J . Phys. Chem. 1992,96, 10302-10307
by photolysis (cf. a), the results obtained by ?radiolysis (cf. b) reveals no clear formation of CA (fromDCA) and AQ (from CA), probably being due to the further radiolysis of the products in accordance with the fact that the absorption increment around 300 nm c a d by radiolysis is much greater than that caused by photolysis. (2) In CH$N without amine at room temperature, pulse radiolysis of bromoanthracenes yields the bromoanthracene radical anions and y-radiolysis causes the debromination.20
Acknowledgment. We wish to express our sincere thanks to Professor Tadamasa Shida of Kyoto University for the use of 6oco y-ray sources. Thanks are also due to Horiba Co. Ltd. for the measurement of fluorescence decay curves using a time-resolved photoluminescenceand fluorescence spectrometer (NAES-700L). This work was financed by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (No. 03303001 and 04303001). K.H. also gratefully acknowledges the generous financial support of Chisso Co. Ltd.
References rod Notes (I).Hamanoue, K.;Tai, S.;Hidaka, T.; Nakayama, T.; Kimoto, M.; Teranishi, H. J. Phys. Chem. 1984, 88, 4380. (2) Soloveichik, 0. M.; Ivanov, V. L.; Kuz'min, M. G. High Energv Chem. 1989, 23, 281 (translated from Khim. Vys. Energii 1989, 23, 351). (3) Ohashi, M.;Tsujimoto, IC.; Seki, K. J. Chem. Soc., Chem. Commun. 1973,384. (4) Tsujimoto, K.; Tasaka, S.;Ohashi, M. J. Chem. Soc.. Chem. Commun. 1975,758. (5) Bunce, N. J.; Pilon, P.; Ruzo, L.0.;Sturch, D. J. J. Org. Chem. 1976, 41, 3023.
(6) Buncc, N. J.; Kumar, Y.; Ravanal, L.; Safe, S.J. Chem. Soc., Perkin Trans. 1978. 2. 880. (7) Chittk, B.; Safe, S.;Bunce, N.; Ruzo, L.; Olie, K.;Hutzinger, 0. Can. J. Chem. 1978,56, 1253. (8) Davidson, R. S.;Goodwin, J. W. Tetrahedron Lett. 1981, 22, 163. (9) Saeva, F. D. Top. Curr. Chem. 1990, 156,61. (101 Fulara. J.: Latowski. T. Pol. J. Chem. 1990. 64. 369. (11) Nagamura, T.; Nahyama, T.; Hamanoue,.K. .Chem. Lett. 1991, 2051. (12) Hamanoue,
K.;Nakayama, T.; Ikenaga, K.; Ibuki, K.; Otani, A. J . Photochem. Phorobiol. A: Chem., in press. (13) Nagamura, T.; Nakayama, T.; Hamanoue, K. Chem. Phys. Lett. 1992, 193, 30. (14) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956, A235, 518. (15) Ushida, K.; Nakayama, T.; Nakazawa, T.; Hamanoue, K.; Nagamura, T.; Mugishima, A.; Sakimukai, S . Reo. Sci. Instrum. 1989,60, 617. (16) Murov, S.L. Handbook of Photochemistry; Marcel1 Dekker: New York, 1973. (17) Tanaka, M.; Tanaka, I.; Tai, S.; Hamanoue, K.; Sumitani, M.; Yoshihara, K.J. Phys. Chem. 1983, 87, 813. (1 8) Hamanoue, K.; Hidaka, T.; Nakayama, T.; Teranishi, H.; Sumitani, M.; Yoshihara, K.Bull. Chem. Soc. Jpn. 1983,56, 1851. (19) Hamanoue, K.;Nakayama, T.; Ikenaga, K.; Ibuki, K., paper in preparation. (20) Hamanoue, K.; Kimoto, M.; Nakayama, T.; Teranishi, H.; Tagawa, S.; Tabata, Y . Radiar. Phys. Chem. 1984, 24, 445. (21) Hamil, W. H. Radical Ions; Kaiser, E. T., Kevan, L., Eds.; Wiley: New York, 1968; Chapter 9. (22) Shida, T. Electronic Absorption Spectra of Radical Ions; Elsevier: Amsterdam, 1988. (23) Iwamura, H.; Eaton, D. F. Pure Appl. Chem. 1991, 63, 1003. (24) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 86. (25) Nakayama, T.; Ushida, K.;Hamanoue, K.; Washio, M.; Tagawa, S.; Tabata, Y. J. Chem. SOC.,Faraday Trans. 1990,86,95.
