Origin of difference between one-electron redox potentials of guanosine and guanine: Electrochemical and quantum chemical study

Article abstract of DOI:10.1021/jp0481207

Cyclic voltammetry was used to measure the rates of the chemical oxidation of guanine (G), guanosine (Gs), 2a?2-deoxyguanosine (dG), and 2a?2-deoxyguanosine 5a?2-monophosphate (dGMP) by electrochemically generated tris(2,2a?2-bipyridyl)ruthenium(III). The numeric fit of voltammograms to an ECCCE type of mechanism provided the equilibrium and rate constants of the two-step chemical oxidation of the guanine species. One-electron redox potentials evaluated from the equilibrium constant of the first electron uptake follow the sequence G < Gs a?? dG a?? dGMP, indicating that guanine is oxidized most easily. This sequence is expressed in the rate constant, which apparently follows the expected driving force dependence. Ab initio molecular orbital calculations were carried out using the DFT/B3LYP method with 6-31G* and 6-31G++* basis sets, and also the RI-MP2 method with the cc-pVDZ basis set, so as to clarify the role of various factors contributing to the redox potential. Theoretical results suggest that the difference between the one-electron redox potentials of Gs and G (ca. 0.13 V) originates partly from the higher energy of proton dissociation from the cation radical Gs⁺ and partly from the higher difference in the hydration energy between the deprotonated radical Gs(-H) and the parent Gs, which compensate for the lower ionization potential of Gs compared to that of G.

Full text of DOI:10.1021/jp0481207

15896
J. Phys. Chem. B 2004, 108, 15896-15899
Origin of Difference between One-Electron Redox Potentials of Guanosine and Guanine:
Electrochemical and Quantum Chemical Study
Jan Langmaier,† Zdeneˇk Samec,*,† Eva Samcova´,‡ Pavel Hobza,*,§ and David Rˇ eha§
J. HeyroVsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejsˇkoVa 3,
182 23 Prague 8, Czech Republic, Charles UniVersity, 3rd Faculty of Medicine, Center for Biomedical
Sciences, Ruska´ 87, 100 00 Prague 10, Czech Republic, and Institute of Organic Chemistry and Biochemistry,
FlemingoVo nam. 2, 166 10 Prague 6, Czech Republic
ReceiVed: April 30, 2004; In Final Form: July 21, 2004
Cyclic voltammetry was used to measure the rates of the chemical oxidation of guanine (G), guanosine (Gs),
2′-deoxyguanosine (dG), and 2′-deoxyguanosine 5′-monophosphate (dGMP) by electrochemically generated
tris(2,2′-bipyridyl)ruthenium(III). The numeric fit of voltammograms to an ECCCE type of mechanism provided
the equilibrium and rate constants of the two-step chemical oxidation of the guanine species. One-electron
redox potentials evaluated from the equilibrium constant of the first electron uptake follow the sequence G
< Gs ≈ dG ≈ dGMP, indicating that guanine is oxidized most easily. This sequence is expressed in the rate
constant, which apparently follows the expected driving force dependence. Ab initio molecular orbital
calculations were carried out using the DFT/B3LYP method with 6-31G** and 6-31++G** basis sets, and
also the RI-MP2 method with the cc-pVDZ basis set, so as to clarify the role of various factors contributing
to the redox potential. Theoretical results suggest that the difference between the one-electron redox potentials
of Gs and G (ca. 0.13 V) originates partly from the higher energy of proton dissociation from the cation
radical Gs•+ and partly from the higher difference in the hydration energy between the deprotonated radical
Gs(-H)• and the parent Gs, which compensate for the lower ionization potential of Gs compared to that of
G.
