Stereoselective photoinduced electron-transfer reaction between zinc myoglobin and new chiral quinolinium ions

Article abstract of DOI:10.1246/bcsj.76.561

New chiral quinolinium derivatives, 1-[(S)- or (R)-(1-phenylethyl)carbamoylmethyl]-6-methoxyquinolinium hexafluorophosphate ([PMQ]PF₆, 2) and 1-[(S)- or (R)-(1-phenylethyl)carbamoylmethyl]quinolinium hexafluorophosphate ([PQ]PF₆, 3), were synthesized and characterized. A cyclic voltammetry in MeCN shows an irreversible redox behavior at -0.88 V and -0.85 V vs SCE (saturated calomel electrode) for 2 and 3, respectively. Fluorescence from the quinolinium moiety of 2 and 3 was observed at 455 nm and 440 nm, respectively. The fluorescence lifetime of 2 was longer than that of 3: τ_f = 30 ns (2) and 20 ns (3) in MeCN and τ_f = 26 ns (2) and 16 ns (3) in H₂O). The excited triplet state of zinc-substituted myoglobin (³(ZnMb)*) was quenched by chiral [PMQ]⁺ and [PQ]⁺ ions; thereafter, the back electron-transfer (ET) reaction from a quinoline radical (PMQ^. or PQ^. to a zinc myoglobin radical cation was detected. The stereoselectivity was observed for both ET quenching and back ET reactions; the (S)-isomers preferentially quench ³(ZnMb)* (k_q(S)/k_q(R) = 1.3 and 1.4 at 25°C for [PMQ]⁺ and [PQ]⁺, respectively); in contrast, the (R)-isomers react faster than the (S)-isomers in the back ET reaction (k_b(R)/k_b(S) = 1.3 and 1.4 for PMQ^. and PQ^., respectively). From a comparison of the rate constants with those for the previously reported systems we suggest that both the quenching and back reactions are controlled by ET, but not by conformational gating.

Full text of DOI:10.1246/bcsj.76.561

c. Jpn.
© 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 561–566 (2003)
561
e Chemical
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Stereoselective Photoinduced Electron-Transfer Reaction between Zinc
Myoglobin and New Chiral Quinolinium Ions
Stereoselective ET of ZnMb with Quinolinium
K. Tsukahara et al.
Keiichi Tsukahara* and Rie Ueda
03
1
6
Department of Chemistry, Faculty of Science, Nara Women’s University, Nara 630-8506
(Received October 4, 2002)
SJA8
09-2673
295
New chiral quinolinium derivatives, 1-[(S)- or (R)-(1-phenylethyl)carbamoylmethyl]-6-methoxyquinolinium
hexafluorophosphate ([PMQ]PF6, 2) and 1-[(S)- or (R)-(1-phenylethyl)carbamoylmethyl]quinolinium hexafluorophos-
phate ([PQ]PF6, 3), were synthesized and characterized. A cyclic voltammetry in MeCN shows an irreversible redox be-
havior at −0.88 V and −0.85 V vs SCE (saturated calomel electrode) for 2 and 3, respectively. Fluorescence from the
quinolinium moiety of 2 and 3 was observed at 455 nm and 440 nm, respectively. The fluorescence lifetime of 2 was
longer than that of 3: τf = 30 ns (2) and 20 ns (3) in MeCN and τf = 26 ns (2) and 16 ns (3) in H2O). The excited triplet
state of zinc-substituted myoglobin (3(ZnMb)*) was quenched by chiral [PMQ]+ and [PQ]+ ions; thereafter, the back
electron-transfer (ET) reaction from a quinoline radical (PMQ· or PQ·) to a zinc myoglobin radical cation was detected.
The stereoselectivity was observed for both ET quenching and back ET reactions; the (S)-isomers preferentially quench
3(ZnMb)* (kq(S)/kq(R) = 1.3 and 1.4 at 25 °C for [PMQ]+ and [PQ]+, respectively); in contrast, the (R)-isomers react
faster than the (S)-isomers in the back ET reaction (kb(R)/kb(S) = 1.3 and 1.4 for PMQ· and PQ·, respectively). From a
comparison of the rate constants with those for the previously reported systems we suggest that both the quenching and
back reactions are controlled by ET, but not by conformational gating.
02
02
.1
Specific recognition and binding between a biomacromole-
cule, such as protein and enzyme, and a small molecule or ion
must play an important role in biological functions. Especial-
ly, stereoselectivity in the electron-transfer (ET) reactions be-
tween metalloproteins and chiral materials is one of the impor-
tant problems to elucidate the mechanisms of redox catalysis
Chart 1. Structure.
in biological systems. Although a number of studies on stereo-
selective ET reactions between metalloproteins and chiral met-
al complexes have been reported,1–6 there are no reports on the
stereoselectivity in the ET reactions of metalloproteins with
chiral organic redox reagents, except for ours on chiral violo-
gens.7–10 One of the reasons is a lack of systematic synthesis
of such chiral materials.
