When aqueous DNA solution was irradiated with 1.2 MHz continuous ultrasound in the presence of cysteamine, the number of ultrasound-induced double-strand breaks of DNA was not influenced, but the number of ultrasound-induced single-strand breaks of DNA was reduced to about one-fifth that of the irradiated control. When the effect of cysteamine on the template activity of the ultrasound-irradiated DNA was investigated, the cysteamine was found to exert a leveling effect on the linear decrease of the template activity against ultrasonic intensity. Since cysteamine was known as an effective radical scavenger, the results of the experiment were regarded to suggest that (1) the double-strand breaks were exclusively induced by the mechanical effect of ultrasound, (2) the majority of single-strand breaks were produced by water radicals arising from cavitation, (3) the initial part in the decrease of the template activity was due to the double-strand breaks arising from mechanical effect, and (4) the further decrease of the template activity depended mainly on the single-strand breaks arising from water radicals.

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Free radicals produced by the reactions of hydrated electrons with pyrimidine nucleosides halogenated at the sugar moiety (2'-chloro-2'-deoxyuridine and 2'-chlorothymidine) were studied by e.s.r. and spin-trapping. 2-Methyl-2-nitrosopropane was used as the spin-trap. The usual spin-trapping technique was extended to frozen and deoxygenated systems to avoid contamination of the trapped radicals with side-products by spin-trapping 2-methyl-2-nitrosopropane itself. When this method was applied to 2'-chloro-2'-deoxyuridine, a free radical at the C-2' position of the sugar moiety was spin-trapped together with a free radical at the C-5 position of the base moiety. This indicates that hydrated electrons both add to the base moiety and eliminate halogen anions from the halogenated sugar moiety. In the case of 2'-chlorothymidine, however, only a free radical attributed to H-addition at the C-6 position of the thymine base was observed. No radicals produced by the reaction of hydrated electrons with the halogenated sugar could be spin-trapped.

Free radicals produced by the reactions of OH radicals with ribose phosphates, pyrimidine nucleosides and nucleotides in aqueous solutions have been investigated by e.s.r. and spin-trapping techniques. OH radicals were generated by U.V. -photolysis of hydrogen peroxide and short-lived free radicals of the samples were spin-trapped by t-nitrosobutane and identified by e.s.r. spectroscopy. For ribose-5'-phosphate and 2' -deoxyribose-5' -phosphate, e.s.r. spectra observed consisted mainly of singles due to -C(5')H2 radicals which were initiated by H-abstraction from the sugar at the C-4' position and formed by the radical transfer to the C-5' position. It has also been shown that OH radicals abstract a hydrogen atom from the sugar at C-1' and C-5' positions. For cytidine, deoxycytidine, 5'-CMP, 3' -CMP and 5' -dCMP, four radicals have been observed and for uridine, deoxyuridine, 5' -UMP, 3' -UMP and 5' -dUMP, the existence of at least three radical species have been established. In contrast to the case of ribose phosphates, no signals due to -C(5')H2 radicals were detected for pyrimidine nucleosides and nucleotides. The results are discussed in relation to a recent mechanism which described OH-induced strand breaks of DNA.

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