Direct EPR detection of a hydrogen-bonded complex between a semiquinone radical anion and a protonated amino acid, and electron transfer driven by hydrogen bonding

Article abstract of DOI:10.1002/anie.200700157

Strong hydrogen bonding in a complex of a semiquinone radical anion and protonated histidine (His·2 H⁺), detected for the first time by EPR spectroscopy during photoinduced electron transfer from 10,10′-dimethyl-9,9′-biacridine (D) to 1-(p-toly

Full text of DOI:10.1002/anie.200700157

Angewandte
Chemie
DOI: 10.1002/anie.200700157
Hydrogen-Bonded Complex
Direct EPR Detection of a Hydrogen-Bonded Complex between a
Semiquinone Radical Anion and a Protonated Amino Acid, and
Electron Transfer Driven by Hydrogen Bonding**
Junpei Yuasa, Shunsuke Yamada, and Shunichi Fukuzumi*
In the photosynthetic reaction center (RC), two quinones
[
1]
denoted QA and QB act as electron acceptors. The RC
catalyzes light-induced two-electron reduction of Q with two
B
protons to yield hydroquinone Q H , which is released from
B
2
the RC and replaced by another quinone molecule from the
[
2]
ꢀ
A
pool. Although the first electron transfer, Q C !Q , does
B
ꢀ
[3,4]
not involve direct protonation of Q C ,
the electron
B
transfer (ET) is coupled with protonation of a nearby amino
ꢀ
[5–7]
acid residue through hydrogen-bond formation with Q C .
B
Specific hydrogen bonds with nearby amino acid residues
enable the quinone to perform a specific function, and hence
differences in the nature of the hydrogen-bonding interac-
tions of QA and QB have been suggested to result in their
[
8]
differing functions. Since hydrogen bonds are largely
[
9]
electrostatic in nature, protonation of amino acids that
leads to a change in fractional charge on one of the
components in a hydrogen bond will remarkably affect the
strength of hydrogen bonding. Such a change in hydrogen-
bonding strength would have a significant effect on the one-
The EPR spectrum of a hydrogen-bonded complex of a
semiquinone radical anion with protonated histidine
(TolSQC /His·2H ) was detected in photoinduced ET from
ꢀ
+
[
10–13]
electron reduction potential of the quinone.
Direct EPR
10,10’-dimethyl-9,9’-biacridine [(AcrH) ] to TolSQ in the
2
detection of a hydrogen-bonded complex between a semi-
quinone radical anion and a protonated amino acid would
afford valuable insight into the hydrogen-bond strength and
electronic structure of the hydrogen-bonded complex. How-
ever, protonation of singly reduced species of carbonyl
compounds acting as strong bases is generally too fast to
detect any intermediate in organic solvent in the presence of
presence of His·2HClO in MeCN at 298 K [Eq. (1); see
experimental details in the Supporting Information].
4
[
16]
Biacridine (AcrH) is known to act as a two electron donor
2
to produce two equivalents of the radical anion of an electron
[
17]
ꢀ
+
acceptor. Since the TolSQC /His·2H complex is unstable,
ꢀ
2
the EPR spectrum of a solution of (AcrH) (1.6 ” 10 m) and
2
ꢀ3
+
TolSQ (4.0 ” 10 m) in MeCN in the presence of His·2H
[
14]
ꢀ3
proton donors.
(4.0 ” 10 m) was measured under steady-state photoirradia-
tion (Figure 1a). The EPR spectrum is well reproduced by a
simulated spectrum with hfc values of a(3H) = 0.88, 5.31, and
6.08 G and superhyperfine splitting due to one nitrogen atom
and three equivalent protons [a(N) = 1.35 G and a(3H) =
We report herein the first successful EPR detection of a
hydrogen-bonded complex between a semiquinone radical
+
[15]
anion and protonated histidine (His·2H )
using 1-(p-
tolylsulfinyl)-2,5-benzoquinone (TolSQ). Effects of hydrogen
bonding between TolSQCꢀ and His·2H on the one-electron
reduction potential of TolSQ and the rate of ET reduction
were examined to reveal how the ET rate is controlled by
hydrogen bonding.
