Lifetimes and reaction pathways of guanine radical cations and neutral guanine radicals in an oligonucleotide in aqueous solutions

The exposure of guanine in the oligonucleotide 5'-d(TCGCT) to one-electron oxidants leads initially to the formation of the guanine radical cation G(?+), its deptotonation product G(-H)(?), and, ultimately, various two- and four-electron oxidation products via pathways that depend on the oxidants and reaction conditions. We utilized single or successive multiple laser pulses (308 nm, 1 Hz rate) to generate the oxidants CO(3)(?-) and SO(4)(?-) (via the photolysis of S(2)O(8)(2-) in aqueous solutions in the presence and absence of bicarbonate, respectively) at concentrations/pulse that were ～20-fold lower than the concentration of 5'-d(TCGCT). Time-resolved absorption spectroscopy measurements following single-pulse excitation show that the G(?+) radical (pK(a) = 3.9) can be observed only at low pH and is hydrated within 3 ms at pH 2.5, thus forming the two-electron oxidation product 8-oxo-7,8-dihydroguanosine (8-oxoG). At neutral pH, and single pulse excitation, the principal reactive intermediate is G(-H)(?), which, at best, reacts only slowly with H(2)O and lives for ～70 ms in the absence of oxidants/other radicals to form base sequence-dependent intrastrand cross-links via the nucleophilic addition of N3-thymidine to C8-guanine (5'-G*CT* and 5'-T*CG*). Alternatively, G(-H)(?) can be oxidized further by reaction with CO(3)(?-), generating the two-electron oxidation products 8-oxoG (C8 addition) and 5-carboxamido-5-formamido-2-iminohydantoin (2Ih, by C5 addition). The four-electron oxidation products, guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp), appear only after a second (or more) laser pulse. The levels of all products, except 8-oxoG, which remains at a low constant value, increase with the number of laser pulses.     

Site-selective electron transfer from purines to electrocatalysts: voltammetric detection of a biologically relevant deletion in hybridized DNA duplexes.

BACKGROUND: The one-electron oxidation of guanine nucleobases is of interest for understanding the mechanisms of mutagenesis, probing electron-transfer reactions in DNA, and developing sensing schemes for nucleic acids. The electron-transfer rates for oxidation of guanine by exogenous redox catalysts depend on the base paired to the guanine. An important goal in developing the mismatch sensitivity is to identify a means for monitoring the current resulting from electron transfer at a single base in the presence of native oligonucleotides that contain all four bases. RESULTS: The nucleobase 8-oxo-guanine (8G) is selectively oxidized by the redox catalyst Os(bpy)(3)(3+/2+) (bpy = 2,2'-bipyridine) in the presence of native guanine. Cyclic voltammograms of Os(bpy)(3)(2+) show current enhancements indicative of nucleobase oxidation upon addition of oligonucleotides that contain 8G, but not in the presence of native guanine. As expected, similar experiments with Ru(bpy)(3)(2+) show enhancement with both guanine and 8G. The current enhancements for the 8G/Os(III) reaction increase in the order 8G-C approximately 8G.T < 8G.G < 8G.A < 8G, the same order as that observed for guanine/Ru(III). This site-selective mismatch sensitivity can be applied to detection of a TTT deletion, which is important in cystic fibrosis. CONCLUSIONS: The base 8G can be effectively used in conjunction with a low-potential redox catalyst as a probe for selective electron transfer at a single site. Because of the high selectivity for 8G, rate constants can be obtained that reflect the oxidation of only one base. The mismatch sensitivity can be used to detect biologically relevant abnormalities in DNA.     

Photooxidative damage of guanine in DG and DNA by the radicals derived from the alpha cleavage of the electronically excited carbonyl products generated in the thermolysis of alkoxymethyl-substituted dioxetanes and the photolysis of alkoxyacetones.

