Electron adducts of adenine nucleosides and nucleotides in aqueous solution: protonation at two carbon sites (C2 and C8) and intra- and intermolecular catalysis by phosphate

Tracking Isomerizations of High-Energy Adenine Cation Radicals by UV-Vis Action Spectroscopy and Cyclic Ion Mobility Mass Spectrometry

We report experimental and computational studies of protonated adenine C-8 σ-radicals that are presumed yet elusive reactive intermediates of oxidative damage to nucleic acids. The radicals were generated in the gas phase by the collision-induced dissociation of C-8-Br and C-8-I bonds in protonated 8-bromo- and 8-iodoadenine as well as by 8-bromo- and 8-iodo-9-methyladenine. Protonation by electrospray of 8-bromo- and 8-iodoadenine was shown by cyclic-ion mobility mass spectrometry (c-IMS) to form the N-1-H, N-9-H and N-3-H, N-7-H protomers in 85:15 and 81:19 ratios, respectively, in accordance with the equilibrium populations of these protomers in water-solvated ions that were calculated by density functional theory (DFT). Protonation of 8-halogenated 9-methyladenines yielded single N-1-H protomers, which was consistent with their thermodynamic stability. The radicals produced from the 8-bromo and 8-iodo adenine cations were characterized by UV-vis photodissociation action spectroscopy (UVPD) and c-IMS. UVPD revealed the formation of C-8 σ-radicals along with N-3-H, N-7-H-adenine π-radicals that arose as secondary products by hydrogen atom migrations. The isomers were identified by matching their action spectra against the calculated vibronic absorption spectra. Deuterium isotope effects were found to slow the isomerization and increase the population of C-8 σ-radicals. The adenine cation radicals were separated by c-IMS and identified by their collision cross sections, which were measured relative to the canonical N-9-H adenine cation radical that was cogenerated in situ as an internal standard. Ab initio CCSD(T)/CBS calculations of isomer energies showed that the adenine C-8 σ-radicals were local energy minima with relative energies at 76-79 kJ mol^-1 above that of the canonical adenine cation radical. Rice-Ramsperger-Kassel-Marcus calculations of unimolecular rate constants for hydrogen and deuterium migrations resulting in exergonic isomerizations showed kinetic shifts of 10-17 kJ mol^-1, stabilizing the C-8 σ-radicals. C-8 σ-radicals derived from N-1-protonated 9-methyladenine were also thermodynamically unstable and readily isomerized upon formation.

The DNA Radical Code. Resolution of Identity in Dissociations of Trinucleotide Codon Cation Radicals in the Gas Phase

Sixty DNA trinucleotide cation radicals covering a large part of the genetic code alphabet were generated by electron transfer in the gas phase, and their chemistry was studied by collision-induced dissociation tandem mass spectrometry and theoretical calculations. The major dissociations involved loss of nucleobase molecules and radicals, backbone cleavage, and cross-ring fragmentations that depended on the nature and position of the nucleobases. Mass identity in dissociations of symmetrical trinucleotide cation radicals of the (XXX+2H)^+• and (XYX+2H)^+• type was resolved by specific ¹⁵N labeling. The specific features of trinucleotide cation radical dissociations involved the dominant formation of d₂⁺ ions, hydrogen atom migrations accompanying the formation of (w₂+H)^+•, (w₂+2H)⁺, and (d₂+2H)⁺ sequence ions, and cross-ring cleavages in the 3'- and 5'-deoxyribose moieties that depended on the nucleobase type and its position in the ion. Born-Oppenheimer molecular dynamics (BOMD) and density functional theory calculations were used to obtain structures and energies of several cation-radical protomers and conformers for (AAA+2H)^+•, (CCC+2H)^+•, (GGG+2H)^+•, (ACA+2H)^+•, and (CAA+2H)^+• that were representative of the different types of backbone dissociations. The ion electronic structure, protonation and radical sites, and hydrogen bonding were used to propose reaction mechanisms for the dissociations.

