Crystallography data collection

Acknowledgments The Australian Research Council (ARC) is thanked for financial support. Dr. Jonathan White (The University of Melbourne) is gratefully acknowledged for the X-ray crystallography data collection. 

When a diffracted X-ray beam hits a data collection device, only the intensity of the reflection is recorded. The other vital piece of information is the phase of the reflected X-ray beam. It is the combination of the intensity and the phase of the reflections that is needed to unravel the contributions made to the diffraction by the electrons in different parts of the molecule in the crystal. This so-called phase problem has been a challenge for theoretical crystallographers for many decades. For practical crystallography, there are four main methods for phasing the data generated from a particular crystal. 

The major advances in crystallographic methods were both experimental and theoretical. In experimental terms, there was widespread availability of synchrotron data collection resources and the emergence of CCD detectors that dramatically increased the speed at which data could be collected. A particularly important advance was the development of cryocrystallography methods [39] that revolutionized crystallography by making crystals essentially immortal. 

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. 

Time-Resolved Crystallography. Time-resolved crystallography (TC) uses an intense synchrotron X-ray source and Laue data collection techniques to greatly reduce crystallographic exposure times. Normal time resolution for X-ray... 

A number of synchrotrons (including the National Synchrotron Light Source, New York, the Advanced Light Source, Berkley, and soon the Canadian Light Source, Saskatoon) operate mail-in crystallography services where a scientist can mail in crystals (prefrozen and mounted on loops) and data will be collected, processed (sometimes), and returned. This is becoming a method of choice, as it eliminates the need to travel to a synchrotron and speeds up the data collection procedure at the synchrotron also. 

Muchmore, S. W., Olson, J., Jones, R., Pan, J., Blum, M., Greer, J., Merrick, S. M., Magdalinos, P and Nienaber, V. L. (2000). Automated crystal mounting and data collection for protein crystallography. Structure 8, R243-R246. 

Muchmore, S. W.,etal. (2000). Automated crystal mounting and data collection for protein crystallography. Structure Fold Des. 8, R243-246. 

This structure was confirmed by x-ray crystallography (Figure 3) of acetylated III. A crystal of dimensions 1.8 x 0.5 X 0.4 mm was used for data collection on a Enraf-Nonlus CAD-4... 

As is evident from Eqs. (1.36) and (1.37), and from the classical treatment as well, the effect of resonance on the intensity of X-ray scattering is pronounced when E E0, that is, in the vicinity of the absorption edges. Even for data collected at other wavelengths, it is necessary to correct the structure factors for anomalous scattering before the electron density can be calculated by the Fourier inversions of Eqs. (1.22) and (1.26), as further discussed in chapter 5. The anomalous scattering factors needed for this purpose are available in the literature (International Tables for X-ray Crystallography 1974, Kissel and Pratt 1990). 

All the above techniques use incident monochromatic radiation, usually focus in one or two dimensions. However for cases a) and d) the reduction of radiation damage and more particularly in kinetic crystallography the use of polychromatic data collection is yielding promising results. This technique makes combined use of the intensity and collimation of the SR beam with a large wavelength spread for Laue data colla tion from protein single crystals. 

Further reductions in exposure time and hence radiation damage in virus crystallography may accrue from the use of white beam (modified) Laue methods preliminary work on this is in progress (Bloomer and Helliwell (1985), unpublished at the SRS and Rossmann et al. at Cornell unpublished (1986)). Data collection on some virus crystals is virtually impossible in the home laboratory. 

It took the short time of one year or so to solve the structure of rhinovirus which causes the common cold. This relied on two major advances in methods. The first was the use of synchrotron radiation in data collection. Nearly a million reflections were collected on the protein crystallography facility at the Cornell Synchrotron source in a matter of days. This conveyed a speed advantage over data collection on a conventional source and also ameliorated an otherwise impossible problem of radiation damage when long exposure times were used. The far greater rate of radiation damage in the X-ray beam in relation to plant viruses is symptomatic of an inherently less stable protein capsid and the absence of quasi-symmetry. The capsid consists of 60 copies each of four proteins and the virus with about 30 % RNA has a total molecular weight of about 8.5 million. 

Time-resolved x-ray crystallography (TC) is a more recent advanced application of x-ray crystallography. It uses an intense synchrotron x-ray source and data collection methods to reduce crystallographic exposure times. This allows multiple exposures to be taken over time at near-physiological, crystalline conditions to determine the structures of intermediates. A typical problem with this method is that the existence of the intermediates is brief, resulting in difficulty in interpreting the resulting electron density maps. 

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