Proton-coupled electron transfer from tryptophan: a concerted mechanism with water as proton acceptor.
ABSTRACT The mechanism of proton-coupled electron transfer (PCET) from tyrosine in enzymes and synthetic model complexes is under intense discussion, in particular the pH dependence of the PCET rate with water as proton acceptor. Here we report on the intramolecular oxidation kinetics of tryptophan derivatives linked to [Ru(bpy)(3)](2+) units with water as proton acceptor, using laser flash-quench methods. It is shown that tryptophan oxidation can proceed not only via a stepwise electron-proton transfer (ETPT) mechanism that naturally shows a pH-independent rate, but also via another mechanism with a pH-dependent rate and higher kinetic isotope effect that is assigned to concerted electron-proton transfer (CEP). This is in contrast to current theoretical models, which predict that CEP from tryptophan with water as proton acceptor can never compete with ETPT because of the energetically unfavorable PT part (pK(a)(Trp(•)H(+)) = 4.7 ≫ pK(a)(H(3)O(+)) ≈ -1.5). The moderate pH dependence we observe for CEP cannot be explained by first-order reactions with OH(-) or the buffers and is similar to what has been demonstrated for intramolecular PCET in [Ru(bpy)(3)](3+)-tyrosine complexes (Sjödin, M.; et al. J. Am. Chem. Soc.2000, 122, 3932. Irebo, T.; et al. J. Am. Chem. Soc.2007, 129, 15462). Our results suggest that CEP with water as the proton acceptor proves a general feature of amino acid oxidation, and provide further experimental support for understanding of the PCET process in detail.
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ABSTRACT: Redox-active tryptophans are important in biological electron transfer and redox biochemistry. Proteins can tune the electron transfer kinetics and redox potentials of tryptophan via control of the protonation state and the hydrogen-bond strength. We examine the local environment of two neutral tryptophan radicals (Trp108 on the solvent-exposed surface and Trp48 buried in the hydrophobic core) in two azurin variants. Ultrahigh-field EPR spectroscopy at 700 GHz and 25 T allowed complete resolution of all of the principal components of the g tensors of the two radicals and revealed significant differences in the g tensor anisotropies. The spectra together with (2)H ENDOR spectra and supporting DFT calculations show that the g tensor anisotropy is directly diagnostic of the presence or absence as well as the strength of a hydrogen bond to the indole nitrogen. The approach is a powerful one for identifying and characterizing hydrogen bonds that are critical in the regulation of tryptophan-assisted electron transfer and tryptophan-mediated redox chemistry in proteins.Journal of the American Chemical Society 11/2011; 133(45):18098-101. · 10.68 Impact Factor
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ABSTRACT: Click chemistry was used as an efficient method to covalently attach a chromophore to an amino acid. Such easily prepared model systems allow for time-resolved studies of one-electron oxidation reactions by the excitation of the chromophore by a laser flash. The model complex ruthenium-tryptophan () has been synthesised and studied for its photophysical and electrochemical properties. Despite a small driving force of less than 100 meV, excitation with a laser flash results in fast internal electron transfer leading to the formation of the protonated radical (Trp˙H(+)). At neutral pH electron transfer is followed by deprotonation to form the neutral Trp˙ radical with the rate depending on the concentration of water acting as the proton acceptor. The formation of the tryptophan radical was confirmed by EPR.Photochemical and Photobiological Sciences 04/2013; · 2.92 Impact Factor