+
+
2.97 G] of His·2H (Figure 1b). The complete agreement of
the observed EPR spectrum (Figure 1a) with the simulated
spectrum (Figure 1b) clearly indicates formation of the
ꢀ
+
TolSQC /His·2H complex. The g value (2.0026) and the hfc
ꢀ
+
values [a(3H) = 0.88, 5.31, and 6.08 G] of TolSQC /His·2H
are drastically changed from those of TolSQC in the absence
ꢀ
[
*] Dr. J. Yuasa, S. Yamada, Prof. Dr. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering
+
of His·2H , which are g = 2.0057 and a(3H) = 2.00, 2.20, and
[
18,19]
3
.35 G.
Osaka University and SORST (JST)
Suita, Osaka 565-0871 (Japan)
Fax: (+81)6-6879-7370
ꢀ
+
The optimized structure of TolSQC /His·2H was obtained
by density functional calculations with the BLYP/6-31G**
basis set (see experimental details in the Supporting Infor-
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[
20]
mation).
Hydrogen bonds are found between the C=O
[
**] This workwas partially supported by Grants-in-Aid (No. 16205020)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
ꢀ
oxygen atom of TolSQC and the COOH proton as well as the
NH3 protons of His·2H , and also between the S=O oxygen
atom of TolSQC and the NH proton of the imidazole ring of
His·2H (Figure 1c; for hydrogen-bond lengths, see the
+
+
ꢀ
+
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
+
Angew. Chem. Int. Ed. 2007, 46, 3553 –3555
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3553

Communications
ꢀ
+
ꢀ2
ꢀ3
Figure 1. a) EPR spectrum of TolSQC /His·2H produced by photoinduced ET from (AcrH) (1.6”10 m) to TolSQ (4.0”10 m) in the presence
2
ꢀ3
ꢀ3
ꢀ
+
of His (4.0”10 m) and HClO (8.0”10 m) in deaerated MeCN at 298 K. b) Spectrum simulated with the hfc values of TolSQC /His·2H .
DHmsl(maximum slope linewidth)=30 G. c) Optimized structure of TolSQC /His·2H calculated by DFT at the BLYP/6-31G** level (calculated hfc
4
ꢀ
+
values are given in parentheses).
[
21]
Supporting Information).
Such
bonding
multiple
hydrogen
ꢀ
+
between TolSQC and His·2H may
ꢀ
+
stabilize TolSQC /His·2H . The hfc
values calculated with the opti-
mized structure are given in paren-
theses in Figure 1c. The superhy-
perfine coupling due to the hydro-
+
gen-bonded NH3
proton of
+
His·2H is estimated as 6.61 G.
The average of the calculated hfc
values (2.20 G) due to the three
+
NH3 protons agrees with the
[
22]
observed value.
This indicates
ꢀ
2
+
Figure 2. a) Cyclic voltammogram of TolSQ (1.0”10 m) in the absence of His·2H . b) Cyclic
that the hydrogen-bonded proton
is rapidly exchanged among the
NH3 protons on the EPR time-
ꢀ
3
voltammogram and c) second-harmonic alternating-current voltammogram of TolSQ (5.0”10 m)
in the presence of His·2H (5.0”10 m) in deaerated MeCN containing tetrabutylammonium
perchlorate (TBAP, 0.10m) with a Pt working electrode at 298 K. d) Dependence of k on [His·2H ]
+
ꢀ2
+
+
et
scale. The existence of superhyper-
fine coupling due to the hydrogen-
bonded protons and nitrogen atoms
ꢀ4
+
for ET from Me Fc (1.0”10 m) to TolSQ in the presence of His·2H in deaerated MeCN at 298 K.