Reactlon of Hydrated Electrons with Guanine Nucleosides: Fast Protonation on Carbon of the Electron Adduct Luis Pedro Camleias,' Petra Wolf? Peter O'NeU,* and Steen Steenken**' Max- Planck- Institut f%r Strahlenchemie, 0-4330 Mtilheim, Germany, and MRC Radiobiology Unit, Chilton, Didcot, Oxon, OX1I ORD, UK (Received: May 28, 1992; In Final Form: September 3, 1992)
The reactions of hydrated electrons (eq-) and hydrogen atoms (IT) with guanosine, Z'deoxyguanosine, and 1-methylguanosine (G) were studied by pulse radiolysis with optical and conductometric detection. In neutral solution, the reaction of both e, - and H' with these purines leads to the same neutral radical. This is suggested to be the H adduct to the C8 position (G(C8H)') formed by H'addition and by protonation at C8 of the electron adducts, via tautomerization (k = 1.2 X lo6 s-I for the case of guanosine) following protonation of the radical anion at a hetero atom (k 2 lo7 s-l). The C-protonated radical is only a weak reductant: It does not react (k Ilo7 M-'s-') with the oxidant methylviologen (E = -0.44 V/NHE), but with the stronger oxidant Fe(CN)?- (E = 0.36 V/NHE) there is some reactivity (k = 5.9 X lo8 M-'PI). In acidic solution G(C8H)' protonates at a heteroatom to give a radical cation (pKa = 5.4-5.5). The same radical cation results also from the reaction of H' with G at pH 3. The C8 protonated electron adduct of 1-methylguanosinedeprotonates (from the amino function at C2) with a pKa of 13.3. C-protonated electron adducts of the purines have so far not been detected in irradiated DNA. Since the tendency for their formation (by protonation by the water of hydration of DNA) is probably very high, the failure to detect these radicals in DNA can be taken as indication that electron transfer from the purine radical anions to pyrimidines in their neighborhood is faster than their C protonation.
btrduction According to the two-component model,3 the ESR spectrum of irradiated DNA in the solid phase4 at room temperature can be explained on the basis of only two radical species: the guanine radical cation and the CCprotonated thymine radical anion. This situation, as well as the results of various studies with model compounds,5 can be explained as resulting from electron and electron hole migration in DNA, by which the randomly distributed initial damage is concentrated at specific bases. These ideas have recently suffered some criticism,4 arising especially from the realization that proton-transfer reactions may play an important role in the fixation of the radical sites.68 These pro0022-3654/92/2096-10302$03.00/0
ton-transfer reactions include reversible (de)protonation of heteroatoms but also irreversible protonations at carbon.ei1 Ultimately, the electrons result in the formation of 5,6-dihydrot hymin-5-yl radical. An example of the importance of proton transfer in aqueous solution is the competition between deoxycytidine (C) and thymidine (T) for e-, produced in ionizing events.'* Whereas in the gas phase T is more electron affinic than C,13 in aqueous solution the higher basicity of the radical anion of C (pK 1 13) relative to that of T (pK = 6.9) has a leveling effect, making the (aqumus = -1.1 solution) reduction potentials of C and T equal V/NHE).'* In agreement with this concept, recent ESR ex@ 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10303
Guanine Nucleosides periments on irradiated DNA at S77 K showed that the electrons are captured preferentially by C rather than by T.14 However, upon warming, the electrons are mobilizad and migrate along the DNA chain where they are finally fixed at T by irreversible protonation at C6 of this molec~le.'~*'~ It was recently observed" that in aqueous solution the electron adducts of adenine nucleosides and nucleotides undergo irreversible protonation at carbon with a rate constant at least 2 orders of magnitude higher than that9for the C protonation of the thymidine electron adduct under the same conditions. If this difference in reactivity applies also to the situation in DNA, the question arises why the C-protonated electron adduct of the adenine moiety has so far not been found in DNA.16 For this reason, it is pertinent to search for more information on the behavior of the DNA bases upon one-electron reduction. The radical anions of guanine derivatives (G) have been studied by FSR/ENDOR" in the solid
0
RI = H; R, = ribose: guanosine R1= H; R9 = 2'-deoxyribose: 2'-deoxyguanosine R1= CH,;R9 = ribose: 1-methylguanosine
(1)
state, but their behavior in aqueous has so far received little attention. It was therefore considered neceSSary to investigate in detail the reaction of guanine nucleosides (guanosine, 2Jdeoxyguanosine, 1-methylguanosine, see formula 1) with the hydrated electrons (eq-) and hydrogen atoms (H') generated by pulse radiolysis of aqueous solutions.
