One-Electron Reduction of 8-Bromoisoguanosine and 8

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One-Electron Reduction of 8-Bromoisoguanosine and 8-Bromoxanthosine in the Aqueous Phase: Sequential versus Concerted Proton-Coupled Electron Routes Chryssostomos Chatgilialoglu,* Mila D'Angelantonio, Panagiotis Kaloudis, Quinto G. Mulazzani, and Maurizio Guerra* ISOF, Consiglio Nazionale delle Ricerche, Via P. Gobetti 101, 40129 Bologna, Italy

ABSTRACT The reactions of hydrated electrons (eaq-) with 8-bromoisoguanosine and 8-bromoxanthosine were studied by pulse radiolysis techniques and addressed computationally by means of hybrid meta DFT calculations at the B1B95/6-31þG** level. The one-electron oxidized purine derivative is formed either by oxidation of the corresponding purine with SO4•- or by reduction of the corresponding 8-bromopurine with eaq- at pH 7. The reactivity of 8-bromoxanthosine depends on its protonation state. In agreement with the experimental findings, DFT calculations suggest that the reaction of the eaq-/Hþ couple with 8-bromoisoguanosine and 8-bromoxanthosine (monoanion) involves sequential electron-transfer-proton-transfer (ET-PT) and concerted electron-proton-transfer (EPT) pathways, respectively. SECTION Biophysical Chemistry

O

ur understanding of the one-electron reduction of 8bromopurines 1 has been improved in recent years (Scheme 1). The nature of the substituents X and Yat positions 6 and 2 of the nitrogenous six-member ring plays an important role in the reaction outcome. 8-Bromo20 deoxyadenosine (X = NH2, Y = H)1 and 8-bromo-20 -deoxyinosine (X = O, Y = H)2 were found to capture hydrated electrons and rapidly lose bromide ions to give the corresponding σ-type radicals 2 as expected for delocalized halide (Cl, Br) π anion radicals.3 Radical translocation to the sugar moiety affords C50 radicals and allows for the detailed study of these species. On the other hand, electron transfer (ET) to 8bromoguanine (X = O, Y = NH2)4,5 and 8-bromo-2-aminoadenine (X = Y = NH2)6 derivatives was found to give radical anion 3, which undergoes a fast protonation at C8 by proton transfer (PT) from solvent to afford the complex 4. This mechanism is similar to the step-by-step ET-PT reactions observed in pulse radiolysis of adenine7 and guanine8 nucleosides. The successive debromination allowed studying for the first time the tautomerization of one-electron-oxidized guanosine.4,5 This Letter reports on the reactivity of 8-bromoisoguanosine (X = NH2, Y = O) and 8-bromoxanthosine (X = Y = O) as well as on the dynamics of the proton-coupled electron-transfer (PCET) reactions,9-11 in particular, on the sequential electron-transfer-proton-transfer (ET-PT) versus concerted electron-proton-transfer (EPT) mechanism.9,10 Radiolysis of neutral water leads to eaq- (0.27), HO• (0.28), and H• (0.062), where the values in parentheses represent the radiation chemical yields in units of μmol J-1. The reactions of eaq- with the substrates were studied in O2-free solutions containing 0.25 t-BuOH. With this amount of t-BuOH, HO•

r 2009 American Chemical Society

Scheme 1. Reaction of Hydrated Electrons with 8-Bromopurines Exhibiting Two Independent Reaction Pathways

and H• are scavenged efficiently.12 The optical absorption spectrum obtained after the reaction of eaq- with 8-bromoisoguanosine (5) at pH 7 is shown in Figure 1 (black circles). The time profile of the formation of this transient at 340 nm (upper inset) or the disappearance of eaq- at 720 nm (lower inset) leads to the same pseudo-first-order rate constant (kobs), indicating that if any intermediate exists, its half-life will be below a few ns. From the slope of the linear plot between kobs and [5], the bimolecular rate constant was found to be (1.5 ( 0.1) 1010 M-1 s-1. Furthermore, this spectrum is very similar to that obtained after the oxidation of isoguanosine (7) by SO4•- at pH ∼ 7 (Figure 1, red triangles). The spectra in Figure 1 are assigned to radical 6, which can be obtained either from the reduction Received Date: October 16, 2009 Accepted Date: November 9, 2009 Published on Web Date: November 12, 2009

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DOI: 10.1021/jz9001532 |J. Phys. Chem. Lett. 2010, 1, 174–177

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Scheme 3. Reaction of eaq- with 8-Bromoxanthosine at pH 4.2 and 7 Affording the Isoelectronic σ-Type 12 and π-Type 14 Radicals, Respectively