Quinoline is a basic unit of an antimalarial agent, such as
quinoline alkaloid found in citrus fruits. N-Substituted quino-
linium compounds are useful agents to detect intracellular Cl−
ions by using fluorescence quenching of the quinolinium moi-
ety through an electron-transfer (ET) mechanism.11 Zwitter
ionic [1-(6-methoxy)quinolinio]propyl-3-sulfonate (SPQ)12–16
and halide salts, 1-ethyl-6-methoxyquinolinium iodide
([MEQ]I)17,18 and 1-(ethoxycarbonylmethyl)-6-methoxyquino-
linium bromide ([MQAE]Br)18,19 have been widely used to
measure intracellular Cl− activity. Studies on 1-methylquino-
linium perchlorate ([MQ]ClO4) and 1-methyl-6-methoxyquin-
olinium perchlorate ([MMQ]ClO4) have also been report-
ed.11,20–24 However, because few examples of biologically im-
portant chiral quinolinium ions have been reported, it is neces-
sary to develop artificial chiral agents.
It has been suggested that a conformational gating controls the
intermolecular ET reactions of ZnMb, zinc hemoglobin, and
the other metalloproteins.25–35 Is there any ET-controlled
quenching reaction for ZnMb other than intramolecular ET
reactions? If the driving force of the reaction becomes small
and, therefore, the ET rate decreases, it might be possible that
the reaction is not controlled by a conformational change, but
by ET. We have chosen quinolinium compounds to find the
ET-controlled photoinduced reaction of ZnMb, because the
redox potential of the quinolinium ions is much lower than that
of viologens and close to that of the couple of the radical
cation (ZnMb·+) with the excited triplet state of ZnMb
(3(ZnMb)*).
Experimental
Materials. Horse heart metmyoglobin (metMb, Sigma) was
purified as previously described.36,37 Zinc-substituted myoglobin
(ZnMb) was prepared at 4 °C in the dark by the same method as
reported previously.28,29,38,39 The concentrations of ZnMb were
determined spectrophotometrically (ε428 = 1.53 × 105 dm3 mol−1
cm−1).39 Chiral quinolinium compounds were synthesized by the
reaction of quinoline or 6-methoxyquinoline with chiral iodo[(1-
phenylethyl)carbamoyl]methane. Hexafluorophosphate salts of
In this work we report on new chiral quinolinium derivatives
(Chart 1) which show interesting luminescence properties and
stereoselectivity in both ET quenching and back ET reactions.

562 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
Stereoselective ET of ZnMb with Quinolinium
chiral quinolinium compounds were converted to chloride salts by
ion-exchange chromatography for kinetic and the other measure-
ments in aqueous solutions. All other chemicals used were of re-
agent grade. All of the aqueous solutions were prepared from re-
distilled water. The ionic strength (I) of the solution was adjusted
with NaCl.
fluorophosphate ((S)-[PQ]PF6, 3a) and 1-[(R)-(1-Phenyleth-
yl)carbamoylmethyl]quinolinium Hexafluorophosphate ((R)-
[PQ]PF6, 3b). Yield, 1.75 g (40.1%) for 3a, 1.65 g (37.8%) for
3b. 1H NMR (270 MHz, DMSO-d6, TMS) δ 1.45 (3H, d, J = 6.8
Hz, CH3), 4.88–4.94 (1H, m, CH), 5.88 (2H, s, CH2), 7.25–7.37
(5H, m, C6H5), 8.03 (1H, d, J = 9.8 Hz, 8-quinolinium), 8.06 (1H,
dd, J = 4.8, 9.8 Hz, 6-quinolinium), 8.23 (1H, dd, J = 5.8, 8.3 Hz,
3-quinolinium), 8.27 (1H, dd, J = 4.8, 9.8 Hz, 7-quinolinium),
8.49 (1H, d, J = 4.8 Hz, 5-quinolinium), 9.23 (1H, br, NH), 9.35
(1H, d, J = 8.3 Hz, 4-quinolinium), 9.48 (1H, d, J = 5.8 Hz, 2-
quinolinium). UV-vis (MeCN, λmax/nm (ε/dm3 mol−1 cm−1)) 237
(3.68 × 105), 317 (8.21 × 104). ORD (c 0.25 in MeCN, 20 °C)
[Φ]589 91° for 3a, −91° for 3b. IR (KBr, ν/cm−1) 847 (PF), 1532
(CwC), 1699 (CwO), 3422 (NH). Found: C, 52.39; H, 4.33; N,
6.41% (3a), C, 52.19; H, 4.23; N, 6.40% (3b). Calcd for
C19H19N2OPF6: C, 52.30; H, 4.39; N, 6.42%.
