Radiationhyphen;Induced Free Radicals in Sulfur Compounds. ESR Studies in the Polycrystalline State

摘要

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 77deg;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 77deg;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 77deg; to about 110deg;K. Depending upon the compound, decay processes seem to dominate in the temperature range varying from about 110deg;K and up to about 250deg;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 170deg;K for simple thiols and disulfides, 240deg;K for peptides like glutathione, and about 280deg;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 295deg;K exceeds the initial value at 77deg;K by about 70.Evidence is presented that diffusionhyphen;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 Rmdash;CH2sngbnd;Smiddot; (e.g., cysteine, cysteamine, glutathione, proteins) was found to exhibit a temperature dependent hyperfine splitting which is not found for radicals of the type Rmdash;CX2mdash;Smiddot; (e.g., penicillamine). This observation supports the model for organic sulfur radicals proposed by Kurita and Gordy.