Introduction
wave potentials10,11 lead the authors to propose a multistep
mechanism initiated by concerted electron and proton uptake
from the guanine moiety.13 The intermediate formation of 8-oxo-
7,8-dihydroguanine (8OG) was supposed to occur but was not
proved, because 8OG is oxidized far more easily than the parent
guanine base.12 On the other hand, chemical oxidation of 2′-
deoxyguanosine (dGs) in neutral aqueous solutions indicated
that the product of the first electron transfer is uncharged,8,15
i.e., the electron-transfer reaction is accompanied by a fast proton
dissociation from N(1) of the radical cation G•+, pKa ) 3.9, to
form the neutral radical G(-H)•.15 The existence of proton-
coupled electron transfer (PCET) from G is supported by kinetic
studies of oxidation of mononucleotides16,17 and single- and
double-stranded DNA.17 Several studies of the molecular
mechanism point to a second electron-transfer step leading to
8OG.18,19 A model with two-step guanine oxidation was used
to simulate the catalytic oxidation of metal complexes, e.g., Ru-
(bpy)32+ at an indium tin oxide (ITO) electrode,20,21 though the
contribution from the second electron transfer is likely to be as
low as 1% or less.22 Molecular calculations suggest that water
addition to G(-H)• on the C(8) site leading to 8OG is
energetically much less favorable than in the case of G•+.14
Hence, if 8OG were the intermediate of the pathway involving
the neutral radical, the water addition to G(-H)• should be
preceded or concerted with the electron transfer.
Guanine (G) is most easily oxidized of the nucleic acid bases,
as indicated by the lowest values of ionization1,2 and one-
electron redox3,4 potentials. The ionization potential of guanine
is further lowered for stacked bases, e.g., GG2,5 and GGG6,7
sequences in DNA, which thus appear to be the sites of the
specific DNA cleavage. These observations are of relevance to
oxidative degradation of nucleic acids in mutagenesis, carcino-
genesis, and aging.8,9 Measurement of the electron-transfer
equilibrium in solutions indicates that the redox potential of
guanine depends also on the substitution at the N(9) site, e.g.,
for guanosine (Gs) it is by ca. 0.1 V more positive compared
to that for G.3 These results are supported by voltammetry of
G and Gs oxidation at a graphite electrode.10-13 Although the
overall irreversibility of the electrode reaction and the adsorption
effects preclude evaluating of the one-electron redox potentials,
it is clear that Gs is oxidized at pH 7 at a potential by ca. 0.1
V more positive than G.11 Apart from the computational method
used, the substitution at the N(9) site can be responsible for the
difference in the ionization potentials calculated for 9-methylG,3
Gs,5 and G.14
Both G10-12 and Gs11,12 undergo four-electron oxidation at a
graphite electrode, leading to a series of products including
guanidine, parabanic acid, and oxalyl guanidin.10,13 Their
identification13 and the observed pH dependence of the half-
In this paper, we propose an explanation for the difference
between the one-electron oxidation-reduction (redox) potentials
of G and Gs. First we shall employ the experimental approach
and procedure20-23 to evaluate the equilibrium constants and
redox potentials for the first one-electron step in the oxidation
of G, Gs, dGs, and 2′-deoxyguanosine 5′-monophosphate
* To whom correspondence may be addressed. E-mail: zdenek.samec@
jh-inst.cas.cz. Fax: +420-286582307.
† Academy of Sciences of the Czech Republic.
‡ Charles University.
§ Institute of Organic Chemistry and Biochemistry.
10.1021/jp0481207 CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/15/2004

Redox Potentials of Guanosine and Guanine
J. Phys. Chem. B, Vol. 108, No. 40, 2004 15897
(dGMP) by electrochemically generated Ru(bpy)33+. Second,
we shall make a theoretical estimate of the difference in the
redox potentials with the help of ab initio molecular orbital
calculations to clarify the roles of various factors such as
ionization potential, deprotonization, and hydration energy.
and nucleosides,25 the diffusion coefficients of all guanine
species were fixed to 1 × 10-5 cm2 s-1. Each numeric fitting
then provided the values of four parameters: kf1, K1, kf2, and
K2.