On thermolysis of the methoxy (MeO-TMD), tert-butoxy (tBuO-TMD), and hydroxy (HO-TMD) derivatives of 3,3,4,4-tetramethyl-1,2-dioxetane (TMD) in the presence of dG and calf-thymus DNA, the guanine is oxidized considerably more efficiently than the parent TMD. The same trend in the oxidative reactivity is observed for the photolysis of the corresponding oxy-substituted ketones versus acetone. The oxidative reactivity order in the dioxetane thermolysis, as well as in the ketone photolysis, parallels the ability of the excited ketones to release radicals (determined by spin trapping with DMPO and EPR spectroscopy) upon alpha cleavage (Norrish-type-I reaction). In the presence of molecular oxygen, the carbon-centered radicals are scavenged to produce peroxyl radicals, which are proposed as the reactive species in the oxidation of the guanine in dG and calf-thymus DNA.     

Possible cause of G-C-->C-G transversion mutation by guanine oxidation product, imidazolone

BACKGROUND: The genome is constantly assaulted by oxidation reactions which are likely to be associated with oxygen metabolism, and oxidative lesions are generated by many types of oxidants. Such genotoxin-induced alterations in the genomic message have been implicated in aging and in several pathophysiological processes, particularly those associated with cancer. The guanine base (G) in genomic DNA is highly susceptible to oxidative stress due to having the lowest oxidation potential. Therefore, G-C-->T-A and G-C-->C-G transversion mutations frequently occur under oxidative conditions. One typical lesion of G is 8-oxo-7,8-dihydro-guanine (8-oxoG), which can pair with A. This pairing may cause G-C-->T-A transversion mutations. Although the number of G-C-->C-G transversions is rather high under specific oxidation conditions such as riboflavin photosensitization, the molecular basis of G-C-->C-G transversions is not known. RESULTS: To determine which oxidative products are responsible for G-C-->C-G transversion mutations, we photooxidized 5'-d(AAAAAAGGAAAAAA)/5'-d(TTTTTTCCTTTTTT) using either riboflavin or anthraquinone (AQ) carboxylate under UV irradiation. Prolonged low-temperature (4 degrees C) enzymatic digestion of photoirradiated sample indicated that under both conditions the amount of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) initially increased with decreasing amounts of 2'-deoxyguanosine (dG), then decreased with the formation of 2-amino-5-[(2-deoxy-beta-D-erythro-pentofuranosyl)amino]-4H-imidazol-4-one (dIz), suggesting that nascent 8-oxoG was further oxidized to 2,5-diamino-4H-imidazol-4-one (Iz) in duplex DNA. Photoirradiation of an AQ-linked oligomer with a complementary strand containing 8-oxoG indicated that 8-oxoG residues were oxidized to Iz. These results indicate that Iz is formed from 8-oxoG through long-range hole migration. Primer extension experiments using a template containing Iz demonstrated that only dGTP is specifically incorporated opposite Iz suggesting that specific Iz-G base pairs are formed. The 'reverse' approach consisting of DNA polymerization using dIzTP showed that dIzTP is incorporated opposite G, further confirming the formation of a Iz-G base pair. CONCLUSIONS: HPLC product analysis demonstrated that Iz is a key oxidation product of G through 8-oxoG in DNA photosensitized with riboflavin or anthraquinone. Photoreaction of AQ-linked oligomer confirmed that Iz is formed from 8-oxoG through long-range hole migration. Two sets of primer extension experiments demonstrated that Iz can specifically pair with G in vitro. Specific Iz-G base pair formation can explain the G-C-->C-G transversion mutations that appear under oxidative conditions.     

Inhibitory effects of the guanine moiety on Suzuki couplings of unprotected halonucleosides in aqueous media

In the Suzuki arylations of unprotected halonucleosides in aqueous media, 8-bromo-2'-deoxyguanosine (8BrdG) couplings were slower to reach completion than the corresponding 8-bromo-2'-deoxyadenosine (8BrdA) couplings. The guanine moiety has an acidic proton, which under our Suzuki conditions (pH congruent with 10) may be deprotonated to give an anion that can coordinate to palladium. The possibility that guanine coordination was responsible for the observed slower rates was explored using additive experiments in which nonhalogenated nucleosides were added to the Suzuki coupling reaction of 8BrdA or 4-bromotoluene and PhB(OH)2 and the reaction progress monitored by HPLC or GC. Adding dG slowed these reactions, and an induction period was observed. The addition of dA or 1-methyl-2'-deoxyguanosine (1MedG) to these couplings did not affect the rate of conversion to product. Guanine coordination was further explored using 13C and 31P NMR spectroscopy, which implies that guanine is coordinating to palladium through N-1 or O-6, or both. Furthermore, the presence of dG inhibited the formation of the active palladium(0) catalytic species, which may account for both the observed induction period and the sluggishness of reactions where guanine is involved.     