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ABSTRACT: We report the photophysical and electrochemical properties of phenol-pyrrolidinofullerenes 1 and 2, in which the phenol hydroxyl group is ortho and para to the pyrrolidino group, respectively, as well as those of a phenyl-pyrrolidinofullerene model compound, 3. For the ortho analog 1, the presence of an intramolecular hydrogen bond is supported by (1)H NMR and FTIR characterization. The redox potential of the phenoxyl radical-phenol couple in this architecture is 240 mV lower than that observed in the associated para compound 2. Further, the C(60) excited-state lifetime of the hydrogen-bonded compound 1 in benzonitrile is 260 ps, while the corresponding lifetime for 2 is identical to that of the model compound 3 at 1.34 ns. Addition of excess organic acid to a benzonitrile solution of 1 gives rise to a new species, 4, with an excited-state lifetime of 1.40 ns. In nonpolar aprotic solvents such as toluene, all three compounds have a C(60) excited-state lifetime of ∼1 ns. These results suggest that the presence of an intramolecular H-bond in 1 poises the potential of phenoxyl radical-phenol redox couple at a value that it is thermodynamically capable of reducing the photoexcited fullerene. This is not the case for the para analog 2 nor is it the case for the protonated species 4. This work illustrates that in addition to being used as light activated electron acceptors, pyrrolidino fullerenes are also capable of acting as built-in proton-accepting units that influence the potential of an attached donor when organized in an appropriate molecular design.Photochemical and Photobiological Sciences 02/2012; 11(6):1018-25. · 2.92 Impact Factor
Published:April 18, 2011
r2011 American Chemical Society
dx.doi.org/10.1021/ja201536b|J. Am. Chem. Soc. 2011, 133, 8806–8809
Proton-Coupled Electron Transfer from Tryptophan: A Concerted
Mechanism with Water as Proton Acceptor
Ming-Tian Zhang and Leif Hammarstr€ om*
Department of Photochemistry and Molecular Science, Uppsala University, Box 523, SE-751 20 Uppsala, Sweden
S Supporting Information
ABSTRACT: The mechanism of proton-coupled electron
transfer (PCET) from tyrosine in enzymes and synthetic
model complexes is under intense discussion, in particular
the pH dependence of the PCET rate with water as proton
acceptor. Here we report on the intramolecular oxidation
kinetics of tryptophan derivatives linked to [Ru(bpy)3]2þ
methods.It is shown that tryptophan oxidation can proceed
not only via a stepwise electron?proton transfer (ETPT)
also via another mechanism with a pH-dependent rate and
higher kinetic isotope effect that is assigned to concerted
electron?proton transfer (CEP). This is in contrast to
current theoretical models, which predict that CEP from
tryptophan with water as proton acceptor can never com-
part (pKa(Trp•Hþ) = 4.7 . pKa(H3Oþ) ≈ ?1.5). The
moderate pH dependence we observe for CEP cannot be
explained by first-order reactions with OH?or the buffers
and is similar to what has been demonstrated for intramo-
lecular PCET in [Ru(bpy)3]3þ?tyrosine complexes
(Sj€ odin, M.; et al. J. Am. Chem. Soc. 2000, 122, 3932. Irebo,
T.; et al. J. Am. Chem. Soc. 2007, 129, 15462). Our results
general feature of amino acid oxidation, and provide further
experimental support for understanding of the PCET
process in detail.
