CRYOPROTECTION OF PROTEIN CRYSTALS AGAINST RADIATION-DAMAGE IN ELECTRON AND X-RAY-DIFFRACTION

By using the fading of electron diffraction patterns during electron irradiation of protein or other organic crystals as a benchmark to measure destruction of the crystalline atomic arrangement by ionizing radiation, calculations suggest that protein crystals in laboratory X-ray beams might last for about five years when the specimen is cooled to liquid nitrogen temperatures or below. It is suggested that all crystals should be equally stable to X-irradiation at this temperature, as they are to electron irradiation. The calculation, which depends on the assumption that electrons and X-rays are more or less equally damaging to the structure giving rise to the diffraction at very low temperature, supports experimental observations that X-ray diffraction from protein crystals seems to last indefinitely at liquid nitrogen temperatures, even in the most powerful beams available at synchrotron X-ray sources. An attempt is made to explain the relation between the present analysis and other ways of viewing radiation damage.