Using the technique of pulse radiolysis with spectrophotometric detection, OH radical (•OH)-induced electron transfer by intramolecular processes was studied in aqueous solutions containing either equimolar binary mixtures of deoxynucleosides or di- and oligodeoxynucleotides at pH 7.4. The time-resolved optical absorbance changes in mixtures of monodeoxynucleosides did not reveal significant electron transfer, indicating the lack of intermolecular electron transfer induced by •OH. Of the dinucleotides studied, only 2‘-deoxyadenylyl-(3‘→5‘)-2‘-deoxyguanosine (dApdG) shows •OH-induced intramolecular electron transfer. This reaction involves electron transfer from guanine to the adenine radical, which results from dehydration of its C(4)−•OH adduct and was monitored at 400 nm. The rate-determining step of electron transfer is the dehydration of the C(4)−•OH adduct of adenine. With the single-stranded oligodeoxynucleotides, dAGA, dAAGAA, and dAAGTA, the spectral changes with time are consistent with electron transfer occurring from guanine, but to only one of the possible neighboring adenine radicals produced by dehydration of the adenine C(4)−•OH adduct. In contrast, •OH interactions with dATGAA and dATGTA do not induce electron transfer from guanine to the adenine radical produced by dehydration of the C(4)−•OH adduct. These results indicate that the dehydrated adenine radical 5‘ to the guanine is preferentially involved in the electron-transfer process.

The reactions of •OH, O•- and SO4 •- with 2-, 3-, and 4-cresols were studied by pulse radiolysis, laser flash photolysis, and product analysis techniques. The rates of OH reaction with cresols are very high (k ≈ 1 × 1010 M-1 s-1), whereas O•- was found to be less reactive (k ≈ 2.4 × 109 M-1 s-1). The second-order rate constants for SO4 •- reaction with cresols are in the range (3−6) × 109 M-1 s-1. The transient absorption spectra measured in OH reaction exhibited peaks in the range 295−325 nm with a red shift for the meta isomer. The absorption spectra obtained for O•- reaction with 2-cresol has a peak at 360 nm, which is different from those measured with the m and p isomers (λ max = 310 and 380 nm). The absorption spectra of the transient species in SO4 •- reaction obtained by pulse radiolysis and flash photolysis techniques are similar, with absorption maxima centered around 290 and 390 nm in all three isomers. The intermediates formed in •OH, O•-, and SO4 •- reactions are assigned to OH adducts, substituted benzyl radicals, and radical cations, respectively. The rates for oxygen addition to OH adducts of 2-, 3-, and 4-chlorotoluenes and cresols are high, with k f values lying in the range (1−2.7) × 108 M-1 s-1. The relatively higher stability constants of peroxyl radicals formed with cresols (K = k f/k r = (2−5) × 104 M-1) than with chlorotoluenes suggest that the product-forming reaction competes effectively with the reverse reaction in cresols. Dihydroxytoluenes with OH groups ortho to each other were only formed in oxygenated solutions of cresols, and the mechanism involves the addition of oxygen to 1,3-type OH adducts at the carbon carrying the hydroxyl function.

Oxidants and oxygen enhance the sensitivity of cells to radiation. To understand this effect at the mechanistic level, the kinetics of interaction of the OH adducts of pyrimidines and 2‘-deoxynucleosides with oxidants (quinones, viologens, nitroarenes) of differing one-electron reduction potentials (−447 to 99 mV) have been determined in aqueous solution at pH 7.5−8 using the technique of pulse radiolysis. With quinones and viologens, this interaction produces the one-electron-reduced species of the oxidants, with rate constants (3.0 × 106 to 2.0 × 109 dm3 mol-1 s-1), which depend significantly on the redox potential of the oxidant. This dependence is consistent with an outer-sphere electron-transfer mechanism. In contrast, an addition (nitroxyl) adduct is formed with nitroarenes with rate constants that are weakly if at all dependent on the one-electron redox potentials of the nitroarenes. Using poly C as a probe for strand breakage, the resulting nitroxyl adduct of the nucleobase radical species in the absence of oxygen leads to strand breakage involving a base to sugar transfer of the radical site with a rate constant of 2.7 s-1. In contrast with benzoquinone, the resulting carbocation of the cytosine moiety of poly C does not result in strand breakage but leads to a decrease in the yield of ssb by ∼60%. Therefore nitroarenes mimic the effects of oxygen in leading to ssb on interaction with hydroxyl radical damage of nucleobases.