OH Radical Adduct Research Articles

Fast repair of oxidizing oh radical adduct of dGMP by hydroxycinnamic acid derivatives. A pulse radiolytic study

Using the technique of pulse radiolysis, it has been demonstrated that the interaction of the oxidizing OH radical adduct of 2′-deoxyguanosine-5′-monophosphate (dGMP) with hydroxycinnamic acid derivatives proceeds via an electron transfer reaction. From buildup kinetics of radical species the rate constants of electron transfer from hydroxycinnamic acid derivatives to the oxidizing OH radical adduct of dGMP have been determined to be k ≈ (2–3) x 10 9 dm −3 mol −1 s −1. The yields for oxidizing OH radical adduct of dGMP are determined to be G ≈ 2.8.

Mechanism of free radical oxidation of caffeine in aqueous solution

The oxidation of caffeine with persulfate and hydroxyl radicals in aqueous solution has been studied by EPR spectroscopy and HPLC analysis. In both cases the formation of 1,3,7-trimethyluric acid is observed as the main final product. A C8–OH radical adduct is postulated as the intermediate after reaction with OH˙, and leads to the final product after further oxidation. This radical is too short-lived to be observed by EPR. After oxidation of caffeine with SO4˙- its radical cation is detected by EPR. This radical reacts with water to produce the above mentioned C8–OH radical adduct after deprotonation. In the presence of dibasic phosphate a spectrum attributed to the C8-phosphate radical adduct is observed. This radical results from the nucleophilic attack of the buffer on the radical cation of caffeine. Hydrolysis and oxidation of the phosphate radical intermediate results in the formation of 1,3,7-trimethyluric acid.

The Origin of the Hydroxyl Radical Oxygen in the Fenton Reaction

There is an ongoing discussion in the chemical literature regarding the nature of the highly reactive hydroxyl radical formed from the reaction between ferrous iron and hydrogen peroxide (the Fenton reaction). However, the fundamental experiment of directly determining the source of the hydroxyl radicals formed in the reaction has not yet been carried out. In this study, we have used both hydrogen peroxide and water labeled with 17O, together with ESR spin trapping, to detect the hydroxyl radicals formed in the reaction. ESR experiments were run in phosphate buffer with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap, and either H 2O 2 or H 2O labeled with 17O. The hydroxyl radical was generated by addition of Fe 2+ ion to H 2O 2, or as a control, by photolysis of H 2O 2 in the ESR cavity. Observed ESR spectra were the sum of DMPO/ 16 OH and DMPO/ 17 OH radical adduct spectra. Within experimental accuracy, the percentage of 17O-labeled hydroxyl radical trapped by the DMPO was the same as in the original hydrogen peroxide, for either method of hydroxyl radical generation, indicating that the trapped hydroxyl radical was derived exclusively from hydrogen peroxide and that there was no exchange of oxygen atoms between H 2O 2 and solvent water. Likewise, the complementary reaction with ordinary H 2O 2 and 17O-labeled water also showed that none of the hydroxyl radical was derived from water. Our results do not preclude the ferryl intermediate, [FeO] 2+ reacting with DMPO to form DMPO/ OH if the ferryl oxygen is derived from H 2O 2 rather than from a water ligand. Copyright © 1997 Elsevier Science Inc.

Photoionization of Indole, N‐Methylindole and Tryptophan In Aqueous Solution upon Excitation at 193 nm

Abstract—The 193 nm photoionization of aqueous indole, A'‐meth‐ylindole and tryptophan–as a function of pH and under several saturating gas conditions–has been studied by laser photolysis using optical and conductometric detection methods. Monophotonic ionization leads to production of the cation radicals and hydrated electrons, the quantum yield of electron ejection is 0.3–0.4. The cation radicals have pKavalues of 4.5, <5 and 4.5 for indole,N‐methylindole and tryptophan, respectively. Above these pH values, the cation radical deprotonate rapidly, having lifetimes of 1.0, ≅6 and 1.1 μs, respectively. Under N2O, neutral indolyl radical production is accompanied by formation of an OH adduct radical (<1 μs). The conductivity results in Ar‐ and N2O‐saturated solution support the deprotonation mechanism and indicate that in the acidic pH range, the cation radical decays by release of protons with kinetics on the millisecond time scale.