On the evolution of one-electron-oxidized deoxyguanosine in damaged DNA under physiological conditions: a DFT and ONIOM study on proton transfer and equilibrium

Different deprotonation paths of the radical cation formed by one-electron oxidation of 2'-deoxyguanosine (2dG) sites in DNA have been studied using Density Functional Theory (M05-2X/6-31+G(d,p)) and ONIOM methodology (M05-2X/6-31+G(d,p):PM6) in conjunction with the SMD model to include the solvent effects. Models of increased complexity have been used ranging from the isolated nucleoside to a three unit double-stranded oligomer including the sugar units, the base pairing with cytidine, and the phosphate linkage. The reported results correspond to aqueous solution, at room temperature, and pH = 7.4. Under such conditions it was found that the proton transfer (PT) within the base pair is a minor path compared to the PT between the base pair and the surrounding water. It was also found that the deprotonation of ground-state 2dG˙(+) sites mainly yields C centered radicals in the sugar unit, with the largest populations corresponding to C4'˙ and C5'˙, followed by C3'˙. The different aspects of the presented theoretical study have been validated with experimental results.     

Redox pathways in DNA oxidation: kinetic studies of guanine and sugar oxidation by para-substituted derivatives of oxoruthenium(IV)

The oxidation of nucleotides and DNA by a series of complexes based on Ru(tpy)(bpy)O2+ (1) was investigated (tpy = 2,2':6',2"-terpyridine; bpy = 2,2'-bipyridine). These complexes were substituted with electron-donating or-withdrawing substituents in the para positions of the polypyridyl ligands so that the oxidation potentials of the complexes were affected but the reaction trajectory of the oxo ligand with DNA was the same throughout the series. The prepared complexes were (with E1/2(III/II) and E1/2(IV/III) values in volts versus Ag/AgCl) Ru(4'-EtO-tpy)(bpy)O2+ (2; 0.47, 0.60), Ru(4'-Cl-tpy)(bpy)O2+ (3; 0.55, 0.63), Ru(tpy)(4,4'-Me2-bpy)O2+ (4; 0.48, 0.62), and Ru(tpy)(4,4'-Cl2-bpy)O2+ (5; 0.58, 0.63). The complexes oxidized deoxycytosine 5'-monophosphate at the sugar moiety (k = 0.24-0.47 M-1 s-1) and guanosine 5'-monophosphate at the base moiety (k = 6.1-15 M-1 s-1). The rate constants increase across these ranges in the order 3 > 1 > 4 > 2, which is the same order as the redox potentials of the complexes. The effect of the base on these reactions was also studied, and xanthine was found to react with 1 much faster than guanine while hypoxanthine was less reactive than the sugar moiety. The complexes all oxidized oligonucleotides to generate base-labile lesions at guanine and a combination of spontaneous and base-labile scission at the sugar functionalities. The selectivity of cleavage in duplex and single-stranded DNA was not a strong function of the substituents on the metal complex.     

Effect of base sequence and deprotonation of Guanine cation radical in DNA

The deprotonation of guanine cation radical (G+*) in oligonucleotides (ODNs) was measured spectroscopically by nanosecond pulse radiolysis. The G+* in ODN, produced by oxidation with SO4-*, deprotonates to form the neutral G radical (G(-H)*). In experiments using 5-substituted cytosine-modified ODN, substitution of the cytosine C5 hydrogen by a methyl group increased the rate constant of deprotonation, whereas replacement by bromine decreased the rate constant. Kinetic solvent isotope effects on the kinetics of deoxyguanosine (dG) and ODN duplexes were examined in H2O and D2O. The rate constant of formation of G(-H)* in dG was 1.7-fold larger in H2O than D2O, whereas the rate constant in the ODN duplex was 3.8-fold larger in H2O than D2O. These results suggest that the formation of G(-H)* from G+* in the ODN corresponds to the deprotonation of the oxidized hydrogen-bridged (G+*-C) base pair by a water molecule. The characteristic absorption maxima of G+* around 400 nm were shifted to a longer wavelength in the order of G相似文献   

