Abstract
Many advances in the understanding of radiation damage to protein crystals, particularly at cryogenic temperatures, have been made in recent years, but with this comes an expanding literature, and, to the new breed of protein crystallographer who is not really interested in X-ray physics or radiation chemistry but just wants to solve a biologically relevant structure, the technical nature and breadth of this literature can be daunting. The purpose of this paper is to serve as a rough guide to radiation damage issues, and to provide references to the more exacting and detailed work. No attempt has been made to report precise numbers (a factor of two is considered satisfactory), and, since there are aspects of radiation damage that are demonstrably unpredictable, the 'worst case scenario' as well as the 'average crystal' are discussed in terms of the practicalities of data collection.
Highlights
The specific chemical changes induced by radiation damage can change the structure from the biologically relevant form, and this sometimes leads to wrong conclusions about function
A factor of two roughly doi:10.1107/S0909049509004361 133 radiation damage corresponds to the decision thresholds that must be faced in data collection strategy
As long as the sample is free of heavy atoms, the dose deposited by a given beam of X-rays in a thin layer of protein crystal is within 5% of that deposited in a thin layer of pure water for all photon energies between 5 and 50 keV
Summary
Diffraction spots can fade away before the data set is complete and heavy atoms sites can become disordered before sufficient anomalous signal is measured. The former is easy to detect by eye during data collection, but the latter is more insidious (Holton, 2007; Olieric et al, 2007). Increasing all three linear dimensions of a crystal by 26% will double the volume of scattering matter (1.263 = 2) Such a change in size appears to be a typical ‘error bar’ when examining crystals under a microscope, as most crystallographers will not distinguish between an 88 mm crystal and a 110 mm crystal, but the latter has twice the volume of the former and the number of photons a crystal will diffract before it is ‘dead’ is proportional to volume (see Appendix A). A factor of two roughly doi:10.1107/S0909049509004361 133 radiation damage corresponds to the decision thresholds that must be faced in data collection strategy
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