Radiation-Induced Free Radicals in Sulfur Compounds. ESR Studies in the Polycrystalline State

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Abstract
The secondary processes occurring in organic sulfur compounds, irradiated with ionizing radiation in the dry state, have been followed by ESR spectroscopy. Samples were irradiated in vacuum at 77°K and the ESR spectra recorded at this temperature before and after stepwise heat treatment at successively increasing temperatures. The resonance spectra seem to indicate that after irradiation at 77°K, several kinds of ESR centers are formed. No or very few sulfur radicals seem to be formed at this temperature. The results show that few secondary processes proceed efficiently in solids in the temperature range from 77° to about 110°K. Depending upon the compound, decay processes seem to dominate in the temperature range varying from about 110°K and up to about 250°K. Upon further heat treatment, secondary reactions take place which lead to the formation of sulfur radicals. The threshold temperature for these reactions is about 170°K for simple thiols and disulfides, 240°K for peptides like glutathione, and about 280°K for proteins. The formation of sulfur radicals is found to coincide with an increase in the total number of observable ESR centers. Thus, in the case of penicillamine, the number of unpaired spins observed after annealing to 295°K exceeds the initial value at 77°K by about 70%. Evidence is presented that diffusion‐controlled intermolecular reactions take place in irradiated solids, and it is suggested that the observed formation of sulfur radicals may in part be explained by such a mechanism. The resonance pattern due to polyoriented radicals of the type R—CH2–S· (e.g., cysteine, cysteamine, glutathione, proteins) was found to exhibit a temperature dependent hyperfine splitting which is not found for radicals of the type R—CX2—S· (e.g., penicillamine). This observation supports the model for organic sulfur radicals proposed by Kurita and Gordy.