectors has resulted in a major change in structure determination by single crystal X-ray diffraction, which accounts for most of the structures represented in Figures 1 and 2. CCD detectors are more sensitive than scintillation detectors, permitting data to be collected on smaller crystals. The current generation of CCD detectors permits data collection and structure determination in favorable cases for crystals with dimensions of 10-100 xm, whereas 200-500 xm were more typical crystal dimensions for use with scintillation detectors. As area detectors permit the simultaneous measurement of many diffraction intensities rather than sequential measurement... 

The first, and earliest, application of CSP was to aid in crystal structure determination of an nncharacterized material, combined with experimental observations that, on their own, are insufficient to provide a structure. The idea is that CSP calculations can provide a set of plausible, energetically feasible structures that could be used as starting models to refine against experimental data. Under the assumption that the set of predicted structures is complete, and sufficiently accurate that the experimental... 

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