2
+
of NH3 (Figure 1) strongly sup-
+
ports the presence of a strong hydrogen bond.
between neutral TolSQ and His·2H . In such a case, the
positive shift in the one-electron reduction potential of TolSQ
in the presence of His·2H is expressed by Equation (2),
When TolSQ is replaced by p-benzoquinone (Q), only the
protonated species (semiquinone radical QHC) is detected by
+
EPR in photoinduced ET from (AcrH) to Q in the presence
of His·2H (see the Supporting Information). Thus, the S=O
2
+
0
red
þ
Ered ¼ E þ ð2:3 R T=FÞlog fKred½His Á 2 H Šg
ð2Þ
oxygen atom in TolSQ plays a crucial role in multiple
ꢀ
+
0
red
hydrogen bonding between TolSQC and His·2H .
where E is the reduction potential of TolSQ in the absence
ꢀ
+
ꢀ
The strong hydrogen bonding between TolSQC and
of His·2H , and K the formation constant of the TolSQC /
red
+
+
His·2H is expected to increase the electron-acceptor ability
His·2H complex. The Kred value was determined as 4.2 ”
[
11]
10 ꢀ1
of TolSQ. The positive shift of the one-electron reduction
10 m from the E value of TolSQ in the presence of 5.0 ”
red
+
ꢀ2
+
ꢀ
potential E of TolSQ in the presence of His·2H (5.0 ”
10 m of His·2H . Such a large K value of the TolSQC /
red
red
ꢀ2
+
1
0
m) was verified by electrochemical measurements. The
His·2H complex clearly indicates strong hydrogen bonding
ꢀ
+
cyclic voltammogram of TolSQ exhibits a reversible redox
wave (Figure 2a). In contrast, the cyclic voltammogram of
between TolSQC and His·2H .
The positive shift of the Ered value of TolSQ should
enhance ET from an electron donor to TolSQ.
confirmed by ET from 1,1’-dimethylferrocene (Me Fc) to
TolSQ in the presence of His·2H . No ET from Me Fc (E =
+
[12,13]
TolSQ in the presence of His·2H exhibits an irreversible
This was
ꢀ
+
cathodic wave due to the instability of the TolSQC /His·2H
2
+
complex (Figure 2b). Thus, the Ered value of TolSQ in the
2
ox
+
presence of His·2H was determined by second-harmonic
0.26 V vs SCE) to TolSQ (Ered = ꢀ0.26 V vs SCE) occurs,
alternating-current voltammetry (Figure 2c). The Ered value
of TolSQ [ꢀ0.26 V vs saturated calomel electrode (SCE)] is
because the free-energy change of ET is highly endergonic
ꢀ2
+
(DG = 0.52 eV). In the presence of 5.0 ” 10 m of His·2H ,
however, efficient ET from Me Fc to TolSQ occurs to yield
Me Fc [Eq. (3)], as expected from the negative free-energy
change of electron transfer (DG = ꢀ0.03 eV).
et
ꢀ
2
shifted to 0.29 V versus SCE in the presence of 5.0 ” 10 m of
2
+
+
His·2H . In contrast to the strong hydrogen bonding between
2
ꢀ
+
TolSQC and His·2H , there is virtually no interaction
et
3
554
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Angew. Chem. Int. Ed. 2007, 46, 3553 –3555

Angewandte
Chemie
Me Fc þ TolSQ þ His Á 2 H ƒ!Me Fc þ TolSQ =His Á 2 Hþ
þ
ket
þ
Cꢀ
ð3Þ
2
2
[9] K. Morokuma, Acc. Chem. Res. 1977, 10, 294 – 300.
10] H. Ishikita, G. Morra, E.-W. Knapp, Biochemistry 2003, 42,
882 – 3892.
[11] For effects of hydrogen bonding on Ered of quinones, see: a) K.
Okamoto, K. Ohkubo, K. M. Kadish, S. Fukuzumi, J. Phys.
Chem. A 2004, 108, 10405 – 10413; b) Y. Ge, R. R. Lilienthal,
D. K. Smith, J. Am. Chem. Soc. 1996, 118, 3976 – 3977; c) N. A.
Macias-Ruvalcaba, N. Okumura, D. H. Evans, J. Phys. Chem. B
2006, 110, 22043 – 22047; d) N. Gupta, H. Linschitz, J. Am.