Oxidized intermediates formed from α- and β-galactosyl ascorbic acid by pulse radiolysis

The formation of OH-adduct radicals and the one-electron oxidation radicals of 6- O- α- and β-galactosyl ascorbic acid (α- and β-GA) were studied by pulse radiolysis. The OH-adduct radicals formed by the reaction between α- or β-GA and OH radicals were converted to their dehydrated radicals in neutral and alkaline solutions. The rate constants for reactions between OH radicals and α- or βGA were slightly smaller than that of ascorbic acid (AsA). The rate constants for the formation of oxidized radicals of GA with N 3 radicals were smaller than those of OH-adduct radicals. The rate constants for the decay of the α- or β-forms of galactosyl ascorbate radicals were twice as fast as that of AsA. The oxidized radicals of GA were found to decay by a disproportionation reaction on γ-ray irradiation.

Amflutizole, a xanthine oxidase inhibitor, inhibits free radical generation in the ischemic/reperfused rat cerebral cortex

Free radical generation and release from the cerebral hemispheres of rats subjected to four-vessel occlusion elicited cerebral ischemia/reperfusion was monitored using a cortical cup technique with the spin-trapping agent α-(4-pyridyl-1-oxide)- N-tert-butylnitrone (POBN). Electron spin resonance (ESR) was used to detect the presence of free radical adducts of POBN in the cortical superfusates. Thirty min of ischemia plus reperfusion resulted in the release of ·OH radical adducts during ischemia and especially in the initial stages of reperfusion. Pretreatment with the xanthine oxidase inhibitor, amflutizole (30 mg/kg) virtually abolished free radical formation and release. These results are consistent with earlier evidence that xanthine oxidase activity contributes to free radical formation in the ischemic/reperfused rat brain.

Free radical induced oxidation of phloroglucinol. A pulse radiolysis and EPR study

Phloroglucinol-derived radicals have been studied using pulse radiolysis and EPR spectroscopy. Phloroglucinol 1a(pKa= 8.0) and its anion 1b(pKa= 9.2) have phenolic structures while the 3,5-dihydroxycyclohexa-2,5-dienone structure predominates in the dianion 2c. The neutral OH-adduct radicals (λmax= 345 nm) rapidly eliminate water (k= 2 × 105 s–1) yielding the 3,5-dihydroxyphenoxyl radical 4a(λmax= 495 nm, pKa= 6.5). This radical as well as its monoanion 4b(λmax= 550 nm, pKa= 8.6), its isomer 5b, derived from 2c(λmax > 800 nm), and the dianion 4c(λmax= 640 nm) can be generated directly with the N3 radical (k= 1.4 × 109 dm3 mol–1 s–1 at pH 6). All four radicals have been characterized by EPR spectroscopy. Radical 5b reacts with the phloroglucinol monoanion 1b with a rate constant of 2 × 107 dm3 mol–1 s–1. Formation of an adduct is excluded by EPR. Therefore, electron transfer from the phloroglucinol monoanion 1b to radical 5b is favoured as an explanation for this reaction.While 4a does not react with O2(k < 4 × 105 dm3 mol–1 s–1), the anions 4b and 5b do so quite rapidly (k= 2.1 × 108 and 1.9 × 108 dm3 mol–1 s–1, respectively) and at pH 7, O2 is consumed with G= 15 × 10–7 mol J–1.Although Br2˙– mainly produces radicals 4a and b, bromination occurs with an efficiency of at least 10%.

Oxidation intermediates of l-ascorbic acid 2-glucoside by pulse radiolysis

Radical species generated from l-ascorbic acid 2-glucoside (2-G-AH) and OH radicals (OH•) or azide radicals (N3•) were investigated by pulse radiolysis. The OH adduct radicals formed by the reaction of 2-G-AH with OH• were converted into the ascorbic acid radical (A•−) in alkaline solutions with the elimination of glucose. Oxidation radicals with a broad absorption spectrum were produced directly upon oxidation of 2-G-AH by the oxidizing N3•, and some of these radicals were converted into A•− in strongly alkaline solutions. Ascorbic acid was detected from 60Co γ ray irradiation on strongly alkaline N2O-saturated solutions of 2-G-AH owing to disproportionation of the A•−.