Computational Methods. The molecular geometries of
guanine and guanosine and their cation and neutral radicals were
optimized using the DFT/B3LYP method with 6-31G** and
6-31++G** basis sets and also using a more reliable resolution
of the identity MP2 (RI-MP2) method with the cc-pVDZ basis
set. Ionization potentials of the neutral parent systems were
determined as a difference between the energy of the parent
(neutral) system and the respective cation radical (in both cases,
the geometry was fully optimized). Deprotonization energies
of cation radicals were calculated as a difference between the
energies of cation radicals and deprotonated neutral radicals
(again both geometries were fully optimized). Hydration-free
energies of neutral parent systems and of the respective neutral
deprotonated radical were obtained with self-consistent relax-
ation field method using the COSMO methodology (HF/6-
31G*), as implemented in the Gaussian 03 suits of codes.26 All
other calculations were performed with Gaussian 0326 and
Turbomol27,28 program packages.
Experimental Section
Reagents. Guanine (2-amino-6-hydroxypurine), guanosine
hydrate, 2′-deoxyguanosine hydrate, 2′-deoxyguanosine 5′-
monophosphate disodium salt, 8-oxo-7,8-dihydro-2′-deoxygua-
nosine, tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate,
and the supporting electrolytes were purchased from Sigma-
Aldrich. Aqueous solutions of the bases and of Ru(bpy)3Cl2 (bpy
) 2,2′-bipyridine) in 1/15 M McIlvaine (pH 6.24) were prepared
using highly purified and deionized water (Milli-Q, Millipore).
Solutions were deoxygenated prior to measurements by argon
bubbling.
Voltammetry and Digital Simulation. Voltammetric mea-
surements were carried out in an all-glass cell by using a three-
electrode potentiostat (Model 273A, EG&G PAR), which was
equipped with an operating software (M270, EG&G PAR).
F-doped tin oxide (FTO) coated-glass electrode (0.14 cm2,
Nippon Sheet Glass, 10 Ω/0) was cleaned following the same
procedure as that used for an ITO electrode,22 i.e., by sonifi-
cation and washing with water and with supporting electrolyte
solution prior to use. A Pt wire and an Ag|AgCl|sat. KCl
electrode (0.197 V vs a saturated hydrogen electrode (SHE))
were used as the auxiliary and the reference electrode, respec-
tively. All measurements were carried out at ambient temper-
ature of 22 ( 2 °C.
Results and Discussion
2+
Voltammetry of Ru(bpy)3 and 8OdGs. Cyclic voltam-
metry was used first to remeasure the parameters of the initial
two steps in the reaction sequence, eqs 1 and 2. At sweep rates
2+
higher than 50 mV s-1, the voltammetric behavior of Ru(bpy)3
can be simulated assuming a simple one-electron-transfer
0
Cyclic voltammograms were corrected for background current
and fitted to an ECCCE type of mechanism consisting of a
sequence of electrochemical (E) and chemical (C) reactions by
using Digisim 3.03 software (BAS, USA). The mechanism was
a modification of one adopted previously20
mechanism for the values of the parameters kel,M ) 0.1 cm
s-1, RM ) 0.5, and EM ) 1.06 V. At lower sweep rates, the
0
effect of conversion of the higher to lower oxidation state of
the metal complex described by eq 2 is apparent, cf. inset A of
Figure 1. A numeric fit of five cyclic voltammograms (CVs)
of Ru(bpy)32+ (5-100 mV/s) to an EC mechanism comprising
the thermodynamically superfluous reaction (TSR), eq 2, yields
the forward rate constant kf0 ) 0.044 ( 0.006 s-1, the
equilibrium constant K0 ) 6 × 107, and the diffusion coefficients
0
M
3+ + e- / M2+ (kel,M0,RM,EM
3+ / M2+ (kf0,K0)
3+ + G / M2+ + G′ (kf1,K1)
)
(1)
(2)
(3)
(4)
(5)
M
DM ) (6.6 ( 1.1) × 10-6 cm2 s-1 and DM ) (4.3 ( 0.4) ×
10-6 cm2 s-1, which agree well with literature data.20,21,29
Voltammetric behavior of 8OdGs at FTO electrode was
simulated to the simple E mechanism described by eq 5, which
yields kel,P0 ) 5.7 × 10-7 cm s-1, RP ) 0.55, EP0 ) 0.56 V. In
an agreement with the reported behavior on the ITO electrode,30
the rate of Gs oxidation at the FTO electrode is far lower, cf.
