The following dipeptides have been irradiated with60 Co γ-rays in liquid nitrogen: glycyl-L-phenylalanine, glycyl-L-tyrosine, glycyl-L-tryptophane, L-phenylalanyl-glycine, L-tyrosyl-glycine, L-tryptophyl-glycine, L-alanyl-L-phenylalanine, L-alanyl-L-tyrosine, L-analyl-L-tryptophane, L-phenylalanyl-L-alanine, L-tyrosyl-L-alanine, and L-tryptophyl L-alanine. Their EPR spectra have been recorded at - 180 or - 160°C and at room temperature. Detectable free radicals are formed at low temperature only on glycine and alanine and are of the types -$\dot{{\rm C}}{\rm H}_{2}$ and -$\dot{{\rm C}}{\rm H}\ ({\rm CH}_{3})$ respectively. Upon warming to room temperature the EPR signals of peptides containing glycine or alanine in the N-terminal position change to mostly unresolved spectra. For the C-terminal peptides, L-Phe-Gly and L-Tyr-Gly, warming results in a spectrum attributed to a$-{\rm NH}-\dot{{\rm C}}{\rm H}-{\rm COO}^{-}$ radical, while for L-Phe-L-Ala, L-Tyr-L-Ala and L-Trp-L-Ala a spectrum believed to result from a -NH-$\dot{{\rm C}}({\rm CH}_{3})-{\rm COO}^{-}$ radical appears after standing at room temperature. The radicals observed in peptides at room temperature after γ-irradiation at low temperature are the same as after x- or γ-irradiation at room temperature.

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