Proton-coupled electron transfer from tyrosine: A strong rate dependence on intramolecular proton transfer distance

Article abstract of DOI:10.1021/ja203483j

Proton-coupled electron transfer (PCET) was examined in a series of biomimetic, covalently linked Ru^II(bpy)₃-tyrosine complexes where the phenolic proton was H-bonded to an internal base (a benzimidazyl or pyridyl group). Photooxidation in laser flash/quench experiments generated the Ru^III species, which triggered long-range electron transfer from the tyrosine group concerted with short-range proton transfer to the base. The results give an experimental demonstration of the strong dependence of the rate constant and kinetic isotope effect for this intramolecular PCET reaction on the effective proton transfer distance, as reflected by the experimentally determined proton donor-acceptor distance.

Full text of DOI:10.1021/ja203483j

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pubs.acs.org/JACS
Proton-Coupled Electron Transfer from Tyrosine: A Strong Rate
Dependence on Intramolecular Proton Transfer Distance
Ming-Tian Zhang, Tania Irebo, Olof Johansson, and Leif Hammarstr€om*
Department of Photochemistry and Molecular Science, Uppsala University, Box 523, SE-751 20 Uppsala, Sweden
S Supporting Information
b
Chart 1. Biomimetic Systems of Hydrogen-Bonded Tyrosine
Linked to a Photosensitizer
ABSTRACT: Proton-coupled electron transfer (PCET)
was examined in a series of biomimetic, covalently linked
RuII(bpy)3ꢀtyrosine complexes where the phenolic proton
was H-bonded to an internal base (a benzimidazyl or pyridyl
group). Photooxidation in laser flash/quench experiments
generated the RuIII species, which triggered long-range
electron transfer from the tyrosine group concerted with
short-range proton transfer to the base. The results give an
experimental demonstration of the strong dependence of
the rate constant and kinetic isotope effect for this intramo-
lecular PCET reaction on the effective proton transfer
distance, as reflected by the experimentally determined
proton donorꢀacceptor distance.
studies of phenols with external7g or internal7b,c bases have
shown a dependence of the rate on the driving force (given by
the pKa value for the base) but reported no distinguishable effects
of, for example, the H-bond length that determines the proton
transfer distance. Another study compared two phenols having a
pyridine base that was either conjugated with the phenol or
linked via a methylene group [30 and 40; see the Supporting
Information (SI)]7d and instead attributed the relatively rapid
oxidation rate of the former to the effect of a resonance-assisted
H-bond that was suggested to result in a flatter proton transfer
barrier. Our group compared intramolecular PCET in two
phenols containing conjugated (salicylic acid) or nonconjugated
(o-hydroxyphenylacetic acid) carboxylate groups.8b,c By theore-
tical simulation of the kinetic data, we showed that although the
H-bond length was very similar in the two compounds, there
were differences in, for example, the stiffness of the H-bond that
affected the thermal distribution of tunneling distances
(“promoting vibrations”; see below) and substantially increased
the rate for the salicylic acid-substituted phenol relative to the
other one. There is apparently a need for more experimental and
systematic studies of the effect of H-bond properties on the
PCET kinetics in model compounds. Herein we report PCET
processes in four new H-bonded tyrosines linked to RuII(bpy)3
(Chart 1). The phenol groups have internal H-bonds to neutral
bases as in PSII, and there is a significant variation in their H-bond
lengths. By laser flash/quench kinetic experiments, we examined
the relationship between the rate and kinetic isotope effects and the
H-bond properties, particularly the proton transfer distance.