Experimental Section The aqueous solutions containing 0.1-1 mM of guanosine, 2'-deoxyguanosine, or 1-methylguanosine (from Sigma) were prepared using water purified with Millipore MU-Q systems and deaerated with argon or N2 prior to irradiation. For the determinations of the solvent kinetic isotope effect, heavy water (Merck) with a deuteration degree 299.84%was used. All experiments were carried out at room temperature (20 f 1 "C). In the experiments where fast formation of the guanosine electron adducts was desired, 3 mM solutions of guanosine were prepared by gentle warming (50 "C) in a water bath for a few minutes followed by cooling to room temperature. This way, supersaturated solutions were obtained that could be kept for several hours before crystallization of guanosine occurred. The pulse radiolysis experiments were performed with a computer-controlled 3-MeV Van de Graaff accelerator (pulse width from 25 to 400 ns) with optical and conductometric detection (at MUlheim) and with a 4.3-MeV LINAC with optical detection (at Didcot) as described e l ~ e w h e r e . ' ~ - ~ ~ For the optical measurements, dosimetry was performed by measuring the optical density at 480 nm due to (SCN),+, formed on irradiation of N20-saturated aqueous solutions of 10 mM KSCN, and assuming G((SCN),-) = 6.0 and c((SCN),*) at 480 nm = 7600 M-' cm-l.2' The doses were such that 0.25-2 pM radicals were produced. The conductivity signals were calibrated by comparison with the changes resulting from the reaction of eq- with 5 mM dichloromethane in deaerated aqueous solution containing 0.2 M tert-butyl alcohol (to scavenge OH'). With the solution, at pH 4.3 a conductance increase was observed which is explained by the formation of H+ (A = 350 S cm2 mol-') from the radiolysis of water (eq 3) and the release of C1- (A = 76 S cm2 mol-') in eq 2. Since reaction 2 proceeds to completion, the G value of Clis equal to that of ew- (~2.7).For organic radical ions, equivalent conductances of 35 S cm2 mol-' were assumed.22
+ CH2Cl2
+
I
1oooOr
75000
r
5 \
w
50002500
-
hlnm Figure 1. Absorption spectra obtained by pulse radiolysis of deaerated aqueous solutions of 1 m M guanosine and 2-propanol 0.1 M,4 ps after the pulse at pH 7.1 (circles) and at pH 4.0 (squares). Inset: pH dependence of the absorption at 300 nm (squares), and of the consumption of H+(circles).
R9
e,
lO-fold that of guanosine) to give H' (eq 12). A buildup of
ea;
11
1
13
14
15
PH
+ H+
+
H'
I
t 400
'
500
0
0
hlnm
(k = 1.2 X 106 PI) as compared to adenosine (k = 1 X 104 sd)," but also the rate constant for phosphate caralysis of C8 protonation (5.9 X lo7 M-'s-I compared to" 1.9 X lo6 M-' s-I). The protonation equilibrium observed by optical and conductometric detection (eq 9) is attributed to reversible protonation of G(C8H)' at a heteroatom to give the radical cation G(C8H)H*+. (b) Reaction of H with Cnanasine. The reaction of the hydrogen atom (H') with guanosine at pH 6.5 was investigated by irradiating an aqueous solution of 0.2 mM guanosine saturated with N 2 0 (22 mM) to convert e,; into OH' (eq lo), and containing 64 mM rert-butyl alcohol to scavenge the OH' radicals (q11). Under these conditions, the only reactive species remaining to react with guanosine are hydrogen atoms (H.). eaq- N 2 0
5000-
I
w
Figure 3. Absorption spectra obtained after reaction of H' with guanosine at pH 6.5 in a NQsaturated solution containing 0.2 mM guanosine and 64 mM rert-butyl alcohol, 40 ps after the pulse (circles) and at pH 2.8 in a deaerated solution of 0.1 mM guanosine and 20 mM tert-butyl alcohol, 70 1 s after the pulse (squares). Insets: variation in time of the absorption at 3 15 nm (a) and of the conductance (b) on reaction of H' with 0.1 mM guanosine at pH 2.8.
