Radiation Effects on α-Chymotrypsin in Aqueous Solution: Pulse Radiolysis and Inactivation Studies

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.