Space groups data collection

The molecular structure of 1,2,9,10-tetragerma[2.2]paracyclophane 17 was determined by the X-ray diffraction study. The single crystals of 17 for X-ray crystallography were obtained from a toluene solution. Similar to 12, the crystal belongs to the space group P2Jn, and the data collection was carried out at 13°C. The ORTEP drawing of 17 is shown in Fig. 7. 

ELVES has been developed as an expert system, by James Holton and Tom Alber, to go from data collection frames to structure without human intervention and will obviate the need for intermediate space-group determination described above. Very recently, 12 different European sites have been collaborating to develop a software package known as DNA (automateD collectioN of datA) for the automatic collection and indexing of macromolecular diffraction data. Further information is available at the web site www.dna.ac.uk. 

The cubic space group Pm3m (no systematic absences) was chosen for X-ray diffraction studies for reasons previously cited (9). Preliminary crystallographic experiments and subsequent data collection were performed at 2A°C with an automated, four-circle Syntex PI diffractometer, equipped with a graphite monochromator and a pulse-height analyzer. Molybdenum radiation was used for all experiments (Ka, X 0.70930 a K 2 ... 

As mentioned earlier, the unit-cell space group can be determined from systematic absences in the the diffraction pattern. With the space group in hand, the crystallographer can determine the space group of the reciprocal lattice, and thus know which orientations of the crystal will give identical data. All reciprocal lattices possess a symmetry element called a center cf symmetry or point of inversion at the origin. That is, the intensity of each reflection hkl is identical to the intensity of reflection -h k -1. To see why, recall from our discussion of lattice indices (Section II.B) that the the index of the (230) planes can also be expressed as (-2 -3 0). In fact, the 230 and the —2 -3 0 reflections come from opposite sides of the same set of planes, and the reflection intensities are identical. (The equivalence of Ihkl and l h k l is called Friedel s law,but there are exceptions. See Chapter 6, Section IV.) This means that half of the reflections in the reciprocal lattice are redundant, and data collection that covers 180° about any reciprocal-lattice axis will capture all unique reflections. 

Crystals and X-ray Data Collection. Detailed information concerning protein purification, crystallization, and X-ray data collection can be found in a previous report (Xu et al., 1991) and will be mentioned here in summary form. Recombinant murine apo-ALBP crystallizes in the orthorhombic space group P2j2i2i with the following unit cell dimensions a = 34.4 A, b = 54.8 A, and c = 76.3 A. The asymmetric unit contains one molecule with a molecular weight of 14,500. The entire diffraction data set was collected on one crystal. In the resolution range t -2.5 A, 5115 of the 5227 theoretically possible reflections were measured. Unless otherwise noted the diffraction data with intensities greater than 2a were used for structure determination and refinement. As can be seen in Table 8.2, this included about 96% of the measured data. 

The crystal structure of saccharin was determined (3) by 3-dimensional integrated intensity data collected on a computer-controlled diffractometer operated by an IBM 1620, in a closed-loop manner. The crystals were found to be monoclinic with a 9 552 3, b 6.919 3, c 11.803 + A, and B 103 9°, and the space group is p2] /c. 

Because of the chemical implausibility of some speculations about zeolite A, high-precision X-ray analyses were made of dehydrated crystals whose chemical composition was checked by electron microprobe analysis. Data were collected for the 24 A superstructure. All diffractions are consistent with space group Fm3c except for a few weak ones. Unpublished measurements by J.J. Pluth and G.D. Price of a hydrated Na-A (i.e. as-synthesized) showed strict obeyance of Fm3c except for a very weak (111) diffraction, and the inconsistent diffractions of the dehydrated crystals are attributed tentatively to minor positional disorder of extra-framework cations (1). 

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