Free Radicals from Purine Nucleosides after Hydroxyl Radical Attack

Free radicals from imidazole, caffeine, guanosine, deoxyguanosine, inosine, adenosine, deoxyadenosine, and adenine, as well as β-D-deoxyribose, were generated by reaction with ȮH from <tex-math>${\rm H}_{2}{\rm O}_{2}-{\rm Ti}^{3+}$</tex-math> at pH 1.5-2 and studied by ESR, using a computerized, Q-band, fast-flow system. Purine base radicals, plus sugar radicals in nucleosides, were always observed. Prior deuteration of C8 made possible the assignment of the observed 10-15 gauss doublet splitting to the C8 proton in guanine derivatives, and to the C2 proton in adenine derivatives and inosine. Hückel calculations (and INDO in a few cases) on possible radical species suggest that hydroxyl addition on nitrogens in the purine ring or on C5 may be responsible for the species which are observed. In imidazole, the radical species is a hydroxyl adduct, <tex-math>$ \matrix\format\c\kern.8em&\c\kern.8em&\c\kern.8em&\c\kern.8em&\c\kern.8em&\c\kern.8em&\c\\ {} & {} & {} & {\rm H} & {} & {} & {} \\ {} & {} & {} &| & {} & {} & {} \\ {\rm H} & {} & {} & {\rm N}^{+} & {} & {} & {} \\ {} & \smallsetminus & \diagup & {} & \diagdown \diagdown & {} & {} \\ {} &|^{\cdot} & {} & {} & {} & - & {\rm H} \\ {\rm H} & \smallsetminus & {} & {} & & {} & {} \\ {} & \diagup & \diagdown & {} & \diagup & {} & {} \\ {\rm HO} & {} & {} & {\rm N} & {} & {} & {} \\ {} & {} & {} &| & {} & {} & {} \\ {} & {} & {} & {\rm H} & {} & {} & {} \endmatrix $</tex-math> An anomalous effect of deuteration of C8 on the spectrum obtained with caffeine is also reported.