Charge-Tagged Nucleosides in the Gas Phase: UV-Vis Action Spectroscopy and Structures of Cytidine Cations, Dications, and Cation Radicals

Cytidine ribonucleosides were furnished at O5' with fixed-charge 6-trimethylammoniumhexan-1-aminecarbonyl tags and studied by UV-vis photodissociation action spectroscopy in the gas phase to probe isolated nucleobase chromophores in their neutral, protonated, and hydrogen-adduct radical forms. The action spectrum of the doubly charged cytidine conjugate showed bands at 310 and 270 nm that were assigned to the N3- and O2-protonated cytosine tautomers formed by electrospray, respectively. In contrast, cytidine conjugates coordinated to dibenzo-18-crown-6-ether (DBCE) in a noncovalent complex were found to strongly favor protonation at N3, forming a single-ion tautomer. This allowed us to form cytidine N3-H radicals by electron transfer dissociation of the complex and study their action spectra. Cytidine radicals showed only very weak absorption in the visible region of the spectrum for dipole-disallowed transitions to the low (A and B) excited states. The main bands were observed at 360, 300, and 250 nm that were assigned with the help of theoretical vibronic spectra obtained by time-dependent density functional theory calculations of multiple (>300) radical vibrational configurations. Collision-induced dissociations of cytidine radicals proceeded by major cleavage of the N1-C1' glycosidic bond leading to loss of cytosine and competitive loss of N3-hydrogen atom. These dissociations were characterized by calculations of transition-state structures and energies using combined Born-Oppenheimer molecular dynamics and DFT calculations. Overall, cytidine radicals were found to be kinetically and thermodynamically more stable than previously reported analogous adenosine and guanosine radicals.

Adenine radicals generated in alternating AT duplexes by direct absorption of low-energy UV radiation

There is increasing evidence that the direct absorption of photons with energies that are lower than the ionization potential of nucleobases may result in oxidative damage to DNA. The present work, which combines nanosecond transient absorption spectroscopy and quantum mechanical calculations, studies this process in alternating adenine-thymine duplexes (AT)n. We show that the one-photon ionization quantum yield of (AT)10 at 266 nm (4.66 eV) is (1.5 ± 0.3) × 10-3. According to our PCM/TD-DFT calculations carried out on model duplexes composed of two base pairs, (AT)1 and (TA)1, simultaneous base pairing and stacking does not induce important changes in the absorption spectra of the adenine radical cation and deprotonated radical. The adenine radicals, thus identified in the time-resolved spectra, disappear with a lifetime of 2.5 ms, giving rise to a reaction product that absorbs at 350 nm. In parallel, the fingerprint of reaction intermediates other than radicals, formed directly from singlet excited states and assigned to AT/TA dimers, is detected at shorter wavelengths. PCM/TD-DFT calculations are carried out to map the pathways leading to such species and to characterize their absorption spectra; we find that, in addition to the path leading to the well-known TA* photoproduct, an AT photo-dimerization path may be operative in duplexes.

An Ethenoadenine FAD Analog Accelerates UV Dimer Repair by DNA Photolyase

Reduced anionic flavin adenine dinucleotide (FADH^- ) is the critical cofactor in DNA photolyase (PL) for the repair of cyclobutane pyrimidine dimers (CPD) in UV-damaged DNA. The initial step involves photoinduced electron transfer from *FADH^- to the CPD. The adenine (Ade) moiety is nearly stacked with the flavin ring, an unusual conformation compared to other FAD-dependent proteins. The role of this proximity has not been unequivocally elucidated. Some studies suggest that Ade is a radical intermediate, but others conclude that Ade modulates the electron transfer rate constant (k_ET ) through superexchange. No study has succeeded in removing or modifying this Ade to test these hypotheses. Here, FAD analogs containing either an ethano- or etheno-bridged Ade between the AN1 and AN6 atoms (e-FAD and ε-FAD, respectively) were used to reconstitute apo-PL, giving e-PL and ε-PL respectively. The reconstitution yield of e-PL was very poor, suggesting that the hydrophobicity of the ethano group prevented its uptake, while ε-PL showed 50% reconstitution yield. The substrate binding constants for ε-PL and rPL were identical. ε-PL showed a 15% higher steady-state repair yield compared to FAD-reconstituted photolyase (rPL). The acceleration of repair in ε-PL is discussed in terms of an ε-Ade radical intermediate vs superexchange mechanism.