which both electrons and protons are transferred. Changes in
electron content and oxidation state can profoundly affect
acid?base and other thermodynamic properties. Therefore,
the thermodynamic coupling between electrons and protons is
a universal phenomenon during the electron- and proton-trans-
fer process, generally referred to as proton-coupled electron
the oxidation of TyrosineZby the photo-oxidized primary donor
(P680þ).2TyrosineZoxidation is coupled to deprotonation to a
nearby base, which may occur either as consecutive PT and ET
reactions or as a single, concerted reaction step.3The latter
mechanism avoids the energetic cost of charge formation but is
less robust to structural changes. Insight into the mechanism of
such reactions is necessary for further understanding and mi-
mportant to many significant energy conversion processes in
chemistry and biology are oxidation?reduction reactions in
We previously reported intramolecular PCET from tyrosine to
photo-generated RuIII(bpy)3in a covalently linked complex as a
concerted electron?proton transfer (CEP) reaction4(denoted
CPET or EPT by others1c,f). When water was the proton acceptor,
the rate constant showed an unsusal pH dependence: a plot of
normal first-order dependence (slope = 1 in a plot of logkCEPvs
time-resolved laser spectroscopic measurements, we could resolve
the reaction from tyrosine without interference from the very
reactive fraction of tyrosinate that dominates the pH dependence
in bimolecular studies. These results generated an interesting
specifically the pH dependence.5
The tyrosine redox potential E?0(TyrO•/TyrOH) is pH-
dependent from pH ?2 to 10, so the driving force for the overall
equilibrium potential includes the entropy of proton release to the
bulk. For the Ru?tyrosine reactions it was suggested that the
rate dependence on pH followed a Marcus-type dependence
CEP step is not clear.4b,cAs noted in Krishtalik’s fundamentally
sound analysis of PCET6and emphasized by others,1e,5b?5fthe
relevant driving force in such a case will indeed be independent of
pHif a water molecule or smallwater cluster is theprimaryproton
acceptor. Proton dilution to the bulk is a subsequent process, and
the observed kCEPshould then be independent of pH. When pH-
dependent PCET rate laws do appear, they may have a variety
transfer to OH?.6bHowever, all these possibilities were ruled
out for the Ru?tyrosine reactions.4cAnother interesting predic-
tion of Krishtalik’s analysis6ais that CEP can never compete
with stepwise electron?proton transfer (ETPT) unless the
intermediate radical cation has pKa< 0, as for tyrosine. The
argument is that proton transfer from an oxidized amino acid to
the accepting water cluster is uphill, unless the former is at least
equally as acidic as H3Oþ(pKa≈ ?1.5). Because CEP has
additional proton-coupling constraints compared to the ET step
of an ETPT reaction, CEP would not be able to compete if the
driving force is also lower.
We reasoned that tryptophan linked to RuII(bpy)3units
would be interesting for further study of PCET with water as
acceptor. First, tryptophan is an amino acid that is frequently
Received:February 18, 2011
dx.doi.org/10.1021/ja201536b |J. Am. Chem. Soc. 2011, 133, 8806–8809
Journal of the American Chemical Society
involved in PCET under physiological conditions.7Second,
the pKaof oxidized tryptophanis ∼4.7,4bmuch higher than for
H3Oþ. Third, its radical cation (Trp•Hþ) and neutral radical
(Trp•) have absorption maxima around 570 and 510 nm,
respectively (Figure S2). Thus, it may be possible to observe
and even distinguish directly the ETPT and CEP mechanisms.
We designed the complexes in Scheme 1 for further studies of
bidirectional PCET in water, with special attention to the
proton acceptor in this PCET oxidation process. RuTrpH was
investigated in a previous study and found to undergo a pH-
dependent PCET assigned to CEP, but only in a narrow range
at high pH.4bFollowing the strategy used for Ru?tyrosine
complexes, we designed the new complexes to have a lower
driving force for the ET step, which suppresses the ETPT rate,
with the aim to establish CEP over a wider pH range.
The redox potentials for RuTrpH and RuTrp(Br)H were
determined by differential pulse voltammetry (DPV, Figure 1).
While the RuIII/IIcouple is pH-independent,4bthe tryptophan
potential shows the pH dependence expected for a 1e?/1Hþ
couple above the pKa of the tryptophan radical cation, in
agreement with previous reports.8From our data we determined
pKa≈ 4.7 (Trp•Hþ) and 3.5 (Trp•(Br)Hþ). The difference
between the one-electron tryptophan potentials at low pH is
∼135 mV, but the difference in the proton-coupled potentials is
only ∼65 mV because the electron-withdrawing bromide in-
creases the one-electron oxidation potential but decreases the
pKavalue. The reaction free energies for intramolecular one-
electron oxidation (ΔG?ET) and PCET oxidation (ΔG?0PCET,
see above) of the tryptophan by RuIIIare listed in Table 1.