OH'
0
.
F
hlnm
+
5
7
-
300
7500 -
(12)
optical density (OD) was observed (inset a of Figure 3) with koW = 7.7 X 104 s-l which yields 8 X lo8 M-l s-l as the second-order rate constant for reaction of H'.The absorption spectrum recorded after this reaction (Figure 3) is nearly identical to that obtained under similar conditions but with guanosine present in higher concentration than H+ (guanosine 1 mM, pH = 4), Le., under conditions of predominant reaction with e,;. The reaction of H'with 0.1 mM guanosine at pH 3 and in the presence of 20 mM tert-butyl alcohol was also monitored by conductance detection. A decrease of conductance with the same rate constant as the buildup of absorption was observed (inset b
Figure 4. Absorption spectra recorded by pulse radiolysis of deaerated solutions of 1 mM 1-methylguanosine and 150 mM tert-butyl alcohol, 5 after the pulse, at pH 10.3 (circles) and in the presence of 0.8 M KOH (quarts). Inset: pH dependence of the optical density at 400 nm.
of Figure 3). The decrease of conductance is consistent with the replacement of H+ by a radical cation.28 The similarity of the absorption spectra of the radicals resulting from the reaction of guanosine with ea; and H', at both pH 7 and 3, and the formation of a radical cation by both pathways at pH 3 are good evidence for the formation of the same species on reaction of guanosine with either (e, - and He). We suggest that this radical is the H adduct to the C8 position G(C8H)' (see Scheme I). The addition of the hydrogen atom to a double bond of guanosine is in line with the reactivity of H' with other unsaturated heterocycles such as u r a ~ i l s ' and ~ , ~iso~ uracils.1° In the solid state, the addition of H'ot the C8 position is well documented on the basis of ESR279Mand ENDOR3' results of irradiated single crystals of guanine derivatives. ESR results have also been interpreted in terms of tautomerization of a heteroatom-protonated radical to give a C-protonated radical upon thermal annealing.17 However, in more recent work3' no precursor-successor relation between the radical anion and the H adduct to C8 was found. (c) Reactim of e-, with Guanine Nucleosides io Bepic solutioa. The reaction of e, - with deprotonated guanosine and 2'-deoxyguanosine (pK, ~ 8 . and 2 9.4,respectively) is much slower than with the neutral form. At pH 12, where 2'-deoxyguanosine is present quantitatively as a monoanion, the rate constant was measured to be 1.7 X lo8 M-' s-I. Thus, the reactivity is -35 times less upon deprotonation. The reason for this is (a) electrostatic repulsion and (b) the increased electron density ( d d electron affinity) of the deprotonated molecule. The absorption spectra obtained on reaction of ea; with guanosine or 2'-deoxyguanosine at pH 12 are different from those obtained at either pH 4 or in neutral solution. They have three bands with maxima at 300,355,and 450 nm (c = 4500,2200, and 630 M-' cm-', respectively). The spectral variations on changing pH, monitored by the OD at 300,330,and 470 nm,did nor follow theoretical pK curves. Addition of ea< to the anionic forms of the substrates leads initially to a doubly negatively charged radical (Go2-). As was demonstrated by the conductance experiments in neutral solution, the monoanionic radical is rapidly protonated by water (k 1 lo7PI, see above). It is therefore reasonable to assume that the dianionic radical W2- will undergo even faster protonation by water to give, at least in a first step, a monoanion. It should be noted that this monoanion is not necessarily the same as that resulting from e,; addition to unbut it can be a tautomeric charged (2'-deoxy)guanosine (e-), form. It is conceivable that these two radicals undergo different (fast) reactions in competition with their tautomeric interconversion. With 1-methylguanosine, the absorption spectrum of the electron adduct remained unchanged from pH 7 to 11. However, for pH > 12, spectral changes were detected (see F i i 4), visible as increases of absorption at 400 nm with increasing pH until at