Figure 1. Absorption spectrum (b) obtained from the pulse radiolysis of Ar-purged solutions containing 1 mM 5 at pH 7 (phosphate buffer) and 0.25 M t-BuOH, recorded 5 μs after the pulse; optical path = 2.0 cm, dose per pulse = 22 Gy; the pKa values of 4.3 and 9.8 are determined for the transient species by following the absorption changes at 600 nm and are associated with protonation and deprotonation steps, respectively. Absorption spectrum (4) obtained from the pulse radiolysis of Ar-purged solutions containing 0.1 mM 7 and 10 mM Na2S2O8 at natural pH with 0.1 M t-BuOH, recorded 5 μs after the pulse; optical path = 2.0 cm, dose per pulse = 22 Gy. Insets: Time dependence of absorption at 340 (upper) and 720 nm (lower) from the pulse radiolysis of 5 for the dose per pulse = 12 Gy; the lines represent the first-order kinetic fit to the data.

Scheme 2. Radical 6 Assigned to the Transient Species Observed in the Pulse Radiolysis Studies of eaq- with 5 and of SO4•- with 7 at pH ∼ 7 (cf. Figure 1)a

normal C(sp3)-Br bond length (∼1.85 Å) and that a loose π-complex 9 is formed, where the Br atom is placed above the molecular plane at a distance of 2.12 Å. In order to arrive at the observable transient species 6, a further debromination and deprotonation step is necessary. The loose π-complex is prompt to lose Br-. In analogy with our previous findings,5,6 we suggest the involvement of a transition state, in which the deprotonation at NH by means of H2O could occur during elimination of Br- from the π-complex 9. Xanthosine and 8-bromoxanthosine differ from the analogous guanine and isoguanine derivatives in that, at pH 7, they consist of monoanionic forms. Indeed, the pKa = 5.7 is reported for xanthosine,14 and a pKa = 5.4 is found for 8bromoxanthosine (cf. structures 10 and 11 in Scheme 3). The rate constant for the reaction of eaq- with 10 was found to be (1.4 ( 0.1) 1010 M-1 s-1 at pH 4.2 by measuring the rate of the optical density decrease of eaq- at 720 nm as a function of nucleoside concentration. An aqueous solution of 10 (1 mM) and t-BuOH (0.25 M) at pH 4.2 was prepared. Under Ar-purged conditions, the reaction of 10 with eaq- was complete in ∼300 ns. At this time, no significant absorption was detected in the 300-750 nm region (Figure 2, blue circles). However, a spectrum containing two bands centered at 310 and 470 nm developed in ∼40 μs (Figure 2, black triangles). The time profile for the formation of the transient with λmax = 310 nm (inset) followed first-order kinetics with a rate constant (kexpt) that was independent of [10] in the range of 0.2-1 mM and slightly increased with dose/pulse. This dose dependence was due to the mixing of first-order growth and second-order decay of the species. By extrapolation to zero dose, a rate constant of k = 3.6 104 s-1 was obtained.

a The structure of 9 (where R = CH3) is calculated at the B1B95/631þG** level; d(C8;Br)=2.12 Å. The pKa values of 9.5 and 9.8 are associated with the deprotonation of the bases in 5 and 7.15

of 5 or from the oxidation of 7, as illustrated in Scheme 2. DFT calculations13 carried out on the corresponding 9-methyl derivative of radical 6 fits very well with our assignment. Indeed, the optical spectrum computed at the TD-B3LYP/6311G**//B1B95/6-31þG** level for the radical 6 [R = Me; wavelengths λ(oscillator strengths f): 294(0.115), 348(0.031), 430(0.030), and 695(0.014)] is in good agreement with the spectrum shown in Figure 1. The initial reaction of eaq- with 5 should afford the radical anion 8, which is expected to protonate rapidly (Scheme 2). Protonation at C8 does not produce a “normal” protonated form. Calculations indicate that the C8-Br bond length increases with respect to the


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Figure 4. Structure of the radical dianion of 8-bromo-9-methylxanthine protonated at C8 at the B1B95/6-31þG** level; (left) starting geometry with the C8-H distance fixed at 2.0 Å; (right) fully optimized structure.