Chem. Soc. 1997, 119, 6384 – 6391.
[12] For effects of hydrogen bonding on ET reduction of quinones,
see: a) S. Fukuzumi, H. Kitaguchi, T. Suenobu, S. Ogo, Chem.
Commun. 2002, 1984 – 1985; b) S. Fukuzumi, K. Okamoto, Y.
Yoshida, H. Imahori, Y. Araki, O. Ito, J. Am. Chem. Soc. 2003,
125, 1007 – 1013.
[
3
The ET rates obeyed pseudo-first-order kinetics in the
presence of a large excess TolSQ and His·2H relative to the
concentration of Me Fc (see the first-order plot in the
Supporting Information). The observed pseudo-first-order
rate constant kobs increases proportionally with increasing
+
2
TolSQ concentration. The second-order rate constant k also
increases linearly with [His·2H ] (Figure 2d).
et
+
+
Since His·2H has no effect on the oxidation potential of
Me Fc, the free-energy change of ET from Me Fc to TolSQ in
2
2
+
the presence of His·2H (DG ) can be expressed by
et
0
Equation (4), where DG is the free-energy change in the
et
[
13] For effects of hydrogen bonding on proton-coupled electron
transfer (PCET), see: a) I. J. Rhile, T. F. Markle, H. Nagao, A. G.
DiPasquale, O. P. Lam, M. A. Lockwood, K. Rotter, J. M.
Mayer, J. Am. Chem. Soc. 2006, 128, 6075 – 6088; b) I. J. Rhile,
J. M. Mayer, J. Am. Chem. Soc. 2004, 126, 12718 – 12719; c) A.
Niemz, V. M. Rotello, Acc. Chem. Res. 1999, 32, 44 – 52; d) M.
Gray, A. O. Cuello, G. Cooke, V. M. Rotello, J. Am. Chem. Soc.
2003, 125, 7882 – 7888.
0
et
þ
DGet ¼ DG ꢀð2:3 R T=FÞlog fK ½His Á 2 H Šg
ð4Þ
red
+
absence of His·2H . Such a change in DGet has previously
been reported for metal-ion-promoted ET from Fc to the
naphthoquinone (NQ) moiety of a ferrocene–naphthoqui-
[
23]
none (Fc–NQ) linked dyad.
metal-ion-promoted ET on driving force is well evaluated in
The dependence of ket of
[14] For fast proton transfer from proton donor to semiquinone
radical anions, see: C. G. Schaefer, K. S. Peters, J. Am. Chem.
Soc. 1980, 102, 7566 – 7567.
[
24]
terms of the Marcus theory of electron transfer when the ket
value increases linearly with increasing metal ion concen-
[
23]
+
[15] It has been suggested that His-H126 and His-H128 near Q
B
tration. In the case of His·2H too, the k value increases
et
facilitate proton transfer into the RC: a) M. L. Paddock, M. S.
Graige, G. Feher, M. Y. Okamura, Proc. Natl. Acad. Sci. USA
+
linearly with [His·2H ] (Figure 2d).
In summary, we have detected a hydrogen-bonded com-
plex of a semiquinone radical anion with protonated histidine
1
999, 96, 6183 – 6188; b) P. ꢁdelroth, M. L. Paddock, A. Tehrani,
J. T. Beatty, G. Feher, M. Y. Okamura, Biochemistry 2001, 40,
14538 – 14546.
ꢀ
+
(
TolSQC /His·2H ) by EPR, which reveals strong hydrogen
ꢀ
+
[16] Although His is difficult to dissolve in MeCN, it becomes soluble
in MeCN in the presence of 2 equiv of HClO4.
17] a) S. Fukuzumi, T. Kitano, K. Mochida, J. Am. Chem. Soc. 1990,
bonding between TolSQC and His·2H . This finding provides
valuable insight into the specific function of quinones in the
photosynthetic RC. Strong hydrogen bonding between semi-
quinone radical anion and protonated amino acid residues
would result in a positive shift in the one-electron reduction
potential of quinones and facilitate the ET reduction of
quinones in the RC.