Oxypurinol attenuates hydroxyl radical production during ischemia/reperfusion injury of the rat cerebral cortex: an ESR study

Free radical generation and release from the cerebral hemispheres of rats subjected to four vessel occlusion followed by reperfusion was monitored using a cortical cup technique in conjunction with the spin-trapping agent α(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN). Electron spin resonance (ESR) was used to detect the presence of free radical adducts of POBN in the cortical superfusates. 30 min of ischemia plus reperfusion resulted in the release of OH radical adducts during the period of ischemia and, especially, during the initial phases of reperfusion. No radical adducts were detectable 90 min after the onset of reperfusion. Pretreatment with the xanthine oxidase inhibitor, oxypurinol (40 mg/kg i.p.), virtually abolished free radical formation and release. The results of this study are consistent with earlier evidence of free radical formation during ischemia/reperfusion and suggest that the cerebroprotective actions of oxypurinol may be related to its ability to prevent the cascade of free radical generation.

Phenylalanine: Its ˙OH and SO4˙⁻-Induced Oxidation and Decarboxylation. A Pulse Radiolysis and Product Analysis Study

Phenylamine has been oxidized by radiolytically generated hydroxyl and sulfate radicals, the ensuing intermediates and their reactions have been studied by pulse radiolysis and product analysis in the absence and presence of oxidants such as Fe(CN)6 3- and O2. Upon OH radical attack, hydroxycyclohexadienyl-type radicals are mainly formed while Η-abstraction reactions can be neglected. In the presence of Fe(CN)6 3- these radicals are for the most part oxidized to the corresponding tyrosines (80%), except for the ipso-OH-adduct radicals (≈ 20%). It is concluded that ˙OH-addition is almost random, but with a slight avoidance of the metaposition relative to the ortho-, para- and ipso-positions. Oxygen adds reversibly to the OH-adduct radicals (kf = 1.8 × 108 dm3 mol-1 s-1, kr = 5.4 × 104 s-1). In this case, tyrosine formation occurs by HO2˙-elimination. However, due to side reactions, tyrosine formation only reaches 52% of the OH radical yield. The tyrosine yield drops to 10% in the absence of an oxidant. Upon SO4˙⁻-attack, decarboxylation becomes a major process (33% of SO4˙⁻) alongside the production of tyrosines (43%). Here, with Fe(CN)6 3- as the oxidant the formation of p-Tyr (18.5%) and m-Tyr (16.5%) is preferred over o-Tyr formation (8.5%). It is believed that in analogy to other systems a radical cation is formed immediately upon SO4˙⁻-attack which either reacts with water under the formation of hydroxycyclohexadienyl-type (“OH-adduct”) radicals, or decarboxylates after intramolecular electron transfer. The radical cation can also arise indirectly through H+-catalysed water elimination from the ˙OH-adduct radicals. At pH 2 and a dose rate of 0.0046 Gy s-1 CO2 formation matches the OH radical yield when ˙OH is the attacking radical. Below pH 2, G(CO2) decreases with falling pH. This indicates the occurrence of another, unimolecular, pathway under these conditions competing effectively with decarboxylation. This appears to be a relatively slow deprotonation reaction of the carboxylprotonated phenylalanine radical cation which gives rise to the benzyl-type radical.

Generation of Free radicals in reactions of Ni(II)-thiol complexes with molecular oxygen and model lipid hydroperoxides

The generation of free radicals from reactions of nickel(II)-thiol complexes with molecular oxygen and model lipid hydroperoxides was investigated by electron spin resonance (ESR) utilizing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap. Incubation of nickel(II) [Ni(II)] with cysteine in an aerobic environment generated hydroxyl (·OH) radical, which then reacted with cysteine to generate a carbon-centered alkyl (·R) radical. Radical generation was inhibited under a nitrogen atmosphere. Model lipid hydroperoxides, cumene hydroperoxide, and t-butyl hydroperoxide enhanced the yield of these radicals and also generated an alkoxyl (·OR) radical. Radical yield decreased by approximately half under a nitrogen atmosphere. Although histidine did not cause radical formation in the reaction between Ni(II) and cumene hydroperoxide under aerobic conditions, the addition of histidine to a mixture containing Ni(II), cysteine, and cumene hydroperoxide under the same experimental conditions increased the yield of ·R radical but lowered the yield of ·OR and ·OH radical adducts. It thus appears that histidine caused the ·OH attack to be more site-specific. Similar results were obtained utilizing t-butyl hydroperoxide. Penicillamine or N-acetylcysteine yielded similar results except that under aerobic conditions, reaction between Ni(II) and N-acetylcysteine without hydroperoxide did not generate a significant concentration of free radicals. Under the same experimental conditions, cystine did not generate any detectable free radicals, suggesting an important role of the -SH group in Ni(II)-mediated free radical generation. The results indicate that free radical generation from the reaction of Ni(II)-thiol complexes and molecular oxygen, and/or lipid hydroperoxides, may play an important role in the mechanism(s) of Ni(II) toxicity and carcinogenesis.