Scheme 3. The computed optical spectrum for the 9-methyl derivative of radical 14 [R = Me, λ(f): 281(0.087), 348(0.057), 455(0.014), and 528(0.017)] is in rather good accord with the experimental spectrum (λmax at 300 nm, a shoulder at 370 nm, and a weak broad band at 480 nm). What is the fate of the radical dianion produced by the attachment of an eaq- to 11? DFT calculations on this species (R = Me) show that the C8-Br bond (>40 Å) dissociates completely, producing Br- (Mulliken atomic charge at Br = -1.0e) and a σ-type radical like 2 (cf. Scheme 1). Therefore, R = ribose should lead again to the cyclic aminyl radical 13 like the experiment at pH ∼ 4 (Scheme 3). However, optimization of the dianion that interacts with a proton located 2.0 Å above the C8 atom (Figure 4, left side) leads to radical 14 that interacts with Br- (Figure 4, right side), the C8-Br distance being 2.84 Å and the Mulliken atomic charge at Br computed to be -0.65e. Our experimental findings coupled with theoretical results indicate that the reaction of monoanion 11 with the eaq-/Hþ couple is concerted (eq 1). We suggest that the incoming electron is initially strongly repulsed by the anion; therefore, its motion couples with proton transfer from water to minimize the high-energy barrier for the electron transfer (EPT mechanism).11 This mechanism is not operative in the neutral form 10 since the electron attachment provides a radical anion that loses Br- prior to protonation. In summary, the PCET reactions in one-electron reduction of 8-bromopurine nucleosides have been disclosed, showing that in negatively charged derivatives (like 11), a concerted EPT mechanism operates (eq 1), whereas in neutral derivatives (like 5), a stepwise ET-PT mechanism occurs (eq 2).

Figure 2. Absorption spectra obtained from the pulse radiolysis of Ar-purged solutions containing 1 mM 10 at pH 4.2 with 0.25 M t-BuOH, recorded 2 (b) and 40 μs (2) after the pulse; optical path = 2.0 cm, dose per pulse = 24.4 Gy. Inset: buildup at 310 nm, where the solid line represents the first-order kinetic fit to the data.

Figure 3. Absorption spectrum (b) obtained from the pulse radiolysis of Ar-purged solutions containing 1 mM 11 and 0.25 M t-BuOH, at pH 7, recorded 2 μs after the pulse. Absorption spectrum (3) obtained from the pulse radiolysis of Ar-purged solutions containing 0.5 mM 15 and 10 mM Na2S2O8 with 0.1 M t-BuOH at pH 7, recorded 2 μs after the pulse. In both experiments, optical path = 2.0 cm and dose per pulse = 24.4 Gy.

We assigned the transient shown in Figure 2 to the conjugated aminyl radical 13 and the observed first-order growth to the cyclization of C50 radicals with analogy with our previous findings1,2 (cf. Scheme 3). That is, in this case, Brelimination is faster than protonation at C8. Indeed, DFT calculations at the B1B95/6-31þ(λ=0.7)G** level show that the π radical anion of 10 (R = Me) is unstable and tends to lose Br-, as previously found for the corresponding radical anion of 8-bromo-20 -deoxyadenosine1 and 8-bromo-20 -deoxyinosine.2 The rate constant for the reaction of eaq- with 11 was found to be (4.9 ( 0.1) 109 M-1 s-1 at pH 7.6, which is 3 times slower than its protonated form 10. The optical absorption spectrum obtained after the reaction of eaq- with 11 at pH 7 is shown in Figure 3 (black circles). Furthermore, this spectrum is very similar to that obtained after the oxidation of 15 by SO4•- at pH 7 (Figure 3, red triangles). The spectra in Figure 3 are assigned to radical 14, which can be obtained either from the reduction of 11 or from the oxidation of 15, as illustrated in


EPT

11 þ eaq - =Hþ sf Br - þ 14 ET -PT

5 þ eaq - =Hþ sf Br - þ 6

ð1Þ ð2Þ

Experimental Methods Pulse radiolysis with optical absorption detection was performed by using the 12 MeV linear accelerator, which delivered 20-200 ns electron pulses with doses between 5 and 50 Gy, by which HO•, H•, and eaq- were generated with 1-20 μm concentrations. The compounds 5, 7, and 10 were prepared following known procedures.15,16

SUPPORTING INFORMATION AVAILABLE Materials, pulse radiolysis experiments, DFT calculations, and complete reference 13. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. E-mail: chrys@ isof.cnr.it(C.C.); [email protected] (M.G.). (13)

ACKNOWLEDGMENT Work supported in part by the European

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Community's Marie Curie Research Training Network under Contract MRTN-CT-2003-505086 [CLUSTOXDNA] is acknowledged. The support and sponsorship provided by COST Action CM0603 on “Free Radicals in Chemical Biology (CHEMBIORADICAL)” are kindly acknowledged. We thank Professor Marc Robert for helpful discussions, as well as M. Lavalle, A. Monti, and A. Martelli for assistance with the pulse radiolysis experiments.