was triggered by the “flash-quench method” which was used
extensively in our previous work.4Excitation of [Ru(bpy)3]2þ
with a 5 ns, 460 nm laser pulse followed by oxidative quenching
with methyl viologen (MV2þ) gave the corresponding RuIII
complex, seen from the rapid appearance of MV•þabsorption
around 390 and 600 nm and the bleach of the RuIIground state
around 450 nm (Figure S1). Subsequent intramolecular PCET
the RuIIabsorption recovery at 450 nm at different pH values
(Figure S1). Single-exponential fits gave a first-order rate con-
stant for PCET from tryptophan to RuIIIas plotted in Figure 2.
For the slowest reactions, a mixed first-/second-order fit was
used to account for the competing second-order recombination
of RuIIIwith MV•þ(see SI). Separate experiments with the
irreversible acceptor [Co(NH3)5Cl]2þ, when there was no
recombination, gave the same PCET rate constants. The result-
ing Trp•product, with an absorption around 510 nm, could be
detected using RuIII(NH3)6 as acceptor instead of MV2þ
(Figures 3 and S6). The formation of Trp•from RuIIIwas in
all cases quantitative, as judged from the transient absorption
changes and known extinction coefficients (Figure S2).
For RuTrpH the rate was constant over a large range and
became pH-dependent above pH 10, in agreement with the
pH-dependent term with a slope ≈ 0.5 (logk vs pH). RuTrp-
(Br)H showed a closely parallel behavior, with a consistently
slower rate, except that the reaction stopped below the pKaof
Trp(Br)H•þbecause the overall reaction was endergonic when
Me4RuTrp(Br)H showed very different behavior. For both
complexes the rate was pH-dependent in the entire range and
pH dependence of the potentials. The RuIII/IIpotentials are E?0= 1.26
and 1.10 V for Ru(bpy)3III/IIand Ru(Me-bpy)3III/II, respectively.
Table 1. Free Energies of Tryptophan Oxidation
RuTrpH RuTrp(Br)H Me4RuTrpH Me4RuTrp(Br)H
aΔG?ET = ?e(E?(RuIII/II) ? E?(Trp•Hþ/TrpH)); work term
neglected.bAt pH 7, ΔG?0PCET= ?e(E?(RuIII/II) ? E?(Trp•/TrpH)).
Figure 2. pH dependence of the observed rate constants for intramo-
lecular oxidation of tryptophan to the flash-quench-generated RuIIIin
0.5 mM phosphate/borate buffer. The solid lines are linear fits to the
pH-independent term is also used. Note that for RuTrpH the subse-
quent deprotonation of TrpHþ•(τ ≈ 450 ns) is slower than the initial
ET step (Figure S4).
dx.doi.org/10.1021/ja201536b |J. Am. Chem. Soc. 2011, 133, 8806–8809
Journal of the American Chemical Society
this weaker RuIIIoxidant there was no reaction at pH <5.5
because of the unfavorable driving force. Note that the data were
obtained in 0.5 mM phosphate:borate (1:1) buffer, and it was
checked that thebufferconcentration didnotaffecttheobserved
PCET rate in this range (Figure S5).
The kinetic isotope effects (KIEs) were also very different for the
four complexes: for RuTrpH and RuTrp(Br)H wefoundonlysmall
isotope effects in the pH-independent region at pH <9, kH/kD≈ 1.0
and 1.6, respectively. In contrast, for the pH-dependent reaction of
Me4RuTrpH and Me4RuTrp(Br)H we found a significant isotope
effect: kH/kD≈ 3.5 for both complexes at pH <9. For the former
complexes there was a similarly large effect in the pH-dependent
It isclear that replacementof theRuIIIoxidant unit resultedin
a quite different kinetic behavior, which we propose is due to a
switch from a stepwise to a concerted mechanism. The argu-
ments for this are developed in the following paragraphs.
