The reactions of H atoms with ribonuclease-S-peptide and ribonuclease-S-protein at pH 2.2 and 6.6 have been investigated by means of pulse radiolysis-kinetic spectrometry and compared with those of intact ribonuclease. Initial spectra were observed after 100-nsec pulses of 5 MeV electrons under conditions such that only H atoms reacted with substrate. The resulting spectra are due to addition of one H atom to a substrate molecule. Their subsequent transformations were studied over periods ranging from several microseconds to several seconds. The specific rates of reaction of H atoms with S-protein and with intact ribonuclease (1) are the same, while S-peptide reacts about half as rapidly. For both substrates the subsequent transformations are first order and presumably correspond to intramolecular chemical reactions. The data implicate aromatic and sulfur-containing amino acid residues as transient radical sites. The data for S-peptide establish that intramolecular radical transfer is a reaction of polypeptide as well as of protein. The types of residues involved as radical sites and the intramolecular nature of sequential transformations are similar to those observed (1) in the reaction of intact ribonuclease with H atoms, but significant differences are observed in specific rates and spectra. The sum of the effects of H atoms on S-peptide and on S-protein separately is not identical with the action of H atoms on structurally intact ribonuclease.

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