Pulse radiolysis and steady-state inactivation methods have been used to study the separate roles of H, OH, and <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> in the radiation-induced inactivation of aqueous α-chymotrypsin solutions. The effects of nitrous oxide and t-butanol used as selective free radical scavengers allow the inactivating efficiencies per unit G-value of OH, H, and <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> radicals to be estimated; these are 0.137, 0.13, and 0.055, respectively, in neutral solution. The transient spectra observed show that both OH radicals and hydrogen atoms react mainly with tryptophan residues. Only about 15% of the hydrated electrons react with the cystine disulfide linkages; rupture of some of these linkages may lead to inactivation. It is suggested that the protective action of oxygen is due partly to the reactions of O2 with enzyme radicals formed initially by reaction with OH.

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