Synthesis of Iodo[(S)-(1-phenylethyl)carbamoyl]methane
((S)-PI, 1a) and Iodo[(R)-(1-phenylethyl)carbamoyl]methane
((R)-PI, 1b). To a stirred solution of iodoacetic acid (11.2 g,
0.0602 mol) in dichloromethane (DCM, 60 cm3) was slowly add-
ed N,N-dicyclohexylcarbodiimide (12.4 g, 0.0601 mol) in DCM
(40 cm3) at −5 °C. After the mixture was stirred for 30 min,
chiral 1-phenylethylamine (7.21 g, 0.0600 mol) was added drop-
wise. Stirring was continued for a further 90 min, after which the
mixture was warmed to room temperature. After the mixture was
filtered, 60 cm3 of ice water was added into the DCM solution.
The organic layer was washed successively with 2 mol dm−3 HCl,
saturated NaHCO3, and saturated NaCl aqueous solutions (100
cm3 each). After the DCM solution was dried over anhydrous
Na2SO4 overnight, removal of solvent gave a pale-yellow pow-
der. Recrystallization from MeOH yielded pale-yellow crystals.
Yield, 6.91 g (39.8%) for 1a, 6.32 g (36.4%) for 1b. 1H NMR
(270 MHz, DMSO-d6, TMS) δ 1.51 (3H, d, J = 6.8 Hz, CH3),
3.70 (2H, s, CH2), 5.04–5.14 (1H, m, CH), 6.28 (1H, br, NH),
7.27–7.40 (5H, m, C6H5). IR (KBr, ν/cm−1) 1583 (CwC), 1658
(CwO), 3330 (NH). Found: C, 42.01; H, 4.32; N, 4.93% (1a), C,
42.02; H, 4.27; N, 4.90% (1b). Calcd for C10H12NOI: C, 41.54; H,
4.19; N, 4.85%.
Synthesis of (S)- and (R)-Isomers of Chiral Quinolinium
Compounds. An iodo derivative, 1a or 1b (5.78 g, 0.0200 mol),
in N,N-dimethylformamide (DMF, 6 cm3) was heated at 85 °C
under N2, to which quinoline or 6-methoxyquinoline (0.010 mol)
in DMF (10 cm3) was slowly added over a period of 2 h. After the
solution was further heated for 24 h, the solvent was removed by a
rotary evaporator. The residue was dissolved in MeOH (20 cm3)
and passed through a Dowex 1-X8 (a Cl− form) column to convert
to chloride salts. After the solvent was removed, the residue was
dissolved in water (30 cm3), and then washed with the same vol-
ume of DCM several times. The aqueous phase was evaporated to
dryness and the residue was redissolved in a small amount of wa-
ter. The addition of a saturated aqueous solution of NaPF6 gave a
white powder. Recrystallization from MeOH gave white crystals.
1-[(S)-(1-Phenylethyl)carbamoylmethyl]-6-methoxyqui-
nolinium Hexafluorophosphate ((S)-[PMQ]PF6, 2a) and 1-
[(R)-(1-Phenylethyl)carbamoylmethyl]-6-methoxyquinoli-
nium Hexafluorophosphate ((R)-[PMQ]PF6, 2b). Yield, 1.82
g (39.0%) for 2a, 1.90 g (40.7%) for 2b. 1H NMR (270 MHz,
DMSO-d6, TMS) δ 1.43 (3H, d, J = 6.8 Hz, CH3), 3.99 (3H, s,
OCH3), 4.87–4.93 (1H, m, CH), 5.82 (2H, s, CH2), 7.20–7.39 (5H,
m, C6H5), 7.88 (1H, d, J = 2.4 Hz, 5-quinolinium), 7.90 (1H, dd, J
= 2.4, 9.8 Hz, 7-quinolinium), 8.15 (1H, d, J = 9.8 Hz, 8-quino-
linium), 8.18 (1H, dd, J = 5.8, 8.3 Hz, 3-quinolinium), 9.13 (1H,
br, NH), 9.16 (1H, d, J = 8.3 Hz, 4-quinolinium), 9.28 (1H, d, J =
5.8 Hz, 2-quinolinium). UV-vis (MeCN, λmax/nm (ε/dm3 mol−1
cm−1)) 253 (3.24 × 105), 318 (1.02 × 105), 355 (6.54 × 104).
