Reactions of OH Radicals with Poly(U) in Deoxygenated Solutions: Sites of OH Radical Attack and the Kinetics of Base Release

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Abstract
Summary Pulse radiolysis of N2O-saturated solutions of poly(U) in the presence of tetranitromethane showed that 81 per cent of the radicals formed are reducing in nature. Using data from other sources it has been estimated that 70 per cent of the OH radicals add to the base at C(5) and 23 per cent at C(6) while only 7 per cent abstract an H-atom from the sugar moiety. To a large extent the C(5) OH adduct radicals attack the sugar moiety of poly(U) thereby inducing strand breakage and base release. G (base release) = 2·9 can be subdivided into three components: (a) immediate (20 per cent), (b) fast (50 per cent) and (c) slow (30 per cent). The immediate base release must occur either during the free-radical stage or as a result of the rapid (t½ < 4 min at 0°C) decomposition of a diamagnetic product. The fast and the slow processes are only readily observable at elevated temperatures, e.g. at 50°C the half lives are 83 min and 26 h, respectively (Ea (fast) = 68 kJ mol−1, Ea (slow) = 89 kJ mol−1, A (fast) = 1·5 × 107 s−1, A (slow) = 1·9 × 109 s−1. It is concluded that there are three different types of sugar lesions giving rise to base release, structures for which are tentatively proposed.