Complexes 1ꢀ4 (Chart 1) were prepared from tyrosine
(1 and 4) or tyrosines bearing a halogen substituent (I or Br)
at the ortho position (2 and 3). The synthesis of 1 started with
monoformalation of tyrosine, which was subsequently con-
densed with o-diaminobenzene under an oxygen atmosphere
to introduce the benzimidazole group. The nonconjugated
benzimidazole in compound 2 was introduced by reaction of
roton-coupled electron transfer (PCET)1 is an essential
P
process in chemical and biological catalysis, for example in
water splitting,2 oxygen activation,3 proton reduction,4 nitrogen
fixation,5 and ribonucleotide reductase reactions.6 Furthermore,
utilization of solar energy is an area of intense research that is
viewed as a promising way to solve the energy problem. In fact,
mechanistic insight into PCET is a significant prerequisite for the
effective design of powerful catalysts for water splitting and solar
fuels generation by artificial photosynthesis. Interestingly, photo-
system II (PSII) has provided a template for understanding
successful molecular water oxidation catalysis and PCET
processes.2 Oxidation of tyrosine-Z in PSII, where the phenolic
proton is transferred to the nearby histidine via a hydrogen bond,
supplies the oxidative equivalents to the manganese cluster that
are required for water oxidation. These reactions have inspired
interest in the PCET oxidation of H-bonded phenols.7ꢀ9 There
are two possible types of mechanisms for oxidation of H-bonded
tyrosine and its derivatives: stepwise and concerted pathways. The
latter can avoid the involvement of higher-energy intermediates
but is less robust with respect to structural change in comparison
with the former.10 Thus, perturbation of the proton transfer
distance might provide critical insight into the nature of the
coupling between the electron and the proton. Because the proton
has a much larger mass than the electron, it is likely that proton
tunneling in PCET reactions of synthetic systems needs to be even
more carefully designed and controlled than the electron transfer.
Recent work on the oxidation of H-bonded phenols has
provided important information about PCET.7ꢀ9 However,
there is still a limited amount of experimental data as well as
contrasting reports on the relationship between the physico-
chemical features of the H-bond and the PCET kinetics, such as
the effect of geometry, distance, and H-bond strength. Some
Received: April 15, 2011
Published: August 03, 2011
r
2011 American Chemical Society
13224
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|

Journal of the American Chemical Society
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o-diaminobenzene with 2-bromoethynyltyrosine, which was pre-
pared by a Sonogashira coupling reaction. The pyridyl was
introduced at the ortho position of tyrosine by Stille coupling
and aldol condensation in the syntheses of 3 and 4, respectively.
Finally, the tyrosines bearing different bases were coupled to
Ru(bpy)32+ via an amido linkage. The final complexes 1ꢀ4 were
fully characterized by NMR spectroscopy and mass spectro-
metry, and the synthetic details are shown in the SI.
tyrosines in 2 and 4 is larger than in their conjugated counterparts
in 1 and 3 (Table 1).
Kinetics of intramolecular TyrOH B oxidation. The intra-
3 3 3
molecular PCET reaction between TyrOH B and RuIII was
3 3 3
triggeredbytheflash/quenchmethod, asdescribedinourprevious
work.8 Excitation of the [Ru(bpy)3]2+ unit with a 5 ns, 460 nm
laser pulse followed by oxidative quenching with methyl viologen
[MV(PF6)2] gave the corresponding RuIII complex. This was seen
from the rapid appearance of MV+• absorption at 390 and 600 nm
and bleaching of the RuII ground state at ∼450 nm (Figure 2a).
1
Hydrogen bonding. The H NMR spectra of 1ꢀ4 all show
resonance shifts for the phenolic proton between 10.19 and
14.13 ppm (Table 1), consistent with an intramolecular hydro-
gen bond. The δH for 1 and 3 appear at lower fields than those for
the corresponding nonconjugated compzounds 2 and 4. Similar
results for related compounds were ascribed to a resonance-
assisted H-bond due to conjugation between the phenol and the
basic site.7d On the basis of the 1H NMR shifts, we do conclude,
however, that the H-bond lengths in 1 and 3 are shorter than
those in their nonconjugated counterparts. This is also supported
by the X-ray crystal structures of the analogous phenolꢀbase
compounds 10ꢀ40 that are not linked to a Ru complex11 (see the
SI). Thus, the proton transfer distance in the PCET process
should be shorter in 1 and 3 than in 2 and 4.
