Radiation damage exposure sources

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. 

It has been reported that Linde X maintains its crystallinity, gas absorption, and ion-exchange properties up to about 1019 fast neutrons cm-2, but these properties were rapidly lost at higher exposures (9). Since radiation damage is predominantly caused by fast neutrons, it would be advantageous to use a neutron source with a substantially smaller fast component. Such facilities can be built, and experiments in this direction are planned. 

These experiments confirmed reduced exposure times and reductions in the relative amount of radiation damage over that on a conventional source (Fourme 1978, 1979 Kahn et al 1982a). In the Soviet Union on the VEPP-3 ring some preliminary data were collected by Mokulskaya (1981) using an electronic area detector in an attempt to optimise the anomalous dispersion of a platinum derivative of pea lectin crystals. 

This enzyme is of considerable biological, medicinal and commercial interest. It plays a fundamental role in purine metabolism, it degrades anticancer drugs being targeted to specific cancers and its absence is associated with severe T-cell deficiency. Crystals of the enzyme on a conventional source diffractometer did not diffract beyond 4 A resolution, a resolution which is not adequate to study the structural interactions with various substrates and inhibitors. With intense synchrotron X-radiation at Daresbury, data could be collected to between 3.2 A and 2.8 A resolution the data to 3.2 A (table 10.3) were sufficient to solve the structure. This is another example of time-dependent radiation damage. Very fast data collection methods at the SRS involved collection of 100 reflections per second, where one crystal was used for four minutes of exposure time. The structure has been described in Ealick et al (1990) (figures 10.5 and 2.1). 

Acute radiation damage results from exposure to large amounts of radiation in a short period of time. The main sources of this kind of exposure are nuclear bombs and exposed nuclear reactor cores. These high levels of radiation kill large numbers of cells. Rapidly dividing cells, such as those in the immune system and the intestinal lining, are most susceptible. Consequently, people exposed to high levels of radiation have weakened immune systans and a lowered abihty to absorb nutrients from food. In milder cases, recovery is possible with time. In more extreme cases, death results, often from infection. 

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