Alpha-Phenyl-Tert-Butyl-Nitrone (PBN) Attenuates Hydroxyl Radical Production During Ischemia-Reperfusion Injury of Rat Brain: An EPR Study

Alpha-phenyl-tert-butyl-nitrone (PBN) a spin adduct forming agent is believed to have a protective action in ischemia-reperfusion injury of brain by forming adducts of oxygen free radicals including .OH radical. Electron paramagnetic resonance (EPR) has been used to both detect and monitor the time course of oxygen free radical formation in the in vivo rat cerebral cortex. Cortical cups were placed over both cerebral hemispheres of methoxyflurane anesthetized rats prepared for four vessel occlusion-evoked cerebral ischemia. Prior to the onset of sample collection, both cups were perfused with artificial cerebrospinal fluid (aCSF) containing the spin trap agent alpha-(4-pyridyl-1-oxide)-N-tert butylnitrone (POBN 100 mM) for 20 min. In addition 50 mg/kg BW of POBN was administered intraperitoneally (IP) 20 min prior to ischemia in order to improve our ability to detect free radical adducts. Cup fluid was subsequently replaced every 15 min during ischemia and every 10 min during reperfusion with fresh POBN containing CSF and the collected cortical superfusates were analyzed for radical adducts by EPR spectroscopy. After a basal 10 min collection, cerebral ischemia was induced for 15 or 30 min (confirmed by EEG flattening) followed by a 90 min reperfusion. .OH radical adducts (characterized by six line EPR spectra) were detected during ischemia and 90 min reperfusion. No adduct was detected in the basal sample or after 90 min of reperfusion. Similar results were obtained when diethylenetriaminepenta-acetic acid (100 microM; DETAPAC) a chelating agent was included in the artificial CSF. Systemic administration of PBN (100 mg/kg BW) produced a significant attenuation of radical adduct during reperfusion. A combination of systemic and topical PBN (100 mM) was required to suppress .OH radical adduct formation during ischemia as well as reperfusion. PBN free radical adducts were detected in EPR spectra of the lipid extracts of PBN treated rat brains subjected to ischemia/reperfusion. Thus this study suggests that PBN's protective action in cerebral ischemia/reperfusion injury is related to its ability to prevent a cascade of free radical generation by forming spin adducts.

Detection of oxidation radicals of 2,3-enediol form of 2,3-dioxoguluno-δ-lactone by pulse radiolysis

The OH adduct radicals of the 2,3-enediol form of 2,3-dioxoguluno-δ-lactone (2,3-End.DKGL) have been observed in aqueous solution by puise radiolysis (the adduct radicals were converted into the one-electron oxidation radical in alkaline solutions by dehydration); the oxidation radical has also been produced by oxidation of 2,3-End.DKGL by the azide radical (N3˙).

Hydroxyl radical adduct at sulfur in substituted organic sulfides stabilized by internal hydrogen bond

The stability of an OH radical adduct at sulfur atoms in substituted organic sulfides is greatly enhanced by the formation of an internal hydrogen bond between the hydroxyl hydrogen and an oxygen located either in an adjacent carbonyl or methoxy group; the first absolute rate constants of the reactions of such adducts with molecular oxygen are reported.