Oxidatively induced DNA damage and its repair in cancer

Oxidatively induced DNA damage is caused in living organisms by endogenous and exogenous reactive species. DNA lesions resulting from this type of damage are mutagenic and cytotoxic and, if not repaired, can cause genetic instability that may lead to disease processes including carcinogenesis. Living organisms possess DNA repair mechanisms that include a variety of pathways to repair multiple DNA lesions. Mutations and polymorphisms also occur in DNA repair genes adversely affecting DNA repair systems. Cancer tissues overexpress DNA repair proteins and thus develop greater DNA repair capacity than normal tissues. Increased DNA repair in tumors that removes DNA lesions before they become toxic is a major mechanism for development of resistance to therapy, affecting patient survival. Accumulated evidence suggests that DNA repair capacity may be a predictive biomarker for patient response to therapy. Thus, knowledge of DNA protein expressions in normal and cancerous tissues may help predict and guide development of treatments and yield the best therapeutic response. DNA repair proteins constitute targets for inhibitors to overcome the resistance of tumors to therapy. Inhibitors of DNA repair for combination therapy or as single agents for monotherapy may help selectively kill tumors, potentially leading to personalized therapy. Numerous inhibitors have been developed and are being tested in clinical trials. The efficacy of some inhibitors in therapy has been demonstrated in patients. Further development of inhibitors of DNA repair proteins is globally underway to help eradicate cancer.

Mechanisms of free radical-induced damage to DNA

Endogenous and exogenous sources cause free radical-induced DNA damage in living organisms by a variety of mechanisms. The highly reactive hydroxyl radical reacts with the heterocyclic DNA bases and the sugar moiety near or at diffusion-controlled rates. Hydrated electron and H atom also add to the heterocyclic bases. These reactions lead to adduct radicals, further reactions of which yield numerous products. These include DNA base and sugar products, single- and double-strand breaks, 8,5'-cyclopurine-2'-deoxynucleosides, tandem lesions, clustered sites and DNA-protein cross-links. Reaction conditions and the presence or absence of oxygen profoundly affect the types and yields of the products. There is mounting evidence for an important role of free radical-induced DNA damage in the etiology of numerous diseases including cancer. Further understanding of mechanisms of free radical-induced DNA damage, and cellular repair and biological consequences of DNA damage products will be of outmost importance for disease prevention and treatment.

One-electron reduction of 2-aminopurine in the aqueous phase. A DFT and pulse radiolysis study

The electron affinity and the subsequent proton affinity of the electron adducts of 2-aminopurine (abbreviated 2AP) and adenine (A) are calculated with density functional theory (DFT). According to these calculations 2AP*- and A*- have similar thermochemical parameters leading to the conclusion that their reaction pathways should be close to analogous. Using the pulse radiolysis technique 2AP*- is formed by reaction with the hydrated electron (e(-)aq) and the resulting transient absorption spectrum is assigned to 2AP(NH)*. Additionally, it was found, employing the laser flash photolysis technique, that the excited singlet state of 2AP is incapable of oxidizing guanine in the aqueous phase. However, the one-electron oxidized 2AP (2AP*+) has sufficient energy to ionize guanine according to the DFT results in agreement with experimental data from the literature.

The energetics of rearrangement and water elimination reactions in the radiolysis of the DNA bases in aqueous solution (eaq- and *OH attack): DFT calculations

DFT calculations on the relative stability of various nucleobase radicals induced by e(aq)(-) and (*)OH have been carried out for assessing the energetics of rearrangements and water elimination reactions, taking the solvent effect of water into account. Uracil and thymine radical anions are protonated fast at O2 and O4, whereby the O2-protonated anions are higher in energy (50 kJ mol(-1), equivalent to a 9-unit lower pK(a)). The experimentally observed pK(a)=7 is thus that of the O4-protonated species. Thermodynamically favored protonation occurs slowly at C6 (driving force, thymine: 49 kJ mol(-1), uracil: 29 kJ mol(-1)). The cytosine radical anion is rapidly protonated by water at N3. Final protonation at C6 is disfavored here. The kinetically favored pyrimidine C5 (*)OH adducts rearrange into the thermodynamically favored C6 (*)OH adducts (driving force, thymine: 42 kJ mol(-1)). Very similar in energy is a water elimination that leads to the Ura-5-methyl radical. Purine (*)OH adducts at C4 and C5 (plus C2 in guanine) eliminate water in exothermic reactions, while water elimination from the C8 (*)OH adducts is endothermic. The latter open the ring en route to the FAPY products, an H transfer from the C8(*)OH to N9 being the most likely process.