curves 1 and 2 in the inset B of Figure 1, and the contribution
of the latter to the measured current was neglected.
3+
2+
M
M
3+ + G′ / M2+ + G′′ (kf2,K2)
G′′ f P + e- (kel,P0,RP,EP )
0
where M represents the metal complex, kel0, R, and E are the
standard rate constants, the apparent charge-transfer coefficients,
and the standard redox potentials of the electrochemical
reactions, respectively, and kf and K are the forward rate and
equilibrium constants of the chemical reactions, respectively.
It can be reasonably assumed that the product G′′ of the second
electron transfer is 8OG, which is oxidized irreversibly at the
FTO electrode in the potential range studied.24 Therefore, we
added the electrochemical step, eq 5, describing the one-electron
oxidation of 8OG to a product P. Although the software
employed allows optimizing the values of twelve parameters
given in parentheses and six diffusion coefficients, the reliable
determination of so many parameters in one fit is rather
uncertain. Therefore, the parameters of electrochemical steps
and the diffusion coefficients of species involved were fixed to
the values found in the literature or determined in separate
voltammetric experiments, vide infra. With reference to the
results of polarographic measurements of adenine nucleotides
Catalytic Electrochemistry. CVs of the Ru(bpy)32+ oxidation
in the presence of various guanine derivatives are shown in
Figure 1. The measurements were performed for (a) 10 µM Ru-
(bpy)32+ and four different concentrations of the guanine bases
(20-100 µM) at the sweep rate of 10 mVs-1, (b) 20 µM Ru-
2+
(bpy)3 and 100 µM dGs at four different sweep rates (10-
2+
100 mV/s), and (c) six different concentrations of Ru(bpy)3
(10-60 µM) and 500 µM dGs at the sweep rate of 20 mV/s.
The mean values of the rate and equilibrium constants obtained
at various base concentrations are summarized in Table 1, cf.
the caption for Figure 1. CV’s simulated for these values agree
well with those measured, cf. the solid and dashed lines in Figure
1. The results obtained for dGs are supported by measurements
at various sweep rates yielding kf1 ) (2.4 ( 1) × 107 M-1 s-1
,
K1 ) 13 ( 6, kf2 ) (3.8 ( 2) × 105 M-1 s-1, and K2 ) (2.9 (
2) × 1013 and by measurements at various concentrations of

15898 J. Phys. Chem. B, Vol. 108, No. 40, 2004
Langmaier et al.
guanine species by Ru(bpy)33+, e.g., the second-order rate
constant for the faster reaction component of the oxidation of
2′-deoxyguanosine 5′-triphosphate (dGTP) was found to be 5.4
× 106 M-1 s-1 17
. Fitting of the catalytic CV for guanosine 5′-
triphosphate to the simple EC mechanism was not possible,
while measurements of GMP provided kf1 ) 6.4 × 105 M-1
s-1 30
However, the latter value was obtained assuming an
.
unreliably low value for the diffusion coefficient of mononucleo-
tides, 1.19 × 10-6 cm2 s-1 30
.
