Occurance of radiation damage is a feature normally encountered while collecting X-ray intenslty data 'from single crystals of proteins. It is wel1known that prote! crystals contain large . solvent regions consisting 13; mainly of water. The protein crystals,' like the molecules themselves, are stable only in'aqueous medium and are therefore mounted with tlie solvent in a capillary for data.collection. According to the indirect action theory of radiation effects, the damage to the protein13; molecule is associated with the production of free13; radicais from the aqueous medium. These radicals,in turn13; transfer the energy acquired from the Jncident radiation to the protein molecule. The protein molecple is thus left in an unstable,pnized state leading to significant changes in its.structure and properties. The onset of radiation13; damage is detected by reduction in the intensity13; of the diffraction pattern which proceeds on to its13; total disappearance on prolonged irradiation. The13; amount of radiation dosage required to cause the13; damage varies with substances, some affected within13; hours of irradiation and some others within few13; days. Hence most often, in protein crystaIlography,13; one is compelled to use several crystals to13; collect a complete set of data. Most proteins are13; available only in smaIl quantities and it is often13; difficult to grow a large number of good crystals.13; Also, correlation of different data sets sometimes13; poses problems when several crystals are used for13; data coIlection