ORD (c 0.25 in MeCN, 20 °C) [Φ]589 89° for 2a, −88° for 2b. IR
(KBr, ν/cm−1) 829 (PF), 1534 (CwC), 1686 (CwO), 3417 (NH).
Found: C, 51.51; H, 4.55; N, 5.98% (2a), C, 52.02; H, 4.57; N,
6.00% (2b). Calcd for C20H21N2O2PF6: C, 51.51; H, 4.54; N,
6.01%.
Kinetic Measurements. A ZnMb solution was gently purged
with Ar gas and then carefully degassed by freeze-pump-thaw cy-
cles. Single-flash photolysis was performed in degassed solutions
containing ZnMb (3.00 × 10−6 mol dm−3) and chiral quinolinium
ions (0–2.00 × 10−3 mol dm−3) at 25.0 °C, pH 7.0 (0.010 mol
dm−3 sodium phosphate buffer), and I = 0.020 mol dm−3 using a
Photal RA-412 pulse flash apparatus with a 30 µs pulse-width Xe
lamp (λ > 450 nm; a Toshiba Y-47 glass filter). Absorption spec-
tral changes during the reaction were monitored at 460 nm for the
3
decay of (ZnMb)* and 680 nm for the formation and decay of
ZnMb·+
.
Other Measurements. 1H NMR spectra were recorded on a
JEOL JNM-GX270 FT NMR spectrometer (270 MHz). UV-vis
spectra were measured with Shimadzu UV-240 and MultiSpec-
1500 spectrophotometers. The fluorescence spectra and lifetimes
were measured in Ar saturated solutions with a Shimadzu RF-
5300PC spectrofluorometer and a Horiba NAES-500 nano-second
fluorometer, respectively. The pHs of the solutions were mea-
sured on a Hitachi-Horiba F-14RS pH meter. Cyclic voltammetry
was performed in an N2-saturated 0.050 mol dm−3 tetrabuthylam-
monium perchlorate ([Bu4N]ClO4) MeCN solution and 0.050 mol
dm−3 KCl aqueous solutions with a Yanaco Model P-900 instru-
ment. A three-electrode system (BAS Inc.) was used with a Pt
auxiliary electrode and a glassy carbon working electrode against
Ag/AgClO4 (0.10 mol dm−3 [Bu4N]ClO4 in MeCN) and Ag/AgCl
(3.33 mol dm−3 KCl in water) reference electrodes. The poten-
tials were calibrated by using 1,1ꢀ-dimethyl-4,4ꢀ-bipyridinium per-
chlorate ([MV](ClO4)2 (E0 = −0.45 V vs SCE (saturated calomel
electrode)) in MeCN and its chloride (E0 = −0.45 V vs NHE
(normal hydrogen electrode)) in water.
Results and Discussion
Characterizations of Chiral Quinolinium Compounds.
New chiral quinolinium compounds (2 and 3) were character-
1
ized by elemental analyses and UV-vis, H NMR, and IR
spectroscopies. The redox and photophysical properties are
summarized in Table 1 along with the previously reported
compounds. Cyclic voltammograms of 2 and 3 showed irre-
versible reduction waves with relatively small oxidation peak
currents. Therefore, the redox potentials were estimated from
the mean values between the reduction and oxidation peak po-
tentials: −0.88 V and −0.85 V vs SCE for 2 and 3, respective-
ly. The phenylethylcarbamoylmethyl group shifted the redox
potential slightly positive compared with that of the N-alkyl-
1-[(S)-(1-Phenylethyl)carbamoylmethyl]quinolinium Hexa-

K. Tsukahara et al.
Bull. Chem. Soc. Jpn., 76, No. 3 (2003) 563
Table 1. Redox and Photophysical Properties of Quinolinium Compounds in Acetonitrile
and Aqueous Solutions
Quinolinium
Ea)/V
τf/ns
Ref.
PMQ+ (2)
PQ+ (3)
MMQ+
MQ+
−0.88b)
−0.88c)
−0.85c)
−0.82c)
−0.96b)
30b)
20b)
26c)
16c)
26c)
15c)
This work
This work
Ref. 11
Refs. 21 and 22
This work
−0.85b)
−0.90b)
−0.89b)
20b)
25b)
PrMQ+
a) The potentials are against SCE and NHE in MeCN and aqueous solutions, respec-
tively. b) In MeCN. c) In an aqueous solution.
quinolinium compounds ([MMQ]+, [MQ]+, and 1-(3-propyl)-
6-methoxyquinolinium ([PrMQ]+) ions). The fluorescence
from the quinolinium moiety of 2 and 3 was observed at 455
nm and 440 nm, respectively. The fluorescence lifetime of 2
(30 ns) is longer than that of 3 (20 ns), which is in good agree-
ment with the fact that the fluorescence intensity of 2 is larger
than that of 3. The fluorescence lifetimes in MeCN are longer
than in water (26 ns and 16 ns for 2 and 3, respectively). These
results are the same tendency as those for the N-alkylquinolini-
um ions.