Structural characterization of carcinogen-modified oligodeoxynucleotide adducts using matrix-assisted laser desorption/ionization mass spectrometry

The aim of this study was to determine the chemical structure of in vitro 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-modified oligodeoxynucleotides (ODNs) by exonuclease digestion and matrix-assisted laser desorption/ionization mass spectrometry. A single-stranded 11-mer ODN, 5'-d(CCATCGCTACC), was reacted with N-acetoxy-PhIP, resulting in the formation of one major and eight minor PhIP-ODN adducts. A 10 min treatment of the major and one minor PhIP-ODN adduct with a 3'-exonuclease, bovine intestinal mucosa phosphodiesterase (BIMP), and a 5'-exonuclease, bovine spleen phosphodiesterase, results in inhibition of the primary exonuclease activity at deoxyguanosine (dG) producing 5'-d(CCATCG(PhIP)) and 5'-d(G(PhIP)CTACC) product ions, respectively. Post-source decay (PSD) of these enzymatic end products identifies dG as the sole modification site in two 11-mer ODN-PhIP adducts. PSD of the minor PhIP-ODN adduct digestion end product, 5'-d(CCATCG(PhIP)), also reveals that the PhIP adducted guanine moiety is in an oxidized form. Prolonged treatment of the PhIP-ODN adducts at 37 degrees C with BIMP induces a non-specific, or endonuclease, enzymatic activity culminating in the formation of deoxyguanosine 5'-monophosphate-PhIP (5'-dGMP-PhIP). The PSD fragmentation pattern of the 5'-dGMP-PhIP [M + H](+) ion of the major adduct confirms PhIP binds to the C-8 position of dG. For the minor adduct, PSD results suggest that PhIP binds to the C-8 position of an oxidized guanine, supporting the hypothesis that this adduct arises from oxidative degradation, resulting in a spirobisguanidino structure.     

Selective one-electron oxidation of duplex DNA oligomers: reaction at thymines

The one-electron oxidation of duplex DNA generates a nucleobase radical cation (electron "hole") that migrates long distances by a hopping mechanism. The radical cation reacts irreversibly with H2O or O2 to form oxidation products (damaged bases). In normal DNA (containing the four common DNA bases), reaction occurs most frequently at guanine. However, in DNA duplexes that do not contain guanine (i.e., those comprised exclusively of A/T base pairs), we discovered that reaction occurs primarily at thymine and gives products resulting from oxidation of the T-C5 methyl group and from addition to its C5-C6 double bond. This surprising result shows that it is the relative reactivity, not the stability, of a nucleobase radical cation that determines the nature of the products formed from oxidation of DNA. A mechanism for reaction is proposed whereby a thymine radical cation may either lose a proton from its methyl group or H2O/O2 may add across its double bond. In the latter case, addition may initiate a tandem reaction that converts both thymines of a TT step to oxidation products.     

Regulation of one-electron oxidation rate of guanine by base pairing with cytosine derivatives

Effects of base pairing on the one-electron oxidation rate of guanine derivatives, guanine, 8-bromoguanine, and 8-oxo-7,8-dihydroguanine have been studied. The one-electron oxidation rate of guanine derivatives was determined by triplet-quenching experiments, using N,N'-dibutylnaphthaldiimide (NDI) in the triplet excited state (3NDI*) and fullerene (C(60)) in the triplet excited state ((3)C(60*)) as oxidants. In all three guanine derivatives studied here, acceleration of the one-electron oxidation was observed upon hydrogen bonding with cytosine, which demonstrates lowering of the oxidation potential of guanine derivatives by base pairing with cytosine. When a methyl or bromo group was introduced to the C5 position of cytosine, acceleration or suppression of the one-electron oxidation relative to the guanine:cytosine base pair was observed, respectively. The results demonstrate that the one-electron oxidation rate of guanine in DNA can be regulated by introducing a substituent on base pairing cytosine.     

