J. Am. Chem. Soc. 1997, 119, 617-618
617
radical cations became available,18,19 whose values are in the
range of 1-1.6 V. Using these compounds as references, we
report here the determination of the reduction potentials of the
adenosine and guanosine radicals in neutral and acidic aqueous
solutions and the evaluation of the pH dependence of the
reduction potentials ranging from 0 to 14.
How Easily Oxidizable Is DNA? One-Electron
Reduction Potentials of Adenosine and Guanosine
Radicals in Aqueous Solution
Steen Steenken*,† and Slobodan V. Jovanovic‡
The purine and reference radicals were generated by pulse
radiolysis (using a 3 MeV van de Graaff accelerator and optical
detection)20 in Ar-purged aqueous solutions of 25 mM K2S2O8,
1 M 2-methyl-2-propanol, and millimolar parent compounds at
20 °C. The composition of the aqueous solution ensures the
selective generation of strongly oxidizing SO4•- radicals (G ≈
Max-Planck-Institut fu¨r Strahlenchemie
D-45413 Mu¨lheim, Germany
Department of Chemistry, UniVersity of Ottawa
10 Marie Curie, Ottawa, Canada K1N 6N5
ReceiVed July 3, 1996
-
2-
3.0) by the eaq + S2O8 reaction. The OH radicals (G ≈
2.8) are scavenged by 2-methyl-2-propanol to the inert
•CH2(CH3)2COH. The SO4•- single electron oxidizes the purine
nucleosides and reference compounds at diffusion-controlled
rates (k > 109 M-1 s-1).13 In order to minimize radical-radical
decay rates, low-dose rates (0.8-2 Gy/pulse, corresponding to
0.5-1.2 µM radicals) were used. Computer averaging of
multiple traces (50-200 pulses/trace) improved the accuracy
of the data.
DNA interacts with fluorescent dyes,1,2 intercalating agents,2
and transition metal complexes,3 often through charge transfer
complexes involving the DNA bases. These interactions, which
are also of interest in photodynamic therapy, depend on the
electron-donating abilities of the bases.4 Furthermore, the direct
and indirect effects of ionizing radiation on DNA, particularly
those relating to base alterations and single-strand breaks, are
mediated by (the one-electron deficient) DNA base radicals.5-12
In order to better understand these effects, it is necessary to
know the reduction potentials of the one-electron deficient bases
(radicals), which are a measure of the susceptibility of DNA to
damage by endogenous oxidizing radicals (e.g., peroxyl radicals
from lipids or amino acids, superoxide radical, singlet oxygen)
and exogenous oxidants (UV light, ionizing radiation).
The reduction potential of the neutral guanosine radical was
determined at pH 7 using thioanisole, with E7 ) 1.44 V,19 and
1,2,4-trimethoxybenzene, with E7 ) 1.13 V vs NHE,18 as the
reference redox couples. In the presence of guanosine (Guo,
from 0.018 to 0.175 mM), the absorbance of thioanisole radical
•-
cation (generated by the SO4 oxidation of 2.8-3.4 mM
The ionization of DNA, induced either by ionizing radia-
thioanisole) at 540 nm19 decayed exponentially. Both the rate
of the reaction and the absorbance upon completion of the first-
order decay depended on the ratio of the concentrations of
thioanisole and Guo, which indicates the following electron
transfer equilibrium:
tion,5,7,9 193 nm photolysis,8,12 or chemi-ionization by strong
•-
transient oxidants such as SO4 or Tl(II)6,13-17 results in the
formation of a positive “hole”. ESR experiments show that this
positive “hole” is transmitted to a guanine moiety.6,14 The
driving force for such intramolecular electron “hops” is the
difference in the reduction potentials of the DNA base radicals.
The guanine radical apparently has the lowest reduction
potential. Values of the reduction potentials of the DNA base
radicals in aqueous media do exist.15,16 However, even if
corrected on the basis of improved values for the reduction
potentials of the reference redox couples, these values15,16 do
not appear to be sufficiently accurate or reliable.1 The main
reason for this is that reference redox couples with sufficiently
high potentials (higher than 1.2 V vs NHE) were previously
not available. The situation has now changed, since recently
the reduction potentials of substituted anisole and thioanisole
The reduction potential of the guanosine radical at pH 7 was
also determined via the electron transfer equilibrium with 1,2,4-
trimethoxybenzene (monitored as a buildup of 1,2,4-trimethoxy-
benzene radical cation at 450 nm21 in an aqueous solution of
2.2 mM Guo and from 0.012 to 0.05 mM 1,2,4-trimethoxyben-
zene, see Figure 1):
† Max-Planck-Institut fu¨r Strahlenchemie.
‡ University of Ottawa.
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(4) The reduction potential of the one-electron oxidized base (a radical)
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(6) Steenken, S. Chem. ReV. 1989, 89, 503.
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The absorbance equilibrium constants were determined from
the plot of absorbances of radicals at equilibrium vs the ratio
of concentrations of the parent compounds,19 as illustrated in
Figure 2.
(11) Devasagayam, T. P. A.; Steenken, S.; Obendorf, M. S. W.; Schulz,
W. A.; Sies, H. Biochemistry 1991, 30, 6283.
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Faraday Discuss. Chem. Soc. 1984, 78, 135.
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Phys. Chem. 1993, 97, 11278.
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Perkin Trans. 2 1995, 67.
(20) Jagannadham, V.; Steenken, S. J. Am. Chem. Soc. 1984, 106, 6542.
(21) O’Neill, P.; Steenken, S.; Schulte-Frohlinde, D. J. Phys. Chem. 1975,
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39.
(17) Simic, M. G. Cancer Res. 1993, 122.
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