The sensitivity of Escherichia coli B to x-rays, in the presence and in the absence of added compounds, was studied at different irradiation temperatures in the range 0° to - 196°C. All irradiations were carried out under anoxic conditions. The radiation sensitivity was reduced by approximately 50% when the irradiation temperature was reduced from 0° to - 15°C. Concomitantly, the protective action of ethanol disappeared almost completely. Glycyl-glycine, which offered no protection at 0°C, gave increasing protection at decreasing irradiation temperatures. The protection of E. coli B at 0°C increased with increasing ability of the compounds to react with OH radicals. No correlation was found between protective ability and rate of interaction with${\rm e}_{{\rm aq}}^{-}$ The results indicate that, under anoxic conditions, the portion of the indirect effect in E. coli B that can be abolished by freezing or high concentrations of radical scavengers contributes approximately 50% to the lethal damage and that the indirect effect is due to the action of OH radicals. The protection to be expected at 0°C by different concentrations of ethanol was calculated with the aid of reaction kinetics. The calculated results were in good agreement with those observed. The diffusion length of the OH radicals involved in the destruction of the target molecules was calculated to be about 25 Å. None of the compounds tested protected to a greater extent than could be predicted from their ability to scavenge OH radicals.

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