Photoinduced ET Reaction of 3(ZnMb)* with Chiral
Quinolinium Ions. The excited singlet state of ZnMb was
not quenched by the quinolinium ions (2 and 3) under the
present experimental conditions. In contrast, the excited triplet
3
state, (ZnMb)*, was quenched by 2 and 3. Figure 1a shows
the absorption spectral changes after the irradiation by light of
a degassed solution containing ZnMb and 3 at 25 °C, pH 7.0
(0.010 mol dm−3 phosphate buffer), and I = 0.020 mol dm−3.
3
The decay of (ZnMb)* in the presence of 2 or 3 was of first
order, and was faster than the spontaneous decay of 3(ZnMb)*.
Plots of the first-order rate constant of the quenching of
3(ZnMb)*, kobsd, vs the concentrations of 2 and 3 were linear
(see Fig. 2), indicating no appreciable complex formation be-
tween 3(ZnMb)* and the quinolinium ions. When the reaction
was monitored at 680 nm, we observed the formation and de-
cay of the radical cation of ZnMb·+ (Fig. 1b),26,27,29,30,38 indi-
cating ET quenching followed by a thermal back ET reaction.
Therefore, the photoinduced ET reaction of ZnMb with the
quinolinium ions is represented in Scheme 1.
Fig. 1. Absorption spectral changes after irradiation by light
of a degassed solution containing ZnMb (3.00 × 10−6 mol
dm−3) and 3 ([PQ]+) at 25 °C, pH 7.0 (0.010 mol dm−3
phosphate buffer), and I = 0.020 mol dm−3. (a) Decay of
3(ZnMb)* at 460 nm in the presence of 1.00 × 10−3 mol
dm−3 3a ((S)-[PQ]+) and 3b ((R)-[PQ]+). Dotted lines are
fitted to the first-order kinetics. (b) Decay of ZnMb·+ at
680 nm in the presence of 2.00 × 10−3 mol dm−3 3a ((S)-
[PQ]+) and 3b ((R)-[PQ]+). Dotted lines are fitted to the
second-order kinetics.
The quenching rate constants (kq) obtained from the slope of
the plots of kobsd vs the concentrations of the quinolinium ions
are listed in Table 2. The ET quenching reactions of 3(ZnMb)*
by 2 and 3 are slightly endothermic: ∆G0 = 0.08 eV and 0.05
eV for 2 and 3, respectively, based on the redox potentials of
the couples, [PMQ]+/PMQ· (−0.88 V vs NHE), [PQ]+/PQ·
(−0.85 V), and ZnMb·+/3(ZnMb)* (−0.80 V).40 Under the
present experimental conditions, in which the quinolinium ions
3
are used in large excess over (ZnMb)*, the latter of which
concentration is 1.8 × 10−6 mol dm−3, the ET quenching was
almost completed: > 93% for 2 and > 98% for 3. The thermal
back ET reaction was much slower than the quenching reac-
Here, A0, At, and A are the absorbances at time 0, t, and infin-
∞
ity, respectively, and [ZnMb·+]0 is the initial concentration of
ZnMb·+. The value of kb was determined by using the value of
[ZnMb·+]0 which was estimated from the concentration of
3(ZnMb)* (∆ε428 = ε(ground) − ε(triplet) = 1.00 × 105 dm3
mol−1 cm−1).29,41 The values of kb are listed in Table 2.
3
tion of (ZnMb)*, and the second-order rate constant of the
back ET reaction (kb) was evaluated at the latter portion of the
decay of ZnMb·+ at 680 nm after the quenching reaction was
completed (see Fig. 1b), based on
Mechanisms of ET Quenching and Back ET Reactions.
It has been suggested that a conformational gating controls the
intermolecular ET reactions of ZnMb, zinc hemoglobin, and
At = (A0 + kb[ZnMb·+]0A t)/(1 + kb[ZnMb·+]0t). (1)
∞

564 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
Stereoselective ET of ZnMb with Quinolinium
Table 2. Rate Constants of the Quenching and Back ET Reactions of ZnMb with Chiral Quinolinium
and Viologen at 25 °C, pH 7.0, and I = 0.020 mol dm−3
kq/105 dm3 mol−1 s−1
a)
kb /108 dm3 mol−1 s−1
Quencher
PMQ+ (2)
PQ+ (3)
(S)-isomer (R)-isomer kq(S)/kq(R)
(S)-isomer
1.0 0.1
0.38 0.03 0.53 0.04 0.72
1.4 0.1 0.98 0.08 1.4
(R)-isomer
1.3 0.1
kb(S)/kb(R)
0.77
4.5 0.3
2.1 0.2
390 10
3.4 0.2
1.5 0.1
290 10
1.3
1.4
1.3
OAV2+ b)
a) Determined at four different concentrations of 2 and 3 ((1.25–2.00) × 10−3 mol dm−3).
b) Ref. 8.
between ZnMb·+ and the quinoline radicals by using the
Marcus theory (Eq. 2);42,43 using
k12 = (k11k22f12K12)1/2.
