SUMMARY: Spores of were inactivated by γ-radiation more effectively in the presence than in the absence of a variety of halogen compounds. Where homologous series of compounds were used, activity decreased in the order I > Br > CI and F. Activity did not depend upon affinity of the compounds for thiol groups. Potassium iodate was the most active compound tested. Iodate and iodoacetamide (IAM), and to a lesser extent iodide, were most effective at low pH values, but potentiation by 3,5-diiodo-L-tyrosine was hardly affected by pH value. Survivor curves of spores irradiated in water plus IAM or iodate were convex, suggesting that potentiation increased with increase in γ-radiation dose. The compounds tested were all more effective potentiators of radiation inactivation of spores in water than in sodium phosphate buffers or in complex medium, and potentiation was greater with low than with high spore concentrations. Histidine antagonized the potentiation of radiation inactivation by iodate, iodide and iodoacetamide. Potentiation decreased with decrease in temperature, and was not shown at - 15°. The results suggest that potentiation resulted from toxicity to spores of halogen-free radicals, and to a lesser extent free halogen, formed by reaction of the compounds with the radiolytic products formed from water during irradiation.


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