Chemical radiation studies of 8-bromo-2'-deoxyinosine and 8-bromoinosine in aqueous solutions

The reactions of hydrated electrons (e(aq) (-)) with 8-bromo-2'-deoxyinosine (8) and 8-bromoinosine (12) have been investigated by radiolytic methods coupled with product studies and have been addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reductive cleavage of the C--Br bond gives the C8 radical 9 or 13 followed by a fast radical translocation to the sugar moiety. Selective generation of a C5' radical occurs in the 2'-deoxyribo derivative, whereas in the ribo analogue the reaction is partitioned between the C5' and C2' positions with similar rates. Both C5' radicals undergo cyclizations, 10-->11 and 14-->15, with rate constants of 1.4 x 10(5) and of 1.3 x 10(4) s(-1), respectively. The redox properties of radicals 10 and 11 have also been investigated. A synthetically useful photoreaction has also been developed as a one-pot procedure that allows the conversion of 8 to 5',8-cyclo-2'-deoxyinosine in a high yield and a diastereoisomeric ratio (5'R)/(5'S) of 4:1. The present results are compared with data previously obtained for 8-bromoadenine and 8-bromoguanine nucleosides. Theory suggests that the behavior of 8-bromopurine derivatives with respect to solvated electrons can be attributed to differences in the energy gap between the pi*- and sigma*-radical anions.

Base Release in Nucleosides Induced by Low-Energy Electrons: A DFT Study

Low-energy electrons are known to induce strand breaks and base damage in DNA and RNA through fragmentation of molecular bonding. Recently the glycosidic bond cleavage of nucleosides by low-energy electrons has been reported. These experimental results call for a theoretical investigation of the strength of the C(1)'-N link in nucleosides (dA, dC and dT) between the base and deoxyribose before and after electron attachment. Through density functional theory (DFT) calculations, we compare the C(1)'-N bond strength, i.e., the bond dissociation energy of the neutral and its anionic radical, and find that an excess electron effectively weakens the C(1)'- N bond strength in nucleosides by 61-75 kcal/mol in the gas phase and 76-83 kcal/mol in the solvated environment. As a result, electron-induced fragmentation of the C(1)'-N bond in the gas phase is exergonic for dA (DeltaG=-14 kcal/mol) and for dT (DeltaG=-6 kcal/mol) and is endergonic (DeltaG=+1 kcal/ mol) only for dC. In the gas phase all the anionic nucleosides are found to be in valence states. Solvation is found to increase the exergonic nature by an additional 20 kcal, making the fragmentation both exothermic and exergonic for all nucleoside anion radicals. Thus C(1)'-N bond breaking in nucleoside anion radicals is found to be thermodynamically favorable both in the gas phase and under solvation. The activation barrier for the C(1)'-N bond breaking process was found to be about 20 kcal/mol in every case examined, suggesting that a 1 eV electron would induce spontaneous cleavage of the bond and that stabilized anion radicals on the DNA strand would undergo base release at only a modest rate at room temperature. These results suggest that base release from nucleosides and DNA is an expected consequence of low-energy electron-induced damage but that the high barrier would inhibit this process in the stable anion radicals.

Excess electron transfer in G-quadruplex

The excess electron transfer in a G-quadruplex is successfully probed by using the reaction of hydrated electrons with quadruplex complex of pentamers and the 8-bromoguanine moieties as the detection system.

