Radiation-induced Free Radicals in Solid 5-halouracil Derivatives: Single Crystals of 5-iododeoxyuridine

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Abstract
X-irradiation of single crystals of 5-iododeoxyuridine (IUdR) in the temperature range 8–300 K produces mainly four different radicals which have been studied by electron spin resonance (e.s.r.) and electron nuclear double resonance (ENDOR)-spectroscopy. At low temperatures, a π*-anion is formed which shows predominantly an interaction of the unpaired electron with a proton at carbon C6 of the base (−11·8 G, −23·9 G, −4·6 G). Above 10–20 K, the anion protonates at C6 to yield a RĊ-I(CH2)-R′ radical comprising α-iodo and β-methylene proton hyperfine interactions. The primary oxidation product is an O5′-situated alkoxy radical RCH2Ȯ which shows inequivalent β-proton couplings of about 100 G and 35 G together with a highly anisotropic g-tensor. Upon warming to 265 K, a C2′-located radical on the deoxyribose is formed which is stable at room temperature. A detailed account of its spectral features as obtained by ENDOR exhibits three different α-type couplings, two small β-protons and a dipolar interaction. Other radicals, not reproducibly observed, involve a C5′-hydroxyalkyl radical and a species related to the base cation at low temperatures.