The subsequent intramolecular PCET between the TyrOH
B
3 3 3
and RuIII units was monitored using the recovery of RuII absorp-
tion at 450 nm. The rate constants at different temperatures were
alsodetermined from the kinetic traces(Figure2b) andare plotted
in Figure S3. It is striking that the rate constants for 2 and 4 were
markedly lower than for 1 and 3 in spite of the significantly larger
(0.15ꢀ0.20 eV) reaction driving force for 2 and 4. This will be
discussed further in the following sections.
Mechanistic analysis. There are three possible mechanisms for
the intramolecular PCET process. A first possibility is electron
transfer (ET) from tyrosine to RuIII to form the radical cation
RuIIꢀTyrOH+• B followed by proton transfer (PT) to the
3 3 3
Electrochemistry and PCET driving force. The electrochemical
behavior of 1ꢀ4 was studied by both cyclic voltammetry and
differential pulse voltammetry (DPV) in acetonitrile (Figure 1;
Figure S2 and Table S1 in the SI). On the cathodic side of the
cyclic voltammogram (CV) during the scan down to ꢀ2.5 V (all
potentials are vs Fc+/0), three redox waves typical for the
reduction of the three ligands in [Ru(bpy3)]2+ were observed.
At +0.932 V, the reversible RuIII/II oxidation was observed. No
electronic influence of the H-bonded tyrosine on the potentials
for ligand reduction and RuIII/II oxidation was observed. The
most interesting feature of the CV is the irreversible oxidation
below the RuIII/II potential, corresponding to oxidation of the
phenol group. The peak potentials for this oxidation in differ-
ential pulse voltammetry were +0.680, +0.489, +0.720, and
+0.570 V for 1ꢀ4, respectively. The relative values are in
excellent agreement ((10 mV) with those reported for the
analogous series of phenols 10ꢀ40, which gave reversible oxida-
tions in their CVs.7c,d The shift toward a less positive potential for
1ꢀ4 relative to the methylated Ru-TyrOMe (+1.264 V; Table
S1) can be ascribed to a PCET process on the electrode with
proton transfer to the base, consistent with previous examples of
intramolecular phenolꢀbase systems.7 Notably, the lower po-
tential in the presence of a base is not due to H-bonding per se
but arises from a protonation equilibrium with the nearby base,
for which the pKa of the protonated base is the important
parameter.7b According to the electrochemistry data, the driving
force for the concerted PCET process in the nonconjugated
base in a stepwise mechanism (the ETPT mechanism). A
second stepwise mechanism would involve pre-equilibrium
proton transfer to yield the zwitterion RuIIIꢀTyrOꢀ +HB)
3 3 3
followed by electron transfer (the PTET mechanism). The
third possibility is concerted electronꢀproton transfer (CEPT)
in a single step with a common transition state. The experi-
mental results show that intramolecular PCET in the present
complexes proceeds by the concerted pathway without the
involvement of either of the high-energy intermediates
RuIIꢀTyrOH+• B or RuIIIꢀTyrOꢀ +HB. First, primary
3 3 3
3 3 3
kinetic isotope effects (KIEs) kH/kD = 1.5ꢀ4.0 were found for
1ꢀ4. Neither rate-limiting electron transfer (ETPT) nor pre-
equilibrium proton transfer (PTET) is consistent with the large
values for 2 and 4. Second, the CEPT mechanism is strongly
favored because of its significant driving force (Table 1). In
contrast, for the first step in the ETPT mechanism, ΔG°ET can
be estimated as +33 kJ molꢀ1 (KET = 2.4 ꢁ 10ꢀ6).12 Addition-
ally, the substituent effect makes the conjugated phenols in 1
and 3 harder to oxidize to the radical cation than the other two
phenols. Thus, their PCET rate constants would be smaller
than those in 2 and 4 if that mechanism were operative, but
instead they are larger. Also, the initial PT is strongly endergo-
nic because of the large pKa difference between phenol and the
bases. On the basis of the pKa values of benzimidazole (∼17),
pyridine (12.3), and phenols (∼27),13 KPT is expected to be
10ꢀ14.7 to 10ꢀ10. The PTET rate constant is the product of KPT
and kET, the rate constant for ET from the zwitterion to RuIII.