Base-dependent electron photodetachment from negatively charged DNA strands upon 260-nm laser irradiation

DNA multiply charged anions stored in a quadrupole ion trap undergo one-photon electron ejection (oxidation) when subjected to laser irradiation at 260 nm (4.77 eV). Electron photodetachment is likely a fast process, given that photodetachment is able to compete with internal conversion or radiative relaxation to the ground state. The DNA [6-mer]3- ions studied here show a marked sequence dependence of electron photodetachment yield. Remarkably, the photodetachment yield (dG6 > dA6 > dC6 > dT6) is inversely correlated with the base ionization potentials (G < A < C < T). Sequences with guanine runs show increased photodetachment yield as the number of guanine increases, in line with the fact that positive holes are the most stable in guanine runs. This correlation between photodetachment yield and the stability of the base radical may be explained by tunneling of the electron through the repulsive Coulomb barrier. Theoretical calculations on dinucleotide monophosphates show that the HOMO and HOMO-1 orbitals are localized on the bases. The wavelength dependence of electron detachment yield was studied for dG63-. Maximum electron photodetachment is observed in the wavelength range corresponding to base absorption (260-270 nm). This demonstrates the feasibility of gas-phase UV spectroscopy on large DNA anions. The calculations and the wavelength dependence suggest that the electron photodetachment is initiated at the bases and not at the phosphates. This also indicates that, although direct photodetachment could also occur, autodetachment from excited states, presumably corresponding to base excitation, is the dominant process at 260 nm. Excited-state dynamics of large DNA strands still remains largely unexplored, and photo-oxidation studies on trapped DNA multiply charged anions can help in bridging the gap between gas-phase studies on isolated bases or base pairs and solution-phase studies on full DNA strands.     

Efficient syntheses of the keto-carotenoids canthaxanthin, astaxanthin, and astacene

Three keto-carotenoids were prepared by the oxidation of the stable C(40) trisulfone 6, which has been used as the key compound in our beta-carotene synthesis. The first allylic oxidation to the unsaturated ketone and the second oxidation to the alpha-hydroxyketone produced the C(40) trisulfones 7 and 10, respectively. The Ramberg-Backlund reaction of the oxidized C(40) trisulfone was efficiently effected by the use of a mild base, NaOMe, in the presence of CCl(4) as a halogenating agent to give the C(40) disulfones 8 and 11. Base-promoted dehydrosulfonation reaction of the disulfone compounds produced the fully conjugated polyenes of canthaxanthin (1), astaxanthin (2), and astacene (3).     

Long-range oxidative damage to DNA: effects of distance and sequence

INTRODUCTION: Oxidative damage to DNA in vivo can lead to mutations and cancer. DNA damage and repair studies have not yet revealed whether permanent oxidative lesions are generated by charges migrating over long distances. Both photoexcited *Rh(III) and ground-state Ru(III) intercalators were previously shown to oxidize guanine bases from a remote site in oligonucleotide duplexes by DNA-mediated electron transfer. Here we examine much longer charge-transport distances and explore the sensitivity of the reaction to intervening sequences. RESULTS: Oxidative damage was examined in a series of DNA duplexes containing a pendant intercalating photooxidant. These studies revealed a shallow dependence on distance and no dependence on the phasing orientation of the oxidant relative to the site of damage, 5'-GG-3'. The intervening DNA sequence has a significant effect on the yield of guanine oxidation, however. Oxidation through multiple 5'-TA-3' steps is substantially diminished compared to through other base steps. We observed intraduplex guanine oxidation by tethered *Rh(III) and Ru(III) over a distance of 200 A. The distribution of oxidized guanine varied as a function of temperature between 5 and 35 degrees C, with an increase in the proportion of long-range damage (> 100 A) occurring at higher temperatures. CONCLUSIONS: Guanines are oxidized as a result of DNA-mediated charge transport over significant distances (e.g. 200 A). Although long-range charge transfer is dependent on distance, it appears to be modulated by intervening sequence and sequence-dependent dynamics. These discoveries hold important implications with respect to DNA damage in vivo.     