10306 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
pH EJ 14 a plateau was reached, consistent with previous fidings.18 The dependence of absorption at 400 nm on pH, which can be fitted to a theoretical pKcurve with an inflection point at pH 13.3 (see inset of Figure 4), is explained in terms of deprotonation of G(C8H)' (eq 13). 0
R
R
(d) Redox Properties Of the Electron Adduets Of GUrnine NUcleaek. The redox properties of the electron adducts of purines and pyrimidines have been shown to allow a distinction between The the heteroatom- and the carbon-protonated isomeric for~ns.~J' C-protonated radicals are in all cases weaker reductants than their heteroatom-protonated tautomers. The reactivity of the guanosine electron adducts with the oxidant methylviologen (MV2+,E = -0.44 V/NHE) was investigated. Different concentrations of this weak oxidant (25-100 pM) were added to deaerated, supersaturated solutions of 3 mM guanosine (see Experimental Section), containing 0.25 M terr-butyl alcohol at pH 7. On irradiation, a two-component increase of the a b sorption at 605 nm, characteristic of the methylviologen radical cation (MV'+), was observed. Since the radical of tert-butyl reacts predominantly alcohol does not react with MV2+ and the with guanosine, the formation of MV'+ emonstrates electron transfer between MV2+ and the guanosine electron adducts. However, the yield of MV'+ (corrected for the direct reaction between MV2+and e -) was 42followed by rapid protonation of the resulting cytosine radical anion by its base partner, and that this intra- or interstrand electron transfer is much faster than irreversible protonation on C8 of guanine (which in DNA would involve proton transfer from a water molecule in the hydration shell of the DNA h e l i ~ ) . 4 ~An .~ analogous mechanism could apply to the adenine-thymine base pair. Acknowledgment. L.P.C.thanks the Deutscher Akademischer Austauschdienst (DAAD) for a scholarship. We thank D U D and British Council also and also NATO (85/6748) for travel grants. References and Notes ( 1 ) Max-Planck-Institut.
(2) MRC, Division of Radiobiological Mechanisms. (3) For reviews, see: (a) GrHslund. A.; Ehrenberg, A.; Rupprecht, A.; Strbm, G. Int. J . Rudiut. Biol. 1975, 28, 313. (b) Hlittermann, J.; Voit, K.; Oloff, H.; Kbhnlein, W.; GrHslund, A.; Rupprocht, A. Fwuduy Discuss. Chem. SOC.1984, 78, 135. (c) Cullis, P. M.; Symons, M. C. R. In Simic, M.G., Grossman, L., Upton, A. C., Eds.; Mechuism of DNA Damage and Repuir, Plenum, New York 1986, p 29. (4) For reviews see, e.g.: (a) Bernhard, W. A. Adu. Rudiur. Biol. 1981, 9, 199. (b) HBttennann. J. Ultrumicroscopy 1982,10, 25. (c) Close. D. M.;
Guanine Nucleosides
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10307
Nelson, W. H.; Sagstuen, E. In Electronic Magnetic Resonance of the Solid Sratc; Wcil, J. A,, Ed.; Canadian Society for Chemistry, Ottawa, 1987;p 237. (d) Symons, M.C. R. J. Chem. Soc. Faraday Trans. 1 1987,83,1. (5) (a) SeviUa, M. D.; Failor, R.;Clark, C.; Holroyd, R. A,; Patti, M. J. Phys. Chem. 1976,80, 353. (b) Visscher, K. J.; Hom, M.; Loman, H.; Spoclder, H. J. W.; Verberne, J. B. Radiat. Phys. Chem. 1988, 32, 465. (6)(a) Steenken, S. Chem. Rev. 1989,89,503. (b) Steenken, S.Free Radical Res. Commun. 1989,6, 117. (c) Sttenken, S.Free Radical Res. Commun. 1992,16,349. (7)Rackovsky, S.;Bernhard, W. A. J . Phys. Chem. 1989,93,5006. (8) Colson, A,-0.; Baler, B.; Close, D. M.; Sevilla, M. D. J. Phys. Chem.