Table 1. Experimental Data for 1ꢀ4
compound
b
f
δH (ppm)a
kH (105 sꢀ1
)
KIEc
ΔGr°xn (eV)d
Ea (kcal/mol)e
dO--N (Å)
ꢀ
1
2
3
4
13.04
10.19
14.13
10.71
6.30 ( 0.03
0.93 ( 0.01
3.81 ( 0.01
1.78 ( 0.01
1.42
4.01
2.16
4.02
ꢀ0.252
ꢀ0.443
ꢀ0.212
ꢀ0.363
6.11
8.13
6.62
7.32
2.539g
2.752h
2.567i
2.691i
a 1H NMR shift of the phenolic proton in dry CD3CN. b PCET rate constant at 298 K in acetonitrile with 0.75% H2O (v/v). c Kinetic isotope effect; reactions
with deuterated substrates were performed in acetonitrile with 0.75% D2O (v/v). d Calculated from DPV data using the equation ꢀΔGr°xn = e[E°(RuIII/II) ꢀ
E°(TyrO•
+HB/TyrOH
B)]. e Activation energies obtained from fits to the Arrhenius equation (Figure S3). f Proton donorꢀacceptor
3 3 3
3 3 3
distances from crystallographic data for the related systems 10ꢀ40 (see the SI). g Reference 7c. h This work (see the SI). i Reference 7d.
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Figure 2. Laser flash/quench-induced transient absorption of complex
1 in the presence of 40 mM MV2+(PF6)2. (a) Transient absorption
spectra at 250, 650, 850, 1250, 1850, 2050, 4000, and 20000 ns after the
laser flash. The absorptions at 600 and 390 nm are due to the formation
of MV+• in the first quenching step, and the bleaching at 450 nm is due to
oxidation of RuII to RuIII; this absorption recovers on a microsecond
time scale due to PCET from the appended tyrosine. (b) Transient
absorbance traces at different temperatures showing the rapid genera-
tion of the MV+• radical cation at 600 nm (upper trace) and the kinetics
of RuII ground-state absorption recovery at 450 nm (lower traces).
Figure 1. Cyclic voltammogram (black) and differential pulse voltam-
mogram (red) for 3 in acetonitrile with 0.1 M TBAPF6 as the electrolyte,
AgNO3/Ag as the reference electrode, and a scan rate of 0.1 V/s.
The observed rate constant kobs > 105 sꢀ1 for 1ꢀ4 then would
imply kET > 1015 ꢀ1, which is far too large to be realistic.
s
Moreover, the reaction would be several orders of magnitude
faster with benzimidazole than with pyridine, and this is not
observed. On the basis of this discussion, the concerted
mechanism is the most likely pathway for 1ꢀ4, since this
mechanism avoids the involvement of high-energy intermedi-
ates (i.e., the radical cations or zwitterions).
VPCET ≈ VETVPT
ð2Þ
ð3Þ
ꢀ
ꢁ
β
Effect of the H-bond properties on the PCET kinetics. Although
the PCET driving force in 2 and 4 is significantly larger than in
the corresponding complexes 1 and 3, the observed rate con-
stants are smaller. This cannot be explained simply by the
difference in the strengths of the conjugated and nonconjugated
H-bonds, as discussed in our previous work.8b,c Generally,
H-bonds are often classified as strong or weak, and these two
types of H-bond are also described as short or long, respectively.
However, the reliability of the relationship between the strength
and the length of H-bonds is now in question.14 In the present
VPT ¼ VP0T exp ꢀ ðd ꢀ d0Þ
2
In the present complexes, we have noted the striking trend of
lower rates for the complexes with the larger driving force; this
apparently contradicts eq 1 (with ꢀΔG0 < λ) and requires an
explanation. On the other hand, there are obvious differences in
dO--N in the series, with a parallel trend in the KIEs. The
complexes with longer dO--N values show lower PCET rate
constants (consistent with eqs 1ꢀ3) as well as larger KIEs. To
explain our data, we therefore focus on the dependence of kPCET
on the two experimentally determined parameters ΔG0 and d.