For RuTrpH at pH <4.7 the reaction is a single ET process
because the oxidized tryptophan is not deprotonated (pKa= 4.7);
the transient spectra in Figure 3a show the clear signature of
the Trp•Hþproduct. At pH >4.7 the transient spectra give direct
evidence for an ETPT mechanism: Figure 3b shows the formation
of a Trp•Hþintermediate concomitant with RuIIrecovery (τ ≈
130 ns), which then deprotonates to form Trp•with τ ≈ 490 ns
(Figure S3). The oxidation rate remains constant at pH 3?10,
whichisexpected for an ETPT mechanism where the first step is
effectively irreversible (ΔG?ET= ?100 meV). The driving force
and the rate for the ET step do not depend on pH, so the
the KIE is negligible. At pH >10 the rate instead becomes pH-
dependent, with a significant isotope effect, showing that the
mechanism is different (see below).
For RuTrp(Br)H the data are entirely analogous to those for
RuTrpH except that for kinetic reasons the radical cation
intermediate of the ETPT reaction cannot be detected
(Figure S6a). This is because the observed oxidation is slower
than for RuTrpH and the subsequent deprotonation is expected
to be much faster (pKa= 3.5 instead of 4.7), so very little
intermediate builds up. The relatively slow initial ET rate is
consistent with the higher potential for oxidation of the Trp-
(Br)H unit. In fact, at pH < pKa(Trp•(Br)Hþ), when the radical
cation does not deprotonate, the overall reaction is slightly
endergonic (ΔG?ET= þ0.035 eV; Table 1) and does not occur
to a detectable degree.9In the ETPT region (pH 4?10),
however, the initial ET is stabilized by the following exergonic
deprotonation that drives the overall reaction.
For Me4RuTrpH and Me4RuTrp(Br)H the mechanism is
apparently different, showing a continuously pH-dependent rate
and a significant KIE. We assign this to a concerted mechanism
because the stepwise mechanism can be excluded:
Stepwise, proton-first mechanisms (PTET) with water, OH?,
or buffer species as proton acceptor are too slow to explain the
observed rates, as the pKaof TrpH is ∼1710(∼16 for Trp-
(Br)H). Thus, even deprotonation with OH?is uphill and
therefore slower than diffusion-controlled: with ΔpKa(TrpH ?
and thus give a much stronger pH dependence (slope = 1 in
faster with brominated tryptophans because of the lower pKa
value, but we instead observe lower rates. We do not exclude
contribution from PTET with OH?for the data at pH >11 for
Me4RuTrpH and Me4RuTrp(Br)H, and there is some positive
deviation from the straight line fit in Figure 2. However, we
conclude that PTET mechanisms can be excluded as an explana-
tion for our data below pH 11.
than the initial ET step. For RuTrpH and RuTrp(Br)H we
measure this rate constant at pH <10. The observed PCET rate
consistent with rate-limiting ET in this pH region. For Me4R-
also be excluded for Me4RuTrpH and Me4RuTrp(Br)H because
neither of the reaction steps would give rise to the observed pH
dependence. The potentials for RuIII/IIand Trp•Hþ/TrpH are
independent of pH, and if the electronic coupling or other
parameters governing the pure ET rate would for some unknown
reason depend on pH in the range 3?10, this would have been
the rate of the subsequent Trp•Hþdeprotonation is pH-indepen-
dent in this range, as expected for an Eigen acid with water as
Figure 3. Transient absorption spectra after 460 nm laser flash-quench
in0.5mMphosphate:boratebuffer. (a) RuTrpHatpH3:The product is
tryptophan radical cation (570 nm). (b) RuTrpH at pH 9:
The 10 ns spectrum shows mainly the Ru excited state. The initial
product (80 ns spectrum) is the protonated radical (570 nm) that
deprotonates to give the neutral radical with absorption maximum at
510 nm. (Inset: Magnification of the 80 and 800 ns spectra.) (c)
Me4RuTrpHatpH9:The product is tryptophan radicalformed directly.