(2)
Here, k12 is the rate constant for the cross reaction, k11 and k22
are those for the self-exchange reactions of donor and accep-
tor, respectively, K12 is the equilibrium constant for the cross
reaction, and f12 is given by
ln f12 = (ln K12)2/4ln(k11k22/1022).
Equations 2 and 3 are also represented by
ln k12 = 25.3 − (λ12 + ∆G0)2/4λ12RT,
(3)
(4)
where λ12 is the reorganization energy for the reaction, and
equals the average of those of the donor and acceptor, (λ11
+
λ22)/2. By using the data k11 = 2.6 × 105 dm3 mol−1 s−1 for
ZnMb·+/3(ZnMb)* (λ11 = 1.32 eV),44 k22 = 1.0 × 108 dm3
mol−1 s−1 for both PMQ+/ PMQ· and PQ+/ PQ· couples (λ22
= 0.71 eV),45 and K12 = 4.4 × 10−2 for PMQ+ and 1.4 × 10−1
for PQ+ (the redox potential of ZnMb·+/ZnMb is 0.98 V),40
the calculated rate constants, k12, are 1.0 × 106 dm3 mol−1 s−1
(f12 = 0.88) and 1.9 × 106 dm3 mol−1 s−1 (f12 = 0.95) for
PMQ+ and PQ+, respectively. The calculated rate constants
are close to the observed ones (see Table 2 and Fig. 3 (symbols
ꢀ, ꢁ, ꢂ, and ꢃ)). Similarly we obtained the calculated rate
constants for the back ET reactions, k12 = 1.0 × 108 dm3
mol−1 s−1 (f12 = 1.2 × 10−29) and 1.7 × 108 dm3 mol−1 s−1 (f12
= 1.1 × 10−28) for PMQ· and PQ·, respectively, by using the
Fig. 2. Plots of kobsd vs [Quinolinium]0 for the quenching of
3(ZnMb)* by the chiral quinolinium ions at 25 °C, pH 7.0,
and I = 0.020 mol dm−3. (ꢀ, ꢁ) 2 ([PMQ]+) and (ꢂ, ꢃ)
3 ([PQ]+). Open and closed symbols are for (S)- and (R)-
isomers, respectively.
data k11 = 2.6 × 105 dm3 mol−1 s−1 for ZnMb·+/ZnMb (λ11
=
1.32 eV) and K12 = 3.2 × 1031 for PMQ· and 9.9 × 1030 for
PQ·. These calculated rate constants for the back ET reaction
are also close to those for the observed ones (see Table 2 and
Fig. 3).
Scheme 1.
On the other hand, the viologen system is different from that
for the quinolinium ions. The quenching rate constant for a
1,1ꢀ-bis(1-phenylethylcarbamoylmethyl)-4,4ꢀ-bipyridinium ion
(OAV2+) is calculated to be k12 = 1.9 × 109 dm3 mol−1 s−1 (f12
= 9.6 × 10−4), by using the data k22 = 1.0 × 108 dm3 mol−1
s−1 for OAV2+/OAV·+ (λ22 = 0.71 eV)46–49 and K12 = 1.5 ×
1010 (the redox potential of OAV2+ is −0.20 V).9 The calculat-
ed value is much larger than the experimentally determined
one ((2.9–3.9) × 107 dm3 mol−1 s−1),8 as is shown in Fig. 3
(symbols ꢄ and ꢅ). Moreover, the calculated rate constant for
the back ET reaction (k12 = 7.6 × 1010 dm3 mol−1 s−1) is also
much larger than the observed one (kb = (0.98–1.4) × 108 dm3
the other metalloproteins;25–35 the ET rate is insensitive to the
redox potentials of quenchers. In the case of the quinolinium
ions (2 and 3), the rate constants (kq) are much smaller than
those for the other quenchers, such as viologens (see Table 2).
This may arise partly because the driving force of the reaction
is slightly positive; namely the ET rate is dependent on the
driving force of the reaction, and is not fast enough to be con-
trolled by a conformational change of myoglobin. To confirm
this assumption we calculated the rate constants for the
3
quenching of (ZnMb)* by 2 and 3, and the back ET reaction

K. Tsukahara et al.
Bull. Chem. Soc. Jpn., 76, No. 3 (2003) 565
heme pocket, which is different from the site for the quenching
reaction.