Besides, we have found that
simulation of the catalytic CVs of isolated guanine species
requires including at least two following super-oxidation steps,
such as those described by eqs 4 and 5. The ratio of the rate
constants kf2/kf1 falls in range 0.01-0.1, which is a result
comparable with that for guanine oxidation in DNA.22
Table 1 also lists the values of the one-electron redox
0
potentials E1 obtained using the relationship
E10 ) EM0 + (RT/F) ln K1
(6)
where the standard redox potential for the Ru(bpy)33+/2+ system
0
EM ) 1.06 V (vs Ag/AgCl), i.e., 1.26 V (vs SHE). The
difference of 0.13 V between the redox potentials of G and Gs
is corroborated by the results of chemical3 or electrochemical11
studies. The value of E1 ) 1.16 V for Gs is somewhat lower
Figure 1. Cyclic voltammograms (solid lines) on a tin oxide coated-
0
glass electrode (0.14 cm2) at 10 mV/s and the numeric fit (dotted lines)
2+
for 10 µM Ru(bpy)3 in 1/15 M McIlvaine buffer (pH 6.24) in the
than the 1.29 ( 0.03 V (vs NHE) value obtained from the
measured equilibrium constant of chemical oxidation of guanine
in aqueous solution at pH 7.4 Nevertheless, the former value is
closer to the redox potential of guanine in double-helical DNA
falling between 0.9 and 1.0 V vs Ag|AgCl at pH 7 (i.e., 1.10-
1.20 V vs SHE), which was estimated on the basis of ability of
metal complexes with different redox potentials to oxidize
DNA.23
absence (1) and presence (2-5) of 100 µM guanine base: (2) G (kf1
)
8.8 × 107 M-1 s-1, K1 ) 7.9 × 103, kf2 ) 1 × 106 M-1 s-1, K2 ) 7.5
× 105); (3) dGs (kf1 ) 7.8 × 106 M-1 s-1, K1 ) 36, kf2 ) 4 × 105 M-1
s-1, K2 ) 1012); (4) Gs (kf1 ) 5.8 × 106 M-1 s-1, K1 ) 52, kf2 ) 3 ×
105 M-1 s-1, K2 ) 5 × 1011); (5) dGMP (kf1 ) 2 × 107 M-1 s-1, K1 )
28, kf2 ) 3 × 105 M-1 s-1, K2 ) 8 × 1012). Insets: background-
subtracted cyclic voltammograms (solid lines) and numeric fits (dotted
lines) for (A) 20 µM Ru(bpy)32+ at 5 mV s-1 and (B) 300 µM 8OdGs
(curve 1) and 100 µM dGs (curve 2) at 20 mV s-1, 1/15 M McIlvaine
buffer (pH 6.24).
Theoretical Estimate of the Redox Potential. The first step
in the oxidation of guanine species, eq 3, apparently proceeds
0
as PCET.15,17 Hence, the evaluated redox potential E1 should
TABLE 1: Rate and Equilibrium Constants for Oxidation
3+
of Guanine Bases by Ru(bpy)3
have the physical meaning of standard potential for the electrode
reaction
10-6kf1
10-5kf2
base (M-1 s-1
)
10-1K1 E10 a (V) (M-1 s-1
)
10-12K2
B(-H)• + H+ + e- / B
(7)
G
Gs
dGs
88 ( 14 790 ( 405
1.03
1.16
1.18
1.17
10 ( 6 (8 ( 4) × 10-7
5 ( 2
7 ( 4
4.8 ( 2
2 ( 0.2
3.9 ( 2
4 ( 1
1 ( 0.3
where B ) G or Gs. With the help of a suitable Born-Haber
4 ( 2 29 ( 18
3 ( 1 8 ( 3
dGMP 13 ( 3
cycle,31 the redox potential can be then expressed as
a One-electron redox potentials (vs SHE) evaluated from K1 using
EM ) 1.26 V.
