The radiosensitivity of dry ribonuclease was determined at various temperatures between 120 °K and 440 °K using 2 MeV protons. Within this temperature range the inactivation cross section S (T) of ribonuclease may be described as a function of temperature by the expression S(T) = (1.28 + 16·e-1000/RT+14000·e-6500/7RT)·10-14 cm2 . This result indicates that the observed radiation damage to ribonuclease is produced by three different mechanisms, one being independent of temperature, the other two

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... having apparent activation energies of 1 kcal/mole and 6.5 kcal/mole, respectively. From these relatively small activation energies the conclusion may be drawn that radical reactions contribute to the inactivation of enzymes in the dry state. Experiments with Co gamma radiation showed that the radiosensitivity of ribonuclease at 77 °K depends on dose rate; at room temperature a dose rate effect was not observed.