This research was partly supported by a Grant-in-Aid for
Scientific Research on Priority Areas (417) from the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT). The authors thank Professor K. Kasuga and Dr M.
Handa, Shimane University, for elementary analyses, and Dr.
T. Nakazawa of Nara Women’s University for 2D NMR mea-
surements.
References
1
K. Bernauer and J.-J. Sauvain, J. Chem. Soc., Chem. Com-
mun., 1988, 353.
2
S. Sakaki, Y. Nishijima, H. Koga, and K. Ohkubo, Inorg.
Chem., 28, 4061 (1989).
3
R. A. Marusak, T. P. Shields, and A. G. Lappin, “Electron
Transfer in Biology and the Solid State,” Advances in Chemistry
Series 226, ed by M. K. Johnson, R. B. King, D. M. Kurtz, Jr., C.
Kutal, M. L. Norton, and R. A. Scott, American Chemical Society,
Washington D. C. (1990), p. 237.
Fig. 3. Driving force dependence of the rate constants for the
quenching and back ET reactions of ZnMb with quinolini-
um ions and viologens. (ꢀ) 2a, (ꢁ) 2b, (ꢂ) 3a, (ꢃ) 3b,
(ꢄ) (S,S)-OAV2+, (ꢅ) (R,R)-OAV2+, and (ꢆ) MV2+. The
solid curve is calculated by Eq. 4 with λ12 = 1.02 eV.
4
K. Bernauer, “Electron Transfer Reactions in Metallopro-
teins,” Metal Ions in Biological Systems 27, ed by H. Sigel and A.
Sigel, Dekker, New York (1991), p. 265.
5
J. R. Pladziewicz, M. A. Accola, P. Osvath, and A. M.
Sargeson, Inorg. Chem., 32, 2525 (1993).
K. Bernauer, P. Schürmann, C. Nusbaumer, L. Verardo,
and S. Ghizdavu, Pure Appl. Chem., 70, 985 (1998).
K. Tsukahara, C. Kimura, J. Kaneko, and T. Hara, Chem.
Lett., 1994, 2377.
6
mol−1 s−1) with K12 = 9.7 × 1019 and f12 = 2.3 × 10−12. For a
typical viologen, MV2+, the rate constants for the quenching
and back ET reactions were also calculated, and are much dif-
ferent from the observed ones (see the symbol ꢆ in Fig. 3).
From these results we can conclude that the present quinolini-
um system is controlled by an ET step and that the viologen
system is not controlled by an ET, as has been previously
reported.7,8,26–31
7
8
K. Tsukahara, C. Kimura, J. Kaneko, K. Abe, M. Matsui,
and T. Hara, Inorg. Chem., 36, 3520 (1997).
9
K. Tsukahara, J. Kaneko, T. Miyaji, K. Abe, M. Matsuoka,
T. Hara, T. Tanase, and S. Yano, Bull. Chem. Soc. Jpn., 72, 139
(1999).
10 K. Tsukahara, R. Ueda, and M. Goda, Bull. Chem. Soc.
Jpn., 74, 1303 (2001).
11 S. Jayaraman and A. S. Verkman, Biophys. Chem., 85, 49
(2000).
12 O. S. Wolfbeis and E. Urbano, J. Heterocyclic Chem., 19,
841 (1982).
13 O. S. Wolfbeis and E. Urbano, Fresenius Z. Anal. Chem.,
314, 577 (1983).
14 E. Urbano, H. Offenbacher, and O. S. Wolfbeis, Anal.
Chem., 58, 427 (1984).
15 N. P. Illsley and A. S. Verkman, Biochemistry, 26, 1215
(1987).
16 H.-R. Bae and A. S. Verkman, Nature, 348, 637 (1990).
17 J. Biwersi and A. S. Verkman, Biochemistry, 30, 7879
(1991).
18 A. S. Verkman and J. Biwersi, “Methods in Neuroscienc-
es,” ed by J. Kraicer and D. J. Dixon, Academic Press, New York
(1995), Vol. 27, p. 328.
19 A. S. Verkman, M. Sellers, A. C. Chao, T. Leung, and R.
Ketchum, Anal. Biochem., 178, 355 (1989).
20 U. C. Yoon, S. L. Quillen, P. S. Mariano, R. Swanson, J. L.
Stavinoha, and E. Bay, J. Am. Chem. Soc., 105, 1204 (1983).
21 T. M. Bockman and J. K. Kochi, J. Am. Chem. Soc., 111,
4669 (1989).
