The separate contributions of the primary radical species to changes in enzymatic activity, amino acid composition and molecular configuration of RNase S-peptide have been determined by γ-irradiation of dilute solutions in controlled environments. For H· and ·OH, approximately one out of two radicals is effective for the enzymatic inactivation of S-peptide; <tex-math>$e^{-}{}_{{\rm aq}}$</tex-math> is much less effective, only one out of eight or more radicals causing loss of activity. Amino acid analyses of irradiated S-peptide show that only the single sulfur-containing residue, methionine, and the single aromatic residue, phenylalanine, are significantly destroyed. For H atom attack, loss of enzymatic activity correlates with the destruction of methionine; phenylalanine is much less affected and no new products are separated on gel filtration. Hydroxyl radicals cause a significant destruction of phenylalanine, as well as methionine, and gel filtration shows that a new product of higher molecular weight is formed.

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