Evidence against the 1:2:2:1 quartet DMPO spectrum as the radical adduct of the lipid alkoxyl radical

It was reported that the electron paramagnetic resonance (EPR) spectrum of 5,5-dimethyl-1-pyrroline N-oxide (DMPO)/lipid alkoxyl radical exhibited a quartet with 1:2:2:1 relative intensity that is identical to that of DMPO/hydroxyl radical ( K. M. Schaich and D. C. Borg, 1990, Free Radicals Res. Commun. 9, 267–278 ). We repeated these EPR experiments using HPLC separation of radical adducts and isotope substitution. We found that the HPLC EPR chromatogram of the radical adduct with a 1:2:2:1 quartet obtained by the reduction of methyl linoleate hydroperoxide (MLOOH) with Fe 2+ exhibited identical retention time to that of the DMPO . OH radical adduct obtained from the Fenton reaction in two different solvent systems. Upon performing the same reaction in 17O-enriched water, the 17O-hyperfine coupling constants due to DMPO .17 OH were identified. Ultimately, ~80–90% of the total DMPO . OH is derived from water by an iron-dependent nucleophilic addition reaction. Initially, a water-independent mechanism also significantly contributes to DMPO/.OH formation. Regardless of its mechanism of formation, the 1:2:2:1 quartet radical adduct of DMPO formed during the reduction of MLOOH by Fe 2+ is in fact DMPO . OH .

Reaction of dithiothreitol and para-nitroacetophenone with different radical precursors of .OH radical-induced strand break formation of single-stranded DNA in anoxic aqueous solution.

The yields of single-strand breakage (ssb) in single-stranded calf thymus DNA (ssDNA) have been determined after 60Co gamma-irradiation of aqueous anoxic solutions in the presence of different concentrations of dithiothreitol (DTT), ascorbate or trans-4,5-dihydroxy-1,2-dithiane, using low-angle laser light scattering. The influence of DTT on the kinetics of ssb formation has been determined by conductivity measurements in pulse radiolysis. The results suggest that strand breakage in ssDNA proceeds via two modes of about equal contribution and with half-lives of about 7 ms and 0.8s, respectively. Both modes reflect reactions of at least two DNA radicals, which react with DTT by hydrogen-atom transfer reactions with similar rate constants of about 5-9 x 10(5) dm3 mol-1 s-1. These hydrogen-atom transfer reactions inhibit strand break formation. The slow mode is shown to represent the decay of base-radicals to generate sugar radicals. The involvement of the oxidizing .OH adduct radical of guanine in the formation of strand breaks can be ruled out and there is no evidence for a contribution from the anion or radical anion of DTT to the inhibition of strand breaks via electron transfer reactions to DNA radicals.

Pulse radiolysis of trans-1,2-bis(4-pyridyl)ethylene in aqueous solution

Pulse radiolysis with optical and conductometric detection techniques was used to study the reactions of hydrated electrons (e{sub aq}{sup {minus}}), H atoms, 2-hydroxy-2-propyl radicals, and OH radicals with trans-1,2-bis(4-pyridyl)ethylene (neutral form: M) and its conjugate acids HM{sup +} and H{sub 2}M{sup 2+} in aqueous solution. Electron attachment to M leads to a transient with {lambda}{sub max} = 455 nm ({epsilon} = 4.0 {times} 10{sup 4} dm{sup 3} mol{sup {minus}1} cm{sup {minus}1}) at pH 11-12 and to another transient ({lambda}{sub max} = 500 nm, {epsilon} = 5.2 {times} 10{sup 4} dm{sup 3} mol{sup {minus}1} cm{sup {minus}1}) at pH 6-9. The 455-nm transient is assigned to a neutral H-adduct radical (HM{sup {sm bullet}}, H attached to an N atom), formed via reaction of water with the radical anion. The 500-nm transient is assigned to H{sub 2}M{sup {sm bullet}+}, the conjugate acid of HM{sup {sm bullet}}, with both H's at the N's; pK{sub a} = 9.8. At pH 4-6 H{sub 2}M{sup {sm bullet}+} is also formed by reactions of e{sub aq}{sup {minus}} or H atoms with HM{sup +} and H{sub 2}M{sup 2+} (at pH 1-5) quantitatively to H{sub 2}M{sup {sm bullet}+} but does not react with neutral M. Three OH-adduct radicals were observedmore » in different pH regions. M{sup {sm bullet}}OH, a neutral radical ({lambda}{sub max} = 392 nm), and its conjugate acid (HM{sup {sm bullet}+}OH, {lambda}{sub max} = 410 nm) have a pK{sub a} of about 6; at pH {le} 4.5 the latter radical is converted by acid catalysis into an isomer with {lambda}{sub max} = 510 nm. Structural assignments are presented for all radicals observed. Some results with trans-1,2-bis(n-pyridyl)ethylene (n = 2 or 3) are also reported.« less