The effect of oxidation on the stability of G:C base pair: a MP2 study

The base-pairing energies of eleven oxidized G:C base pairs were characterized by Møller–Plesset perturbation theory. The analysis was focused on the base pairs consisting of guanine and one of the following cytosine derivatives: G:C1 (5-hydroxycytosine); G:C2 (5-hydroxyuracil); G:C3 (5,6-dihydroxy-cytosine); G:C4 (5,6-dihydroxy-uracil); G:C5 (cytosine glycol);G:C6 (isodialuric acid); G:C7 (uracil glycol), and the base pairs between cytosine and one of the following guanine derivatives: G8:C (8-oxo-guanine); G9:C (6-enol-8-keto-guanine); G10:C (xantho-sine); and G11:C (8-hydroxy-guanine). Full geometry optimizations have been performed for the studied complexes by MP2 method. The interaction energies were corrected for the basis-set superposition error (BSSE), using the full Boys-Bernardi counterpoise correction scheme. The results obtained show that the interaction energies of the base pairs decrease in the following order: G8:C > G:C5 ~ G:C1 > G:C3 ~ G:C ~ G11:C > G:C2 > G:C4 > G:C7 > G:C6 > G10:C > G9:C.     

Oxidation of guanosine derivatives by a platinum(IV) complex: internal electron transfer through cyclization

Many transition-metal complexes mediate DNA oxidation in the presence of oxidizing radiation, photosensitizers, or oxidants. The DNA oxidation products depend on the nature of the metal complex and the structure of the DNA. Earlier we reported trans-d,l-1,2-diaminocyclohexanetetrachloroplatinum (trans-Pt(d,l)(1,2-(NH(2))(2)C(6)H(10))Cl(4), [Pt(IV)Cl(4)(dach)]; dach = diaminocyclohexane) oxidizes 2'-deoxyguanosine 5'-monophosphate (5'-dGMP) to 7,8-dihydro-8-oxo-2'-deoxyguanosine 5'-monophosphate (8-oxo-5'-dGMP) stoichiometrically. In this paper we report that [Pt(IV)Cl(4)(dach)] also oxidizes 2'-deoxyguanosine 3'-monophosphate (3'-dGMP) stoichiometrically. The final oxidation product is not 8-oxo-3'-dGMP, but cyclic (5'-O-C8)-3'-dGMP. The reaction was studied by high-performance liquid chromatography, (1)H and (31)P nuclear magnetic resonance, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The proposed mechanism involves Pt(IV) binding to N7 of 3'-dGMP followed by nucleophilic attack of a 5'-hydroxyl oxygen to C8 of G and an inner-sphere, 2e(-) transfer to produce cyclic (5'-O-C8)-3'-dGMP and [Pt(II)Cl(2)(dach)]. The same mechanism applies to 5'-d[GTTTT]-3', where the 5'-dG is oxidized to cyclic (5'-O-C8)-dG. The Pt(IV) complex binds to N7 of guanine in cGMP, 9-Mxan, 5'-d[TTGTT]-3', and 5'-d[TTTTG]-3', but no subsequent transfer of electrons occurs in these. The results indicate that a good nucleophilic group at the 5' position is required for the redox reaction between guanosine and the Pt(IV) complex.     

