Cryo preservation and decay
Opening the session on cryo-preservation methods, D. Thiel (Cornell) explained that even at cryo temperatures all crystals are mortal, that fully focused undulator radiation is lethal, that determination of cryo conditions can be problematic, particularly for membrane protein and virus crystals, and that damage mechanisms are not completely understood. G. Bunick (Oak Ridge) described the technique of crystal annealing. If cryo cooling has reduced room temperature mosaicity, crystals may be warmed to room temperature in their crystallization media for 3 minutes and flash cooled with partial recovery of the original mosaicity. S. McSweeney (EMBLGrenoble) presented a systematic study of the structural changes induced in acetylcholinesterase by radiation damage. Successive data sets were collected from the same crystal and the structures from each were refined. The unit cell gradually increased and the molecules in the cell rotated and translated as damage increased. Disulphide bonds broke first. The study raised important questions concerning dose rate effects and the use of free-radical scavengers.
The physical processes involved when an X-ray loses energy in a crystal were reviewed by C. Nave (Daresbury), who presented his model calculations on beam heating of frozen crystals. Initially, the temperature rises a few degrees but quickly levels off. T. Tsukihara (Osaka) reported the cryo-conditions and structure determination of the membrane protein cytochrome c oxidase. A combination of 5% PEG 4000 and 35% glycerol and a slow cooling protocol in a sealed capillary tube resulted in lower mosaic spread and higher quality diffraction than fast cooling in a loop. L. Liljas (Uppsala) described the structure of cricket paralyis virus from a frozen crystal despite significant problems in scaling the data due to a sudden decrease in the unit cell size during data collection.