Section 9.1.12.2. Cryogenic freezing

In the early 1990s, the introduction of protein-data collection at cryogenic temperatures, using so-called flash freezing, was a major breakthrough (Garman & Schneider, 1997; Rodgers, 1997). Flash-frozen crystals largely prevented the effects of secondary damage. On the X-ray sources then available, it was in most cases possible to record complete data from a single sample without significant degradation of the diffraction, enormously simplifying the strategy of data collection and merging.

The techniques of macromolecular cryocrystallography have advanced so rapidly that almost all data are currently collected from frozen samples. The key aspects of flash freezing are addressed in Part 10 . The prolonged life of the sample and modest rates of data acquisition, even at second-generation SR sources with imaging plates, allowed enough time for careful analysis of the initial images and optimization of the strategy.

A second major advantage of cryogenic freezing is that it allows crystals to be reused after initial data have been recorded. Two examples show the usefulness of this approach. Firstly, when screening the binding of heavy atoms for phase determination or ligands for complex formation, data can first be recorded to the minimum resolution needed to determine whether the binding is successful. Secondly, a series of frozen crystals can be screened for their degree of order in the home laboratory, and the best stored and retained for subsequent improved collection either in the home laboratory or at a synchrotron site. The ability to transport frozen crystals has proved invaluable in this respect, and leads to optimal use of synchrotron resources.