1992,96,661. (9)Deeble, D. J.; Das, S.;von Sonntag, C. J. Phys. Chem. 1W, 89,5784. (10)Novais, H. M.; Steenken, S.J . Am. Chem. Soc. 1986,108,1. (11) Candeias, L. P.; Sttenken, S.J. Phys. Chem. 1992,96,937. (12)Steenken, S.; Telo, J. P.; Novais, H. M.; Candeias. L. P. J . Am. Chem. Soc. 1992,114,4701. (13) (a) Pullman, B.; Pullman, A. Quanrum Biochemistry; Wiley-Interscience: London, 1963. (b) Bcrthod, H.; Giessner-Rettre, C.; Pullman, A. Theor. Chim. Acto 1966.5.53. (c) Bodor, N.; Dewar, M. J. S.;Harget, A. J. J . Am. Chem. Soc. 1970,92, 2929. (d) Younkin, J. M.; Smith, L. J.; Compton, R. N. Theor. Chim. Acta 1976,41,157. Compton, R. N.; Yoshioka, Y.; Jordan, K. D. Theor. Chim. Acta 1980,54, 259. (e) Rabin, B.; Herak, J. N. Radiat. Phys. Chem. 1983,22, 1043. See also ref 8. (14)(a) Bernhard, W. A. J. Phys. Chem. 1989,93,2187.(b) Sevilla, M. D.; Bccker, D.; Yan, M.; Summerfield, S.R. J. Phys. Ckem. 1991,95,3409. (15) Sagstuen, E.; Hole, E. 0.;Nelson, W. H.; Close, D. M. J. Phys. Sagstuen, E.; Nelson, W. H.; Close, D. Chem. 1989,93,5974.Hole, E. 0.; M. J . Phys. Chem. 1991, 95, 1494. See also ref 42. (16)It has recently been f o w l by product analysis that in aqucous dution the electron adduct of guanosine has no reducing properties with respect to 5-bromouracil. It was concluded that the radical is protonated at carbon: (a) Nese, C.; Yuan, Z.; Schuchmann, M. N.; von Sonntag, C. In?. J. Radiat. Biol., submitted. (17)For reviews see: (a) ref 4a. (b) Hllttermann, J. In: Lund, A,, Shiotani, M., Eds.; Radical Ionic Systems; Kluwer: Dordrecht, 1991;pp 435-62. (18) Moorthy, P. N.; Hayon, E. J . Am. Chem. Soc. 1975,97,3345. (19)Jagannadham, V.; Steenken, S.J . Am. Chem. Soc. 1984,106,6542. (20)ONeill, P.; Chapman, P. W. Int. J. Radiar. Biol. 1985,47,71. The transient digitizer has recently been interfaced to a F C which runs a customized version of ASS4ST software for data acquisition and processing. (21)Schuler, R. H.; Hartzell, A. L.; Behar, B. J . Phys. Chem. 1981,85, 192. (22)The mobility of organic radical ions was assumed to be q u a l to that of acetate at 18 OC (34.6S cm2mol-'). For the mobilty of this and other ions in aqueous solution see: Landolt-B6mstein; Springer: Berlin 1960;Part 7, pp. 264. (23)Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B.J . Phys. Chem. Ref Data 1988,17,513. (24)Due to formation of H* in q 3, the pH of the solution decreases on irradiation. However, the dosar used were such that $2 pM H+ was prcduced, so that the lifetime of the OH- released in reaction 7 is still >3.5 p). (25)For a general discussion of ratedetermining proton-transfer processes, see. e.&: Williams, J. M.; Kretvoy, M. M. In: Advances in Physical Organic