Photochemical and photophysical studies of guanine derivatives: intermediates contributing to its photodestruction mechanism in aqueous solution and the participation of the electron adduct

The low-intensity steady-state (254 nm), microsecond flash and nanosecond (266 nm) laser photolysis of some guanine (Gua) derivatives in aqueous solution were studied. A photodestruction yield between 10(-3) and 10(-2) at a base concentration of 75 microM was determined for 254 nm irradiation at room temperature using high-performance liquid chromatography. This yield decreases with increasing purine concentration. For a similar concentration of the purine bases (2 +/- 1) x 10(-5) M, the yield increases as follows: Gua approximately 9-ethylguanine < deoxyguanosine approximately guanosine (Guo) < guanosine 5'-monophosphate. At concentrations higher than 2 x 10(-4) M the Gua derivatives' photodestruction yield seems to converge to a limiting value of the order of 10(-4). This behavior is explained in terms of self-quenching and aggregation effects which deactivate the excited states of the bases. The yields of electron photoejection have been determined in the nanosecond laser photolysis (0.083) and in the low-intensity steady-state (5.8 x 10(-3)) for Guo. Competition experiments using electron scavengers suggest that the electron adducts of the bases are one of the principal species participating in the photodestruction mechanism of these monomeric Gua. Close to 75% of the total destruction yield has contributions from initial reactions of the photojected electron at neutral pH. The quantum yield of photodestruction of Guo increases when the pH is increased as follows: 4.7 x 10(-3) (pH 1.1), 6.5 x 10(-3) (pH 2.9), 7.7 x 10(-3) (pH 7.5) and 8.1 x 10(-3) (pH 11.9). This dependence on pH and the electron scavenger experiments provide further evidence for the radical anion or its protonated form as one of the principal species involved in the photodestruction of the bases at the different pH. Under oxygen saturated conditions a 22% increase in the destruction yield is observed for Guo. However, for the dinucleotides adenylyl (3'-->5')-guanosine and thymidylyl (3'-->5')2'-deoxyguanosine, the participation of the electron is 41 and 36%, respectively, suggesting that going into a more DNA or RNA-like structure, the participation of the electron adducts species in the photodamage of DNA and RNA decreases. A mechanism of photodestruction for the Gua derivatives is proposed which takes into account these findings.

Photochemistry of DNA and related biomolecules: quantum yields and consequences of photoionization

The reactions of nucleic acids and constituents, which can be induced by laser UV irradiation, are described. Emphasis is placed on the quantum yields of various stable photoproducts of DNA and model compounds upon irradiation at 193, 248, 254 or 266 nm. In particular, those quantum yields and processes are discussed which involve photoionization as the initial step and occur in aqueous solution under well defined conditions, e.g. type of atmosphere. The efficiencies of some photoproducts, with respect to photoionization using irradiation at 193 or 248 nm, are presented. Radical cations of nucleobases are important sources of damage of biological substrates since they can cause lesions other than dimers and adducts, e.g. strand breakage, abasic sites, crosslinks or inactivation of plasmid and chromosomal DNA. While competing photoreactions, such as hydration, dimerization or adduct formation, diminish the selectivity of the photoionization method, a combination with model studies on pyrimidine- and purine-containing constituents of DNA has brought about an enhanced insight into the reaction mechanisms. The knowledge concerning the lethal events in plasmid and cellular DNA has been greatly improved by correlation with the chemical effects obtained by gamma-radiolysis, vacuum-UV (< 190 nm) and low-intensity irradiation at 254 nm.

Addition of e-aq and H atoms to hypoxanthine and inosine and the reactions of alpha-hydroxyalkyl radicals with purines. A pulse radiolysis and product analysis study