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REFERENCES

Ross, A. B.; Mallard, W. G.; Helman, W. P.; Buxton, G. V.; Huie, R. E.; Neta, P. NDRL-NIST Solution Kinetic Database, ver. 3; Notre Dame Radiation Laboratory: Notre Dame, IN and NIST Standard Reference Data: Gaithersburg, MD, 1998, and references therein. Frisch, M. J.; et al. Gaussian 03, Revision D.02; Gaussian, Inc.; Wallingford CT, 2004. (See Supporting Information for the full list of authors) . Kulikowska, E.; Kierdaszuk, B.; Shugar, D. Xanthine, Xanthosine and Its Nucleosides: Solution Structures of Neutral and Ionic Forms, and Relevance to Substrate Properties in Various Enzyme Systems and Metabolic Pathways. Acta Biochim. Pol. 2004, 51, 493–531. Seela, F.; Wei, C.; Kazimierczuk, Z. Substituent Reactivity and Tautomerism of Isoguanosine and Related Nucleosides. Helv. Chim. Acta 1995, 78, 1843–1854. Long, R. A.; Robins, R. K.; Townsend, L. B. Purine Nucleosides. XV. Synthesis of 8-Amino- and 8-Substituted Aminopurine Nucleosides. J. Org. Chem. 1967, 32, 2751–2756.

(1)

Chatgilialoglu, C.; Guerra, M.; Mulazzani, Q. G. Model Studies of DNA C50 Radicals. Selective Generation and Reactivity of 20 -Deoxyadenosin-50 -yl Radical. J. Am. Chem. Soc. 2003, 125, 3839–3848. (2) Russo, M.; Jimenez, L. B.; Mulazzani, Q. G.; D'Angelantonio, M.; Guerra, M.; Miranda, M. A.; Chatgilialoglu, C. Chemical Radiation Studies of 8-Bromo-20 -deoxyinosine and 8-Bromoinosine in Aqueous Solutions. Chem.;Eur. J. 2006, 12, 7684– 7693. (3) Costentin, C.; Robert, M.; Saveant, J.-M. Fragmentation of Aryl Halide π Anion Radicals. Bending of the Cleaving Bond and Activation vs Driving Force Relationships. J. Am. Chem. Soc. 2004, 126, 16051–16057. (4) Chatgilialoglu, C.; Caminal, C.; Guerra, M.; Mulazzani, Q. G. Tautomers of One-Electron Oxidized Guanosine. Angew. Chem., Int. Ed. 2005, 44, 6030–6032. (5) Chatgilialoglu, C.; Caminal, C.; Altieri, A.; Vougioukalakis, G. C.; Mulazzani, Q. G.; Gimisis, T.; Guerra, M. Tautomerism in the Guanyl Radical. J. Am. Chem. Soc. 2006, 128, 13796– 13805. (6) Kaloudis, P.; D'Angelantonio, M.; Guerra, M.; Gimisis, T.; Mulazzani, Q. G.; Chatgilialoglu, C. One-Electron Reduction of 8-Bromo-2-aminoadenosine in the Aqueous Phase: Radiation Chemical and DFT Studies of the Mechanism. J. Phys. Chem. B 2008, 112, 5209–5217. (7) Candeias, L. P.; Steenken, S. 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. J. Phys. Chem. 1992, 96, 937–944. (8) D'Angelantonio, M.; Russo, M.; Kaloudis, P.; Mulazzani, Q. G.; Wardman, P.; Guerra, M.; Chatgilialoglu, C. Reaction of Hydrated Electrons with Guanine Derivatives: Tautomerism of Intermediate Species. J. Phys. Chem. B 2009, 113, 2170– 2176. (9) Huynh, M. H. V.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2007, 107, 5004-5064 and references cited therein. (10) Meyer, T. J.; Huynh, M. H. V.; Thorp, H. H. The Possible Role of Proton-Coupled Electron Transfer (PCET) in Water Oxidation by Photosystem II. Angew. Chem., Int. Ed. 2007, 46, 5284-5304 and references cited therein. (11) Fecenko, C. J.; Thorp, H. H.; Meyer, T. J. The Role of Free Energy Change in Coupled Electron-Proton Transfer. J. Am. Chem. Soc. 2007, 129, 15098–15099.


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One-Electron Reduction of 8-Bromoisoguanosine and 8

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