QUINONE SENSITIZED ELECTRON TRANSFER PHOTOOXIDATION OF NUCLEIC ACIDS: CHEMISTRY OF THYMINE AND THYMIDINE RADICAL CATIONS IN AQUEOUS SOLUTION*

The 2-methyl-1,4-naphthoquinone (MQ) sensitized photooxidation of nucleic acid derivatives has been studied by laser flash photolysis and steady state methods. Thymine and thymidine, as well as other DNA model compounds, quench triplet MQ by electron transfer to give MQ radical anions and pyrimidine or purine radical cations. Although the pyrimidine radical cations cannot be directly observed by flash photolysis, the addition of N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) results in the formation of the TMPD radical cation via scavenging of the pyrimidine radical cation. The photooxidation products for thymine and thymidine are shown to result from subsequent chemical reactions of the radical cations in oxygenated aqueous solution. The quantum yield for substrate loss at limiting substrate concentrations is 0.38 for thymine and 0.66 for thymidine. The chemistry of the radical cations involves hydration by water leading to C(6)-OH adduct radicals of the pyrimidine and deprotonation from the N(1) position in thymine and the C(5) methyl group for thymidine. Superoxide ions produced via quenching of the quinone radical anion with oxygen appear to be involved in the formation of thymine and thymidine hydroperoxides and in the reaction with N(1)-thyminyl radicals to regenerate thymine. The effects of pH were examined in the range pH 5-8 in both the presence and absence of superoxide dismutase. Initial C(6)-OH thymine adducts are suggested to dehydrate to give N(1)-thyminyl radicals.

Formation of radical cations of polyiodomethanes: a pulse radiolysis study

The transient optical absorption band (λmax= 380 nm, t½= 23 µs and ε= 2010 dm3 mol–1 cm–1) produced on pulse radiolysis of N2O-saturated aqueous solution of CH2I2(pH 6.0) has been assigned to the OH adduct radical, CH2I2·OH. In acidic aqueous solutions, CH2I2+(λmax= 380 and 570 nm, t½= 3 µs and ε570= 2 150 dm3 mol–1 cm–1) has been identified as the intermediate. The rate constants for the reaction of OH radicals with CH2I2 to form CH2I2·OH and CH2I2+ have been determined to be 2.1 × 109 and 6.0 × 109 dm3 mol–1 s–1 respectively. Cl2– has been found to oxidize CH2I2 to CH2I2+ with a bimolecular rate constant of 1.7 × 108 dm3 mol–1 s–1. CH2l2+ is a good oxidant and oxidizes I–, Br–, and methyl disulphide with high rate constants close to the diffusion limit. I2 is the main stable end product formed on γ-radiolysis and its yield is equal to ½ G(-CH2I2). The transient optical absorption bands observed on pulse radiolysis of CH2I2 and CHI3 in 1,2-dichloroethane have been assigned to CH2I2+(λmax= 380 and 590 nm) and CHI3+(λmax= 400 and 610 nm).

Pulse-radiolytic studies on the reactions of SO -· 4 with uracil dervivatives

Results of pulse-radiolytic studies on the reaction of SO -· 4 in aqueous solution with different uracil derivatives are reported. From 1-methyluracil, 1,3-dimethyluracil and 1,3,6-trimethyluracil upon reaction with SO -· 4 the C(5)-OH adduct radicals are quickly formed in quantitative yields. With 1,3-dimethylthymine strong evidence is presented that additionally a C(5)-CH 2 allyl-type radical is formed. The latter is thought to be the result of a deprotonation from the methyl group at position C(5) of a short-lived radical cation which is preceded by an SO -· 4 adduct radical whose half-life is 2 μs. When the N(1) position of the uracil carries a sugar moiety, further transient absorbing species were observed, the structures of which are not known at present and which cannot be assigned to any of the known intermediates derived from uracil or its derivatives. The results suggest that the presence of a sugar moiety strongly determines the ultimate fate of the base radicals.