Thus, at the present level of analysis, we ignore possible differences
in the reorganization energy λ and the compressibility (force
constant) of the H-bond8c,10 within this series of complexes
(see the SI). When the driving force is small compared with λ and
the temperature is constant, eq 1 can be rewritten as eq 4.17
Inclusion of the second term on the left-hand side compensates for
the difference in the reaction free energies of the complexes and
allows a better analysis of the dependence of the rate on d.
Furthermore, we use the experimental values of dO--N, which
should closely track the differences in the tunneling distance d
for the compounds. With these approximations, the last termcan be
treated as a constant for this series of complexes at 298 K.
1
series of complexes, both X-ray crystallography and H NMR
analysis showed that the conjugated phenolꢀbase complexes
have shorter H-bonds than their nonconjugated counterparts.
The proton donorꢀacceptor distances dO--N vary from 2.54 to
2.75 Å (Table 1), and typical OꢀH and NꢀH bond lengths are
0.97 and 1.03 Å, respectively, in this type of compound.15 Thus,
the proton tunneling distance (d) should be 0.5ꢀ0.8 Å, increasing
in the order 1 < 3 < 4 < 2.
Concerted PCET can be described as a double tunneling at
the transition state using a formalism corresponding to that for
electron transfer (nonadiabatic regime, eq 1; contributions from
vibrationally excited states of OꢀH and other high-frequency
modes are neglected at the present level of discussion8c,10). The
vibronic coupling VPCET can be treated as the product of the
electron and proton couplings (eq 2), where the latter (VPT) is
determined by the overlap between the donor- and acceptor-state
proton vibrational wave functions. This overlap is strongly depen-
dent on the proton transfer distance d; in a limited range around a
chosen reference distance d0, the overlap decreases exponentially
(eq 3).10,16 Because a deuteron, which is more massive, has a more
localized wave function than a proton, deuteron transfer is typically
slower and shows a stronger distance dependence, giving rise to a
distance-dependent KIE.
ΔG0
þ constant
¼ ꢀ βdO--N
2RT
ð4Þ
ln kPCET
þ
A plot of the data and a linear fit according to eq 4 show very
good agreement (Figure 3). The slope parameter β is ∼27 Åꢀ1
,
which shows the large sensitivity of the PCET rate to dO--N and
thus to the tunneling distance d; the variation in the estimated rate
constant at equal driving force is nearly 3 orders of magnitude for a
difference of only ∼0.2 Å in d. This is in sharp contrast to electron
tunneling, for which the distance-dependence parameter β is
"
#
typically ∼1 Åꢀ1 18
.
The value of β is in very good agreement
2
2
2π VPCET
ðΔG0 þ λÞ
with what is theoretically predicted for proton transfer reactions16a
kPCET
¼
exp ꢀ
ð1Þ
1=2
p
4λRT
and PCET.16c To the best of our knowledge, this is the first direct
ð4πλRTÞ
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Journal of the American Chemical Society
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104, 9370. (c) Mayer, J. M. Annu. Rev. Phys. Chem. 2004, 55, 363.
(d) Reece, S. Y.; Nocera, D. G. Annu. Rev. Biochem. 2009, 78, 673.
(e) Huynh, M. H. V.; Meyer, T. J. Chem. Rev. 2007, 107, 5004. (f)
Costentin, C. Chem. Rev. 2008, 108, 2145. (g) Hammarstr€om, L.;
Styring, S. Philos. Trans. R. Soc., B 2008, 363, 1283.
(2) (a) Hoganson, C. W.; Babcock, G. T. Science 1997, 277, 1953.
(b) Rappaport, F.; Lavergne, J. Biochim. Biophys. Acta 2001, 1503, 246.