The reactions of hydrated electrons e-aq with hypoxanthine and inosine were followed using pulse radiolysis methods. In a neutral solution the electron adduct of inosine is immediately protonated at the heteroatoms of the purine ring by water (k >> 2.5 x 10(6)s-1) to give In(N,O-H).. These N,O-protonated intermediates have a single absorption maximum at 300 nm. In basic solution the protonation of the electron adduct of inosine by water leads to other intermediate products with an absorption maximum at 350 nm. These intermediates are believed to be the C-protonated electron adducts of inosine (In(N,O-H).). In (N,O-H). and In(C-H). differ strongly in their ability to reduce p-nitroacetophenone (PNAP). In(N,O-H). are strong reductants and reduce PNAP quantitatively to PNAP.-. Based on the pH dependence of PNAP.- yields, two types of tautomers of In(C-H). could be distinguished. One of the tautomers can reduce PNAP, albeit with slower rate than In(N,O-H)., the other tautomer has no reducing properties. The latter is the one with the higher pKa and therefore thermodynamically more stable. The absorption spectrum of the intermediates produced in the reaction of e-aq with hypoxanthine at neutral pH is very similar to that of In(N,O-H). with a maximum at 300 nm. However, no build-up at 350 nm was observed in basic solution as in the case of the electron adduct of inosine. The reaction of H atoms with inosine produces in basic solution intermediate radicals with the same absorption spectrum as the C-protonated electron adducts of inosine. It is suggested that both the reactions of e-aq and H. with inosine in basic solution produce the same radical, namely the H-adduct of inosine (In(C-H)) with the highest pKa. alpha-Hydroxyalkyl radicals were found to react very slowly with purine bases and nucleosides in neutral to basic solutions. In acidic solution their reactivity increases and a number of rate constants were determined by pulse radiolysis measurements at pH 0.4. The intermediates from the reaction of 2-hydroxy-2-propyl radicals with inosine could be observed pulse spectrometrically in neutral and in basic solutions. In basic solution this reaction leads to intermediates with the same absorption maximum at 350 nm as that of the H-adduct of inosine. Furthermore, the yield of acetone was found to increase strongly in basic pH.(ABSTRACT TRUNCATED AT 400 WORDS)

Electron migration in oligonucleotides upon gamma-irradiation in solution

Electron migration in irradiated solutions of DNA was investigated using 5-bromouracil synthetically incorporated into oligonucleotides of defined base composition as a molecular indicator of electron interactions. Solvated electrons interact quantitatively with 5-bromouracil, leading to a highly reactive 5-yl radical which can abstract an adjacent hydrogen atom to yield uracil. Yields of uracil, or loss of 5-bromouracil, from irradiated oligonucleotide samples were measured using gas chromatography-mass spectrometric analysis of their trimethylsilylated acid hydrolysates. To examine the effects of base composition and DNA conformation on electron migration, a set of oligonucleotides containing 5-bromouracil at selected positions with three base (guanine, cytosine, thymine or adenine) spacers (e.g. [BrU(GGG)3]3) were irradiated in their single- or double-stranded form following annealing with appropriate complementary sequences. Differences in uracil yields suggested that electron migration occurred to different extents in oligonucleotides containing different base sequences. In irradiated single-stranded oligonucleotides, the yield of uracil decreased in the order A > T > > C approximately G. However, in irradiated double-stranded oligonucleotides, the yield of uracil decreased in the order G > C approximately T > A. These differences were attributed to proton-transfer reactions facilitated by base pairing in double-stranded oligonucleotides. The distance over which the electron would migrate was then determined using a series of oligonucleotides containing 5-bromouracil at selected positions with guanine spacers (i.e. [BrU(G)n]3 (n = 3, 5, 7, 9). Oligonucleotides were irradiated in their double-stranded form following annealing with the appropriate complementary sequences. Analysis of the loss of 5-bromouracil revealed that electron migration occurred efficiently over c. 3-4 guanine bases assuming that migration could occur as efficiently in either direction along the DNA molecule. These data can be compared with studies reporting more extensive migration for electrons generated by direct ionization of DNA.

The photochemistry of adenosine: intermediates contributing to its photodegradation mechanism in aqueous solution at 298 K and characterization of the major product

The steady-state (254 nm) photolysis of 9-(beta-D-erythropentofuranosyl)adenine (adenosine) in aqueous solution was studied. Photodestruction yields on the order of 1.3 x 10(-3) were determined at room temperature by measuring the initial decrease in the absorption maximum as a function of irradiation time. The use of high performance liquid chromatography (HPLC) permitted a more exact determination of the yield (2.5 x 10(-3). The formation of photoproducts was also studied using HPLC. In the photolysis of 50 microM aqueous solutions of adenosine under anaerobic conditions at least 11 stable photoproducts are formed that absorb at 260 nm, the wavelength of maximum absorption of adenosine. The major photoproduct was also isolated and characterized as adenine; its formation yield was determined to be 4.5 x 10(-4). This yield is affected by the presence of oxygen and by the initial concentration of adenosine employed. Fluorescence emission and excitation spectra were used to monitor the formation of highly fluorescent photoproducts that emit with maxima at 365, 398, and 430 nm and absorb in the wavelength region of 240-380 nm. The reactive species in the photodestruction mechanism were established using substrates that react selectively with the respective short-lived species. Photoionization is a primary photoprocess implied by these studies. The triplet state of adenosine also contributes to the photodestruction mechanism.