Stereoselectivity in ET Quenching and Back ET Reac-
tions. The kq values for the (S)-isomers of the chiral quino-
linium ions are larger than those for the (R)-isomers. The
ratios of kq(S)/kq(R) are 1.3 and 1.4 for 2 and 3, respectively.
3
Therefore, the (S)-isomers preferentially quench (ZnMb)*,
which is the same tendency as in the viologen system.8 A sim-
ilar selectivity of quinolinium to viologen may arise from a de-
crease in the charge repulsion and the number of chiral groups
for the former. The latter has a more positive charge and two
chiral moieties. We have suggested from the evidence for the
precursor complex formation in the thermal ET reaction of
myoglobin with chiral viologen radical cations that the amide
bond in the chiral viologen reacts with an amino or carboxylate
group of surface amino-acid residues (Lys, Asp, and/or Glu).10
In the present case, the attacking site by the quinolinium ion
might be Lys, Asp, and/or Glu residues near the heme pocket
of myoglobin.
On the other hand, interestingly, the selectivity in the back
ET reaction was inverted: the ratios of kb(S)/kb(R) are 0.77 and
0.72 for 2 and 3, respectively. The quinoline radical produced
by the ET quenching reaction becomes a neutral molecule;
therefore, the charge repulsion decreases. Then, the quinoline
radical easily attacks myoglobin, especially the hydrophobic
22 S. Fukuzumi, T. Kitano, K. Ishikawa, and T. Tanaka, Chem.

566 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
Stereoselective ET of ZnMb with Quinolinium
Lett., 1989, 1599.
23 S. Fukuzumi and S. Noura, J. Chem. Soc., Chem. Com-
mun., 1994, 287.
24 S. Fukuzumi, M. Fujita, S. Noura, K. Ohkubo, T. Suenobu,
Y. Araki, and O. Ito, J. Phys. Chem. A, 105, 1857 (2001).
25 G. McLendon, K. Pardue, and P. Bak, J. Am. Chem. Soc.,
109, 7540 (1987).
36 K. Tsukahara and Y. Yamamoto, J. Biochem. (Tokyo), 93,
15 (1983).
37 K. Tsukahara, Inorg. Chim. Acta, 124, 199 (1986).
38 H. Zemel and B. M. Hoffman, J. Am. Chem. Soc., 103,
1192 (1981).
39 A. C. Shosheva, P. K. Christova, and B. P. Atanasov, Bio-
chim. Biophys. Acta, 957, 202 (1988).
26 N. Barboy and J. Feitelson, Biochemistry, 28, 5450 (1989).
27 J. Feitelson and G. McLendon, Biochemistry, 30, 5051
(1991).
28 K. Tsukahara and M. Okada, Chem. Lett., 1992, 1543.
29 K. Tsukahara, S. Asami, M. Okada, and T. Sakurai, Bull.
Chem. Soc. Jpn., 67, 421 (1994).
30 K. Tsukahara, Y. Nishikawa, C. Kimura, N. Sawai, and T.
Sakurai, Bull. Chem. Soc. Jpn., 67, 2093 (1994).
31 K. Tsukahara, M. Okada, S. Asami, Y. Nishikawa, N.
Sawai, and T. Sakurai, Coord. Chem. Rev., 132, 223 (1994).
32 B. M. Hoffman and M. A. Ratner, J. Am. Chem. Soc., 109,
6237 (1987).
40 J. A. Cowan and H. B. Gray, Inorg. Chem., 28, 2074
(1989).
41 H. Zemel and B. M. Hoffman, J. Am. Chem. Soc., 103,
1192 (1981).
42 R. A. Marcus, Disscuss. Faraday Soc., 29, 21 (1960).
43 R. A. Marcus, J. Phys. Chem., 72, 891 (1968).
44 J. R. Winkler and H. B. Gray, Chem. Rev., 92, 369 (1992).
45 The self-exchange rate constant for the quinolinium ions is
assumed to be similar to that for viologens:46–49 k11 = 1.0 × 108
dm3 mol−1 s−1
.
46 K. Tsukahara and R. G. Wilkins, J. Am. Chem. Soc., 107,
2632 (1985).
33 B. S. Brunschwig and N. Sutin, J. Am. Chem. Soc., 111,
7454 (1989).
47 A. L. Rieger and P. H. Rieger, J. Phys. Chem., 88, 5845
(1984).
34 J. S. Zhou, and N. M. Kostić, J. Am. Chem. Soc., 115,
10796 (1993).
48 R. F. Anderson and K. B. Patel, J. Chem. Soc., Faraday
Trans. 1, 80, 2693 (1984).
35 V. L. Davidson, Biochemistry, 35, 14035 (1996).
49 K. Tsukahara, Trends Inorg. Chem., 2, 17 (1991).