(c) Dau, H.; Haumann, M. Photosynth. Res. 2007, 92, 327.
(3) (a) Babcock, G. T.; Wikstr€om, M. Nature 1992, 356, 301.
(b) Ferguson-Miller, S.; Babcock, G. T. Chem. Rev. 1996, 96, 2889.
(4) (a) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C.
Science 1998, 282, 1853. (b) Nicolet, Y.; Piras, C.; Legrand, P.;
Hatchikian, E. C.; Fontecilla-Camps, J. C. Structure 1999, 7, 13.
(5) (a) Burgess, B. K.; Lowe, D. J. Chem. Rev. 1996, 96, 2983.
(b) Rees, D. C.; Howard, J. B. Curr. Opin. Chem. Biol. 2000, 4, 559.
(6) Stubbe, J.; Nocera, D. G.; Yee, C. S.; Chang, M. C. Y. Chem. Rev.
2003, 103, 2167.
(7) (a) Rhile, I. J.; Mayer, J. M. J. Am. Chem. Soc. 2004, 126, 12718.
(b) Rhile, I. J.; Markle, T. F.; Nagao, H.; Dipasquale, A. G.; Lam, O. P.;
Lockwood, M. A.; Rotter, K.; Mayer, J. M. J. Am. Chem. Soc. 2006,
128, 6075. (c) Markle, T. F.; Rhile, I. J.; Dipasquale, A. G.; Mayer, J. M.
Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 8185. (d) Markle, T. F.; Mayer,
J. M. Angew. Chem., Int. Ed. 2008, 47, 738. (e) Costentin, C.; Robert, M.;
Saveant, J.-M. J. Am. Chem. Soc. 2006, 128, 4552. (f) Costentin, C.;
Robert, M.; Saveant, J.-M. J. Am. Chem. Soc. 2007, 129, 9953.
(g) Fecenko, C. J.; Thorp, H. H.; Meyer, T. J. J. Am. Chem. Soc. 2007,
129, 15098.
(8) (a) Sj€odin, M.; Irebo, T.; Utas Josefin, E.; Lind, J.; Merenyi, G.;
Åkermark, B.; Hammarstr€om, L. J. Am. Chem. Soc. 2006, 128, 13076.
(b) Irebo, T.; Johansson, O.; Hammarstr€om, L. J. Am. Chem. Soc. 2008,
130, 9194. (c) Johannissen, L. O.; Irebo, T.; Sj€odin, M.; Johansson, O.;
Hammarstro€m, L. J. Phys. Chem. B 2009, 113, 16214.
(9) (a) Sun, L.; Burkitt, M.; Tamm, M.; Raymond, M. K.; Abrahamsson,
M.; LeGourriꢀerec, D.; Frapart, Y.; Magnuson, A.; Kenꢀez, P. H.; Brandt, P.;
Tran, A.; Hammarstr€om, L.; Styring, S.; Åkermark, B. J. Am. Chem. Soc. 1999,
121, 6834. (b) Lachaud, F.; Quaranta, A.; Pellegrin, Y.; Dorlet, P.; Charlot,
M. F.; Un, S.; Liebl, W.; Aukauloo, A. Angew. Chem., Int. Ed. 2005, 44, 1536.
(c) Moore, G. F.; Hambourger, M.; Gervaldo, M.; Poluektov, O. G.; Rajh, T.;
Gust, D.; Moore, T. A.; Moore, A. L. J. Am. Chem. Soc. 2008, 130, 10466.
(d) Moore, G. F.; Hambourger, M.; Kodis, G.; Michl, W.; Gust, D.; Moore,
T. A.; Moore, A. L. J. Phys. Chem. B 2010, 114, 14450.
Figure 3. Dependence of the estimated PCET rate constant at ΔG0 = 0
on the proton donorꢀacceptor distance dO--N. The red line is a fit to eq 4
(see the text).
experimental demonstration of the dependence of the concerted
PCET rate constant on the proton transfer distance for a series of
more than two synthetic model complexes.