Electron transfer from nucleobase electron adducts to 5-bromouracil. Is guanine an ultimate sink for the electron in irradiated DNA?

Electron transfer to 5-bromouracil (5-BrU) from nucleobase (N) electron adducts (and their protonated forms) has been studied by product analysis and pulse radiolysis. When an electron is transferred to 5-BrU, the ensuing 5-BrU radical anion rapidly loses a bromide ion; the uracilyl radical thus formed reacts with added t-butanol, yielding uracil. From the uracil yields measured as the function of [N]/[5-BrU] after gamma-radiolysis of Ar-saturated solutions it is concluded that thymine and adenine electron adducts and their heteroatomprotonated forms transfer electrons quantitatively to 5-BrU. Like the electron adduct of adenine, those of cytosine and guanine are rapidly protonated by water. The (protonated) electron adduct of guanine does not transfer an electron to 5-BrU, and in the case of the (protonated) cytosine electron adduct only partial electron transfer is observed. The results can be modelled if the protonated electron adduct (protonated at N(3) or at the amino group) of cytosine, CH., which can transfer its electron to 5-BrU (k approximately 2 x 10(7) dm3 mol-1 s-1) is transformed in a slow tautomerization reaction (k approximately 2.5 x +/- 10(3) s-1) into another form C'H. (possibly protonated at C(6) or C(5)) which does not transfer an electron to 5-BrU. There is also electron transfer from the electron adduct of thymine to cytosine and guanine which serve as electron sinks. The rate constant of electron transfer from the thymine electron adduct to cytosine is about 250 times greater than that of the reverse reaction. The heteroatom-protonated electron-adduct of thymidine transfers an electron to 5-BrU more slowly (k = 2.3 x 10(7) dm3 mol-1 s-1) than the electron-adduct itself (k = 7.2 x 10(8) dm3 mol-1 s-1). Phosphate buffer-induced protonation of the electron-adduct of thymine at carbon (C(6)) prevents electron transfer to 5-BrU. Such phosphate catalysis is also observed as an intramolecular process (k approximately 2 x 10(4) s-1) with thymidine-5'-phosphate but not with the 3'-phosphate. Phosphate-induced protonation at carbon also reduces transfer efficiency for the electron adducts of dinucleoside phosphates such as dTpdT and dTpdA. The data raise the question whether in DNA the guanine moiety may act as the ultimate sink of the electron in competition with other processes such as protonation at C(6) of the thymine electron adduct.

Electron-transfer-induced acidity/basicity and reactivity changes of purine and pyrimidine bases. Consequences of redox processes for DNA base pairs

Changes in the oxidation state of the DNA bases, induced by oxidation (ionization) or by reduction (electron capture), have drastic effects on the acidity or basicity, respectively, of the molecules. Since in DNA every base is connected to its complementary base in the other strand, any change of the electric charge status of a base in one DNA strand that accompanies its oxidation or reduction may affect also the other strand via proton transfer across the hydrogen bonds in the base pairs. The free energies for electron transfer to or from a base can be drastically altered by the proton transfer processes that accompany the electron transfer reactions. Electron-transfer (ET) induced proton transfer sensitizes the base opposite to the ET-damaged base to redox damage, i.e., damage produced by separation of charge (ionization) has an increased change of being trapped in a base pair. Of the two types of base pair in DNA, A-T and C-G, the latter is more sensitive to both oxidative and reductive processes than the former. Proton transfer induced by ET does not only occur between the heteroatoms (O and N) of the base pairs (intra-pair proton transfer), but also to and from adjacent water molecules in the hydration shell of DNA (extra-pair proton transfer). These proton transfers can involve carbon and as such are likely to be irreversible. It is the A-T pair which appears to be particularly prone to such irreversible reactions.