It is shown that radicals produced from ethanol, formate, and methanol by OH radical attack are able to destroy the thymine chromophore, with a rate constant of the order of <tex-math>$10^{5}\ M^{-1}\,{\rm sec}^{-1}$</tex-math>. Addition of small quantities of cysteine protects thymine almost completely. This is explained by H-atom transfer from the sulfhydryl compound to the organic radical thus restoring the parent organic molecule and preventing the reaction of the organic radical with thymine. It is suggested that this type of indirect protection of a DNA constituent may be of importance for the radioprotection of DNA in living cells.

The reactions of thymine in aqueous solution with radiation-induced radicals OH, H, and <tex-math>${\rm e}_{{\rm aq}}^{-}$</tex-math> were studied under various conditions. Competition studies using scavengers of OH radicals (methanol, ethanol, iodide) or of <tex-math>${\rm e}_{{\rm aq}}^{-}$</tex-math> and/or H atoms (<tex-math>${\rm N}_{2}{\rm O},{\rm H}^{+},{\rm O}_{2}$</tex-math>) led to the conclusion that OH and H radicals destroy the chromophoric group of thymine, but <tex-math>${\rm e}_{{\rm aq}}^{-}$</tex-math> does not. A trace of O 2 proved to be necessary to obtain maximal destruction. Removal of the last traces of O 2 resulted in a decrease of the destruction yield, possibly through restitution reactions. It was found that (1) alcohol radicals destroy thymine, even in the presence of O 2 ; (2) the rate constant, k(OH + thymine) = <tex-math>$4.3\times 10^{9}\ {\rm M}^{-1}\ {\rm sec}^{-1}$</tex-math> (from competition with iodide); and (3) k(H + thymine) = <tex-math>$8\times 10^{8}\ {\rm M}^{-1}\ {\rm sec}^{-1}$</tex-math> (from competition with O 2 in acid solution).

DNA isolated from the bacteriophage ΦX174 was irradiated with γ-rays in a solution of pH 7.0 buffered with 0.01 M phosphate. At concentrations of a few milligrams per milliliter the yield of inactivation is 1.9 molecules of DNA per 100 eV of absorbed energy. The radio-sensitivity in oxygen and nitrogen does not differ significantly. Bubbling with nitrous oxide increases the yield by a factor of 2. These yields were derived from measurements of the 37% survival dose as a function of DNA concentration. Potassium iodide is strongly protective. Because potassium iodide scavenges OH radicals whereas nitrous oxide converts hydrated electrons into OH radicals, it is concluded that in concentrated DNA solutions at least 95% of the inactivations are due to primary attack by OH radicals, although reducing radicals are not completely inactive. At concentrations of a few micrograms per milliliter the yield of inactivation is lower by an order of magnitude and reducing radicals are relatively more important. From the protection afforded by free nucleotides it is concluded that OH radicals react about five times as fast with free nucleotides as with nucleotides in single-stranded DNA. In the presence of free pyrimidine nucleotides oxygen enhances the radiosensitivity considerably.

The effect of soft X-rays on the biological activity of air-saturated aqueous solutions of single-stranded DNA of the bacteriophage φX174 was studied. The inactivation is due to OH radicals, as follows from the large increase of the 37% survival dose if these radicals are scavenged by iodide ions. O 2 - radicals, produced by the reaction of hydrated electrons with oxygen, do not inactivate DNA. In dilute solutions of DNA the inactivation rate at low dose was much smaller than expected from the rate of base destruction as determined from the decrease of optical density at higher dose. This could be explained from the nonuniform nucleotide concentration in such solutions, which enhances the scavenging efficiency of impurities. At DNA concentrations above 1 mg/ml this effect disappears gradually. The protection afforded by various concentrations of a mixture of free nucleotides in a molar ratio corresponding to the base composition of DNA indicates that free nucleotides react faster with free radicals than do nucleotides in DNA.