Chemistry; Gold, V., Ed.; Academic Rare: London, 1968;Vol. 6, p 63. Saunders Jr., W. H.Kinetic Isotope Effcats. In: Techniques of Chemistry; Lewis, E.S.,Ed.; Wiley-Interscience: New York, 1974;Vol. 6,p 21 1. For a recent example of the d v m t kinetic isotope effect for protonationon carbon, see: Bednar. R. A.: Jencks. W. P. J. Am. Chem. Soc. 1985. 107. 7117. (26)Hayon, E.; Lichtin, N.N.; Madhavan, V. J . Am. Ckm. S&. 1973, 95,4762;Radiar. Res. 1973,55, 589. (27)Rakvin. B.; Herak, J. N.; Voit, K.; Hllttermann, J. Radiar. Enu. Biophys. 1987,26, 1. (28)The yield of radical cation could not be determined due to the buffering effect of guanosine. (29)Das, S.;Dwble, D. J.; von Sonntag, C. Z . Naturforsch. 1985,roc,
292. (30)Alexander, C.; Gordy, W. Proc. Nod. Acad. Sci. US.A. 1967,58, 1279. (31) (a) Sagstuen, E.; Hole, E. 0.;Nelson, W. H.; Close, D. M. Radiat. Res. 1988,116, 196. (b) Nelson, W. H.; Hole, E. 0.; Sagstuen, E.; Close, D. M.Int. J. Radial. Biol. 1988,51, 963. (c) Sagstuen, E.; Hole, E. 0.; Nelson, W. H.; Close, D. M.Free Radical Res. Commun. 1989,6,91.(d) Hole, E.0.;Nelson, W. H.; Sagstuen, E.; Close, D. M. Radiat. Res. 1992, 129, 119. (32)This radical has been found in the solid state, see ref 31d. (33) Candeias, L. P. Dissertation, Technical University of Lisbon, 1992. (34)The OH adduct to C5 has also been found to be reducing in the c&pc of N6,M-dimethyladenosine; see: Vieira, A. J. S.C.; Steenken, S . J. Am. Chem. Soc. 1987,109,7441. (35) An attempt was made to measure the rate constant for reaction of the terr-butyl alcohol radical by pulsing a N,O-saturated solution of 0.1 M tert-butyl alcohol and 1.2 mM K3Fc(CN)6. The observed rate of decay of absorption at 420 nm was 4 X lo) s-I, from which the rate constant for reaction of the terr-butyl alcohol radical with FC(CN)6'- is estimated to be =3 X 106 M-' s-l. (36)See the corresponding discussion in ref 10. (37)For a discussion see: Eigen, M. Angew. Chem. 1963,75,489. Eigen, M.;Kruse, W.; Maass, G.; De Maeyer, L. Prog. React. Kinct. 1964,2,287. (38) For cytidine there is no rate constant for C protonation available yet, but the evidence'2Jh is that k is not much larger than lo)s-I. (39)For the values calculated by MO methods, see refs 8 and 13. (40)(a) Hush, N. S.;Cheung, A. S.Chem. Phys. Lett. 1975,31,11. (b) Orlov, V. M.;Smirnov, A. N.; Vanhavsky, Y. M. Tetrahedron Lett. 1976, 4377. (c) Peng, S.;Padva, A.; LeBreton, P. R. Proc. Narl. Acad. Sei. U S A . 1976,73,2966.(d) McGlynn, S.P.; Dougherty, D.; Mathm, T.; Abdulner, S.In: Pullman, B., Goldblum, N., Eds.Excited Srares in Organic Chemistry and Biochemistry; Reidel: Dordrecht, 1977;p 247. (4!) In this connection it is interesting that guanine is also exceptionally sensitive to oxidatiue damage (due to its low ionization potential). (42)Yan, M.; Becker, D.; Summerfield, S.;Renke, P.; Sevilla, M. D. J. Phys. Chem. 1992,96, 1983. (43)It is conceivable that also the heteroatom-protonated electron adduct G(EH)' could act as an electron donor. A case of analogy is the pronounced reducing power of the protonated electron adduct of cytidine (see ref 12). (44)On the basis of luminescence studics on irradiated solid DNA at mom temperature, the rate of electron transfer appears to be 23 X lo7 s-I (AlKazwini, A.; ONeill, p.; Adams, G. E.; Fielden, E. M. Radial. Res. 1990, 121, 149).
