The ..gamma.. irradiation at 77 K of frozen aqueous solutions of over 14 peptides was investigated by ESR (electron spin resonance) spectroscopy. The investigation was composed of three parts. The first part consists of a study of simple dipeptides of glycine and alanine, i.e., Gly-Ala, Ala-Gly, and Ala-Ala. Detailed analyses were made of the radicals produced by the irradiation, their relative concentrations, and their stability as a function of temperature. The results show evidence for anion formation, primary deamination, and decarboxylation initially. This is followed by abstraction from the parent compound to form the ..cap alpha..-carbon radical at the C-terminal residue. In the case of Ala-Ala there is evidence for hole stabilization at low temperature. In the case of Gly-Ala some secondary deamination is noted. In the second part the analysis of the final spectra for 11 dipeptides are reported. The final radicals were all found to be a result of abstraction from the ..cap alpha.. carbon of the C-terminal residue to produce the radical NH/sub 3//sup +/CHRCONHCR'CO/sub 2//sup -/. Radicals with R' = H or CH/sub 2/R'' produced a hyperfine coupling due to one proton of 18 to 19 G. Since 15 of the 20 common amino acid residuesmore » have a structure with R' = H or CH/sub 2/R'', the dominant final radical in proteins should produce a doublet spectrum. In the final portion, three dipeptides with aromatic side groups were studied in frozen H/sub 2/O solutions. It was found that electron attachment to the aromatic ring effectively competed with deamination of the primary amine group. The electron attachment to the aromatic ring was followed by protonation at a carbon site on the ring. The results for Gly-His were typical. Here we found a spectrum indicative of 25% protonation, 30% deamination, and 45% decarboxylation. For Gly-Tyr, phenoxyl radical formation was found to dominate decarboxylation as a decomposition pathway for the hole. 10 figures, 2 tables.« less