Precession photograph

Figure 4.23 Precession camera with mounted goniometer head. To see a precession photograph, refer to Fig. 2.6. Photo courtesy of Charles Supper Company.

Diffractometer software can search for reflections, measure their precise positions, and subsequently compute unit-cell parameters. This search entails complexities we need not encounter here. Instead, I will illustrate the simplest method for determining unit-cell dimensions measuring reflection spacings from an orthorhombic crystal on a precession photograph. 

As discussed earlier, a precession photograph is an undistorted projection of the reciprocal-lattice points onto a flat film. Because reciprocal-lattice spac-... 

Figure 6.5 Precession photographs of the hk0 plane in native (a) and heavy-atom (b) crystals of the MoFe protein from nitrogenase. Corresponding underlined pairs in the native and heavy-atom patterns show reversed relative intensities. Photos courtesy of Professor Jeffrey Bolin.

The images of diffracted crystal lattices can be observed with specialized precession photographic equipment, although the modern day image plate detectors used in most laboratories produce a diffraction image that can be analyzed by computer to provide the indices of the lattice diffraction spots (Fig. ll.l,a-c). 

Some Properties of Xe(OSOtF)2.— Die Raman spectrum is compared with those of FXeOSOtF and StOoFt in Table III. Single crystals of Xe(OS02F)t were obtained by slow evaporation of a HF solution and fragmentary precession photograph data, from such a crystal, provided for the indexing of the X-ray powder data given in Table IV. The unit cell is monoclinic with a = 7.94, b = 13.7, c = 6.84 A, 0 = 96 . A small sample of Xe(OSOtF)2 under a dynamic vacuum at 20 slowly decomposed but none of the compound collected in the limbs of a U tube, cooled at —75°, provided to trap it. 

The previously determined unit-cell and space-group data were confirmed from precession photographs, the 6/wtnwt diffraction symmetry being further confirmed by careful intensity measurements with scintillation counters of several sets of symmetry-equivalent reflections. 

The crystal used for data coUection was transferred to an Enraf-Nonius CAD-4 diffractometer. Automatic peak search and indexing procedures yielded the same monoclinic cell as derived from the X-ray powder diffraction data and precession photographs. Testing showed that the cell was indeed primitive and that there was no superlattice present. Table 5 gives the crystal data and X-ray experimental parameters, and Table 6, the interatomic distances and angles. Positional and thermal parameters are given in Table S3 (Supporting Information)... 

Colorless crystals were formed upon sublimation of the micro-crystalline solid at 40-50 C in 0.7-mm-diameter quartz X-ray capillaries. These had been sealed under an atmosphere of nitrogen. Precession photographs indicated a primidve orthorhombic cell, space group Pnma or Pna2i. 

Single crystals of CgAsKs gave precession photographs showing hexagonal symmetry. There are no systematic absences, hence the unit cell Is primitive. The unit cell dimensions for C8AsF0 are a = 4.92(2) c =... 

Single-crystal precession photograph data for CgOsFe... 

Figure 4.51. Myoglobin Crystal and X-Ray. (A) Crystal of myoglobin. (B) X-ray precession photograph of a myoglobin crystal. [(A) Mel Pollinger/Fran Heyl Associates.]... 

FIGURE 7.14. Two precession photographs of a protein crystal (lac repressor), (a) Okl photograph, (b) hkO photograph. The hkO photograph was also shown in Figure 3.15 (Chapter 3). 

Figure 8 X-ray diffraction images, (a) A precession photograph of muconate lactonizing enzyme. The fourfold symmetry in the diffraction pattern is clearly visible. This gives an undistorted view of the reciprocal lattice but are no longer used because they are not as efficient as rotation images, (b) A rotation image of hen s egg white lysozyme. This easily obtainable image gives a distorted projection of the reciprocal lattice, but this is no obstacle for modern programs.

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