In conclusion, complexes 1 and 3 with a conjugated base
attached to the phenol moiety show a higher rate of concerted
PCET than the corresponding nonconjugated ones 2 and 4, in
spite of a larger driving force in the latter. When corrected for
differences in driving force, the rate constants differ by 2ꢀ3 orders
of magnitude. On the basis of the good correlation in Figure 3, this
is predominantly due to the shorter H-bond length in 1 and 3,
which gives a shorter proton transfer distance and thus better
overlap of the proton wave functions. This is also reflected in the
KIE values, which are significantly higher for 2 and 4. Further
experiments and detailed theoretical analysis of the H-bond proper-
ties and other parameters governing the PCET rate constant are in
progress. The dramatic effect on the PCET rate of small geometric
differences as illustrated here emphasizes the importance of care-
fully controlling the proton transfer component of PCET and
H-bonding in both enzymes and the design of efficient molecular
catalysts for water splitting and solar fuels production.
(10) Soudackov, A.; Hatcher, E.; Hammes-Schiffer, S. J. Chem. Phys.
2005, 122, No. 014505.
(11) HOArꢀBzim (10), HOArꢀpy (30), and HOArꢀCH2py (40)
were reported by Mayer and co-workers,7bꢀd and HOArꢀCH2Bzim
(20) was synthesized and characterized in this study (see the SI).
(12) This estimate uses E(TyrOH+• B/TyrOH B) = 1.264 V
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic details for 1ꢀ4, ex-
b
perimental details for electrochemistry and laser flash photolysis,
and crystallographic data (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
3 3 3
3 3 3
vs Fc+/0, which is taken to be same as E(TyrOMe+•/TyrOMe).
(13) We directly use the pKa values of benzimidazole and pyridine
for the analysis and also assume that the pKa of tyrosine in acetonitrile is
similar to those of the phenols without considering the effect of
intramolecular H-bonding on the pKa's of tyrosine and the base. The
pKa values were taken from: Izutsu, K. Acid-Base Dissociation Constants in
Dipolar Aprotic Solvents; Blackwell: London, 1990.
’ AUTHOR INFORMATION
Corresponding Author
Leif@fotomol.uu.se
(14) Perrin, C. L. Acc. Chem. Res. 2010, 43, 1550.
(15) Allen, F. H.; Kennard, O.; Watson, D. G. J. Chem. Soc. Perkin
Trans. 2 1987, S1.
’ ACKNOWLEDGMENT
(16) (a) Kiefer, P. M.; Hynes, J. T. J. Phys. Chem. A 2004, 108, 11793.
(b) Kiefer, P. M.; Hynes, J. T. J. Phys. Chem. A 2004, 108, 11809.
(c) Skone, J. H.; Soudackov, A. V.; Hammes-Schiffer, S. J. Am. Chem. Soc.
2006, 128, 16655.
We thank Dr. Xue-Li Geng for the help with the growth of a
single crystal of compound 20 and Dr. Marie-Pierre Santoni for
X-ray crystallography. This work was supported by the Swedish
Research Council, the Swedish Energy Agency, and the Knut and
Alice Wallenberg Foundation.
(17) From eq 1, ∂(ln kPCET)/∂(ΔG0) = (2RT)ꢀ1(1+ΔG0/λ) ≈
(2RT)ꢀ1 when ΔG0 ≈ 0.
(18) (a) Paddon-Row, M. N. In Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol. III, Part
2, Chapter 1, pp 201ꢀ215. (b) Gray, H. B.; Winkler, J. R. Proc. Natl.
Acad. Sci. U.S.A. 2005, 102, 3534.
’ REFERENCES
(1) (a) Cukier, R. I.; Nocera, D. G. Annu. Rev. Phys. Chem. 1998,
49, 337. (b) Decornez, H.; Hammes-Schiffer, S. J. Phys. Chem. A 2000,
13227
dx.doi.org/10.1021/ja203483j |J. Am. Chem. Soc. 2011, 133, 13224–13227