ESR Studies on Irradiated Frozen Aqueous Solutions of Sulfur Compounds

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Abstract
Radiation-induced ESR centers in frozen aqueous solutions of sulfur compounds have been studied. Special attention has been focused on secondary radical reactions and the relative importance of direct and indirect effects. Solutions were rapidly frozen at 77°K and irradiated in vacuum at this temperature. All ESR spectra were recorded at 77°K. The irradiated samples were heat treated, either for increasing lengths of time at 110°K, or for the same period of time at successively increasing temperatures. The ESR spectra of aqueous solutions irradiated at 77°K are composed of the ``water resonance'' and a resonance ascribed to radicals in the solute. The latter are found primarily to be induced by the direct action of radiation. This observation implies that H atoms which already have reacted at 77°K have no or very little effect in producing solute radicals. The remaining ESR centers in water, including OH radicals, are released in the temperature range 100° to 130°K. The spectral changes in this temperature range indicate that recombination processes dominate and only a minor fraction of the ``water radicals'' interacts with the solute. Thus, in a 10% solution of glutathione, about 15% of the ``water radicals'' become effective in producing solute radicals. The radicals induced in the solute and the subsequent secondary radical reactions seem to be more or less the same as those found when the solute was irradiated in the solid state. For solutions of some simple thiols like penicillamine the secondary reactions yield sulfur radicals above 170°K, in accordance with the results for solid penicillamine. On the other hand, no sulfur resonance appears for solutions of glutathione in spite of the fact that sulfur radicals are formed in the solid state. The resonance of the solution disappears at a lower temperature than that found necessary for the formation of sulfur raducals in solid glutathione.