Reactions between Nitroxyls and Radiation-induced Long-lived DNA-transients

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Abstract
The reactions of the organic nitroxyl free radical 2,2,6,6-tetramethyl-4-piperidone-N-oxyl (TAN) with radiation-induced DNA transients have been studied under anoxic conditions by means of a special irradiation-mixing technique. Unirradiated 3H-TAN (2 × 10−5 M in 10−2 M NaCl) was mixed in proportion 1 : 1 with irradiated DNA (1 mg/ml in 10−2 M NaCl) at predetermined times (t) after irradiation, with t > 20 sec. After irradiation and mixing, the per cent loss of TAN spins (NSL) was determined by E.S.R. technique, and the per cent 3H-TAN bound to DNA (NB) was determined by column chromatography. The data show that TAN interacts with radiation-induced DNA transients in two different ways,: where DNA· are free radicals and the (DNA−TAN)complexes are void of TAN spins, and The latter reaction involves electron transfer from radiation-induced DNA transients (DNAtr) to TAN, yielding reduced TAN and DNA derivative (DNA+1). The data show that NSL/NB ≈ 2·5 when 20 sec < t < 40 min. About the same ratio was found when anoxic TAN and DNA were mixed before irradiation. It has been shown by pulse radiolysis that radiation-induced DNA transients with a pronounced absorption maximum near 310 nm are formed. For t > 3 min, this transient appears to have decay properties comparable to those of the transients participating in complex-formation and electron-transfer reactions. The life-span of radiation-induced DNA transients is shown to be highly sensitive to the presence of trace amounts of oxygen, which in N2-saturated solution may affect the yield of reduced TAN more than that of complex formation.