Matrix-isolation of free radicals from 5-halouracils. 3. Electron spin resonance of base oxidation in aqueous acidic glasses

Electron spin resonance spectroscopy (ESR) of X-irradiated aqueous acidic glasses (5.3 M H/sub 2/SO/sub 4/, 7.2 to 14.7 M H/sub 3/PO/sub 4/) was used to study the base ..pi.. cations of the nucleic acid constituents uracil (U), thymine (T), and the range of 5-halogen-substituted uracils (FU, ClU, BU, IU) as well as their nucleoside derivatives and to follow their secondary reactions. High solute concentrations (> 50 mM) and the presence of electron scavengers (100 to 200 mM Na/sub 2/S/sub 2/O/sub 8/ or H/sub 2/O/sub 2/) were found to favor the cation formation and stabilization and to suppress the formation and stabilization and to suppress the formation of other solute radicals. The generation of the base cations is shown to result from transfer of a (possibly) excited solvent hole to the pyrimidine base during X irradiation at 77/sup 0/K and from attack of the trapped solvent hole (SO/sub 4//sup -/. or HPO/sub 4//sup -/.) after thermal activation at elevated temperatures (145 to 165/sup 0/K). The fate of the base cations is either deprotonation at N/sub 1/ as in most of the free bases or addition of OH/sup -/ to carbon C/sub 6/ in the nucleosides where the deoxyribose moiety prevents the deprotonation. This hydroxylation is suppressed in a water-deficient system (e.g., 14.7 M H/sub 3/PO/sub 4/) but very efficient in 5.3 M H/sub 2/SO/sub 4/. The structures of both the primary and secondary radicals are confirmed in most cases by a complete simulation of the experimental ESR powder spectra. The ESR parameters obtained display a strong influence of the 5 substituent, e.g., the halogens, on the spin density distribution in the radicals. Characteristic for ..pi.. orbital ranging from 8% in FU to 40% in IU. Moreover, the amount of water in the environment exerts a considerable influence on the halogen spin densities in the charged cation.