[
1
9
12, 3246 – 3247; b) S. Fukuzumi, Y. Tokuda, J. Phys. Chem. 1992,
6, 8409 – 8413.
[18] J. Yuasa, S. Yamada, S. Fukuzumi, J. Am. Chem. Soc. 2006, 128,
14938 – 14948.
ꢀ
+
[
19] The smaller g value of TolSQC /His·2H compared to TolSQCꢀ
(2.0057) indicates that the spin density on oxygen nuclei in
ꢀ
TolSQC is reduced significantly due to strong hydrogen bonding
with His·2H .
Received: January 12, 2007
Published online: March 30, 2007
+
[
[
20] The hfc values of semiquinone radical (QHC) calculated by using
BLYP methods were in good agreement with experimental data:
M. Nonella, J. Phys. Chem. B 1997, 101, 1235 – 1246.
Keywords: amino acids · electron transfer · EPR spectroscopy ·
.
hydrogen bonds · quinones
+
21] The hydrogen-bonded proton of NH is not covalently bound to
3
TolSQCꢀ but electrostatically bound to TolSQC through the
ꢀ
+
hydrogen bond. The NꢀH bond length in NH (1.57 ꢂ) is longer
3
+
3
[
1] Functions of Quinones in Energy Conserving Systems (Ed.: B. I.
Trumpower), Academic Press, New York, 1986.
than that between the NH proton and the C=O oxygen atom of
TolSQCꢀ (1.05 ꢂ) in the optimized structure of TolSQC /His·2H
ꢀ
+
[
2] The Photosynthetic Bacterial Reaction Center—Structure and
Dynamics (Eds.: J. Breton, H. Vermeglio), Plenum, New York,
(see the Supporting Information). This indicates that binding of
+
3
the NꢀH bond in NH is significantly weakened by formation of
ꢀ
1988.
strong hydrogen bonds with TolSQC . The presence of the strong
[
[
3] G. Feher, R. A. Isaacson, M. Y. Okamura, W. Lubitzin Antennas
and Reaction Centers of Photosynthetic Bacteria (Ed.: M. E.
Michel-Beyerle), Springer, Berlin, 1985, pp. 174 – 189.
4] P. J. OꢀMalley, T. K. Chandrashekar, G. T. Babcock in Antennas
and Reaction Centers of Photosynthetic Bacteria (Ed.: M. E.
Michel-Beyerle), Springer, Berlin, 1985, pp. 339 – 344.
hydrogen bond is supported by the existence of superhyperfine
coupling due to the hydrogen-bonded protons and nitrogen atom
+
of NH3 (Figure 1).
ꢀ
+
[22] The calculated hfc values of TolSQC /His·2H agree well with the
observed hfc values within the errors due to the imperfect
density functional method (BLYP/6-31G**).
[
[
5] E. Takahashi, C. A. Wraight, Biochemistry 1992, 31, 855 – 866.
6] M. L. Paddock, S. H. Rongey, G. Feher, M. Y. Okamura, Proc.
Natl. Acad. Sci. USA 1989, 86, 6602 – 6606.
7] P. ꢁdelroth, M. L. Paddock, L. B. Sagle, G. Feher, M. Y.
Okamura, Proc. Natl. Acad. Sci. USA 2000, 97, 13086 – 13091.
8] a) P. J. OꢀMalley, J. Am. Chem. Soc. 1998, 120, 5093 – 5097;
b) P. J. OꢀMalley, J. Phys. Chem. A 1998, 102, 248 – 253.
[23] K. Okamoto, H. Imahori, S. Fukuzumi, J. Am. Chem. Soc. 2003,
125, 7014 – 7021.
[24] R. A. Marcus, Angew. Chem. 1993, 105, 1161 – 1172; Angew.
Chem. Int. Ed. Engl. 1993, 32, 1111 – 1121.
[
[
Angew. Chem. Int. Ed. 2007, 46, 3553 –3555
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3555