A theoretical characterization of reactions of HOOO radical with guanine: formation of 8-oxoguanine

Hydrogen trioxide (HOOO) radical and other polyoxides of general formula, RO_nR (where R stands for hydrogen, other atoms or groups and n?≥?3), are believed to be key intermediates in atmospheric chemistry and biological oxidation reactions. In this contribution, DFT calculations using M06-2X density functional and the 6-31G(d,p) and 6-311+G(d,p) basis sets have been carried out to study different reactions of HOOO radical with guanine such as addition of HOOO radical at the C2, C4, C5, and C8 sites of guanine, abstraction of hydrogen atoms (H1, H2a, and H8) of guanine, and the mechanisms of oxidation of guanine with HOOO radical yielding 8-oxoguanine(a highly mutagenic derivative of guanine) and its radical in gas phase and aqueous media. The polarizable continuum model (PCM) has been used for solvation calculations in aqueous media. Our calculations reveal that the C8 site of guanine is the most reactive site for addition of HOOO radical, and adduct formed at this site would be appreciably stable. The rate constant (\( =\frac{K_bT}{h}{e}^{-\frac{\Delta {E}^b}{RT}} \)) at the C8 site is found to be 6.07?×?10⁷ (2.89?×?10⁷) s^?1 at the M06-2X/6-311+G(d,p) level of theory in gas phase (aqueous media). The calculated barrier energy and heat of formation of hydrogen abstraction reactions show that HOOO radical would not abstract hydrogen atoms of guanine. Oxidation of guanine with HOOO radical can occur following two schemes (Scheme 1 and Scheme 2). It is found that formation of 8-oxoguanine radical via Scheme 1 would predominate over formation of 8-oxoguanine via Scheme 2, in a reaction of HOOO radical and guanine. Thus, HOOO radical can be treated as a member of reactive oxygen species (ROS) which play key roles in biological oxidation reactions, in agreement with previous literature reports.     

One-electron oxidation of the guanine moiety of 2'-deoxyguanosine: influence of 8-oxo-7,8-dihydro-2'-deoxyguanosine

The influence of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) on riboflavin and UVA-mediated one-electron oxidation of an aqueous aerated solution of 2'-deoxyguanosine (dGuo) has been studied. Using labeled experiments, we have demonstrated that, despite not being able to detect significant amounts of 8-oxodGuo upon one-electron oxidation of dGuo, 8-oxodGuo is indeed produced but is further rapidly degraded to oxidized nucleosides. Evidence is provided showing that an efficient electron transfer reaction from 8-oxodGuo to the guanine radical cation or rather its deprotonated form occurs, giving rise to the specific decomposition of 8-oxodGuo together with the restitution of dGuo. It could be concluded that 8-oxodGuo efficiently protects dGuo from decomposition by the one-electron oxidation reaction.     

N(2)-phenyldeoxyguanosine: modulation of the chemical properties of deoxyguanosine toward one-electron oxidation in DNA

We have shown here that (1) substitution of an exocyclic amino group of dG is effective in modulating the chemical properties of dG toward one-electron oxidation and (2) decomposition of the guanine radical cation was effectively suppressed near dPhG. These results indicate that dPhG is a prototype of nucleosides functioning as an intrinsic antioxidant of duplex DNA toward one-electron oxidation.     

Electrochemical Study on DNA Damage Based on the Direct Oxidation of 8‐Hydroxydeoxyguanosine at an Electrochemically Modified Glassy Carbon Electrode

The study of DNA damage induced by Fenton reaction (Fe²⁺/H₂O₂) in vitro was performed based on the direct electrochemical oxidation of 8‐hydroxydeoxyguanosine (8‐OH‐dG), the biomarker of DNA oxidative damage, at an electrochemically modified glassy carbon electrode (GCE). The effects of antioxidants, such as ascorbic acid, and hydroxyl‐radical scavenger (mannitol) on the DNA damage were also investigated. 8‐OH‐dG, the oxidation product of guanine residues in DNA, has shown significantly oxidative peak on the electrochemically modified GCE. The oxidative peak current of 8‐OH‐dG was linear with the damaged DNA concentration in the range of 10–200 mg/L. The experimental results demonstrate that ascorbic acid has ambivalent effect on DNA oxidative stress. It can promote DNA oxidative damage when ascorbic acid concentration is below 1.5 mM and protect DNA from damage in the range of 1.5–2.5 mM. As a hydroxyl‐radical scavenger, mannitol inhibits significantly DNA oxidative damage. The influence of Fe²⁺, as reactant, and EDTA as iron chelator in the system were also studied. The proposed electrochemical method can be used for the estimation of DNA oxidative damage from new point of view.