Temperature-dependent radiation sensitivity and order of 70S ribosome crystals.
All evidence to date indicates that at T = 100 K all protein crystals exhibit comparable sensitivity to X-ray damage when quantified using global metrics such as change in scaling B factor or integrated intensity versus dose. This is consistent with observations in cryo-electron microscopy, and results because nearly all diffusive motions of protein and solvent, including motions induced by radiation damage, are frozen out. But how do the sensitivities of different proteins compare at room temperature, where radiation-induced radicals are free to diffuse and protein and lattice structures are free to relax in response to local damage? It might be expected that a large complex with extensive conformational degrees of freedom would be more radiation sensitive than a small, compact globular protein. As a test case, the radiation sensitivity of 70S ribosome crystals has been examined. At T = 100 and 300 K, the half doses are 64 MGy (at 3 Å resolution) and 150 kGy (at 5 Å resolution), respectively. The maximum tolerable dose in a crystallography experiment depends upon the initial or desired resolution. When differences in initial data-set resolution are accounted for, the former half dose is roughly consistent with that for model proteins, and the 100/300 K half-dose ratio is roughly a factor of ten larger. 70S ribosome crystals exhibit substantially increased resolution at 100 K relative to 300 K owing to cooling-induced ordering and not to reduced radiation sensitivity and slower radiation damage.