1
F
(IP + DP + ∆Ghyd) + ∆E1
(8)
0
0
E1
)
the metal complex yielding kf1 ) (2.8 ( 2) × 106 M-1 s-1, K1
) 14 ( 5, kf2 ) (1.5 ( 0.5) × 105 M-1 s-1, and K2 ) (5.4 (
4) × 1012.
where IP is the ionization potential of B, DP is the difference
in energy between the deprotonated neutral radical B(-H)• and
the radical cation B•+, ∆Ghyd is the difference in the hydration
free energies between B(-H)• and B, and ∆E1 comprises the
constant energy terms associated with proton and electron.
The ionization potentials calculated at different theoretical
levels for G and Gs are collected in Table 2. The data obtained
for G are consistent with literature values calculated by using
the DFT/B3LYP method.14,32 It is evident that the largest IP
values are obtained at the correlated level. However, irrespective
of the theoretical level, IP for G is higher than that of Gs by up
to 0.27 eV (RIMP2/cc-pVDZ), thus exhibiting an opposite
These data point to an appreciable effect of the sugar moiety
on the oxidation of guanine species. As it can be seen from
Figure 1, guanine brings about the largest catalytic enhancement
of the oxidation current, expressed in approximately 1 order of
magnitude higher rate constant kf1 and in 2 orders of magnitude
higher equilibrium constant K1 for the first electron uptake
compared to other guanine species. The difference in the rate
constant between G and Gs, RT∆ ln kf1 ) 0.073 eV can be
related to a higher driving force, RT∆ ln K1 ) 0.13 eV, which
would correspond to the charge-transfer coefficient of 0.56. The
driving force dependence of the rate constant kf1 for oxidation
of guanine in calf thymus DNA by a series of metal complexes
was investigated using catalytic electrochemistry21 and stopped-
flow spectrophotometry17 yielding the charge-transfer coef-
ficients of 0.4921 and 0.8,17 respectively. The latter value was
considered to be evidence for the PCET mechanism.17 There
are few kinetic data published on the oxidation of isolated
0
change than E1 . On the other hand, the calculated DP values
for G are systematically lower than those for Gs, both being
largest again at the correlated level. It appears that the energy
change due to the dissociation of proton presumably from N(1)15
could compensate for the change in the ionization potential.
Indeed, on passing from the B3LYP to the more reliable
correlated level, the sum IP + DP becomes higher for Gs than

Redox Potentials of Guanosine and Guanine
J. Phys. Chem. B, Vol. 108, No. 40, 2004 15899
References and Notes
TABLE 2: Ionization Potentials, Deprotonization Energies,
and Hydratation Free Energies for Guanine (G) and
Guanosine (Gs) Determined at Three Theoretical Levels: (I)
B3LYP/6-31G**; (II) B3LYP/6-31++G**; (III)
RIMP2/cc-pVDZ
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7.65 10.28
7.88 10.65
7.18 10.68
7.51 10.37
7.61 11.09
0.13
17.91
17.93
18.53
17.86
17.88
18.70
18.04
18.06
18.66
18.05
18.07
18.89
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III
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II
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Figure 2. Comparison of the one-electron redox potential of Gs relative
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0
potentials (∆E1 ≈ 0.13 V) at the correlated level, cf. Figure 2.
Conclusions
The sugar moiety has an appreciable effect on both the one-
electron redox potential and the rate of oxidation of guanine
species. Redox potentials for the first electron uptake follow
the sequence G < Gs ≈ dG ≈ dGMP, indicating that guanine
itself is oxidized most easily. This sequence is expressed in the
rate constant, which apparently follows the expected driving
force dependence. Ab initio molecular orbital calculations of
the energy changes in PCET from Gs or G suggest that the
difference between the one-electron redox potentials of Gs and
G (ca. 0.13 V) originates partly from the higher energy of proton
dissociation from the cation radical Gs•+ and partly from the
higher difference in the hydration energy between the depro-
tonated radical Gs(-H)• and the parent Gs, which compensate
for the lower ionization potential of Gs compared to that of G.
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Acknowledgment. Financial support of this work by Grant
Agency of the Czech Republic, Grant No. 203/01/0653, is
gratefully acknowledged.
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