(Inset: Magnification of the 2250 ns spectrum.)
dx.doi.org/10.1021/ja201536b |J. Am. Chem. Soc. 2011, 133, 8806–8809
Journal of the American Chemical Society
acceptor; this is also confirmed by our direct measurements on
RuTrpH (Figure S4).11Note that buffer species and OH?can be
excluded as primary proton acceptors under these conditions, with
the same arguments as for the CEP mechanism discussed next.
Because we can exclude the ETPT and PTET mechanisms for
Me4RuTrpH and Me4RuTrp(Br)H, this leaves the CEP mechan-
ism that we propose is responsible for the pH-dependent PCET
reactions in Figure 2. The KIEs are consistent with CEP where the
function overlap for the two isotopes. It is also consistent with the
lack of a detectable Trp•Hþintermediate for Me4RuTrpH. A key
pointistheidentityoftheprotonacceptor intheCEP process, and
Our control experiments varying the buffer concentration (Figure
S5) show that the rate is independent of buffer in the range
employed in Figure 2. With OH?as acceptor, even a diffusion-
controlledreactioncannotbe fasterthank[OH?] ≈1? 10?3s?1
at pH 7, which is much slower than the observed rate constants.
would have been first-order in [OH?] or [base], with slope = 1
This leaves water as the primary acceptor for CEP.
A pH dependence similar to that shown here was reported for
intramolecularPCET inanalogous Ru?tyrosine complexeswith
water as acceptor (slope ≈ 0.5).4Also in that case the stepwise
mechanisms could be excluded. Lowering the RuIII/IIpotential
decreases the rate of ETPT, so the pH-dependent mechanism
can compete. However, a key point of contention is the relevant
by the lower RuIII/IIpotential. According to Krishtalik’s analysis,
the driving force for CEP with a small cluster of water as proton
acceptor is even 0.3?0.4 eV more endergonic than the pure ET
oxidation of tryptophan, and it is difficult to explain why this
mechanism would be able to compete with ETPT. The model
with irreversible CEP and a small, pH-independent water cluster
such as reversible primary CEP followed by proton migration
away from the initial cluster as part of the rate-determining step.
However, it should be noted that straightforward reversible
reaction schemes invariably give zeroth- or first-order depen-
dence on OH?or other bases, i.e. slope = 0 or = 1 in Figure 2,
which is inconsistent with the present observations.
To conclude, tryptophan oxidation in aqueous solution is a
paradigm of PCET and a good model for further discussion of
how electrons and protons are coupled in PCET. In redox
proteins tryptophan tends to react via pure ET or stepwise
ETPT, and likewise when water is available as proton acceptor,7
while tyrosine more often undergoes CEP.2The competition
between concerted and stepwise reactions may be particularly
intricate in cases like ribonucleotide reductase, where a chain of
several tyrosines and tryptophans are believed to be responsible
for long-range (35 Å) radical transfer.7bOur present data on
model complexes strongly suggest that PCET from tryptophan
with water as primary acceptor can be tuned to occur via either
to previous results for Ru?tyrosine complexes.4This is in
contrast to the expected zeroth- or first-order dependences for
common alternative mechanisms. Our results suggest that
further revision of the models for PCET with water as proton
acceptor may be necessary.
netic data for buffer dependence. This material is available free
of charge via the Internet at http://pubs.acs.org.
Experimental details and ki-
We thank Drs. Tania Irebo and Todd Markle for helpful
discussions. This work was supported by the Swedish Energy
(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,
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)
C. Chem. Rev. 2008, 108, 2145. (g) Hammarstr€ om, L.; Styring, S. Philos.
Trans. B 2008, 363, 1283.
(2) (a) Hoganson, C. W.; Babcock, G. T. Science 1997, 277, 1953.
(b) Ahlbrink, R.; Haumann, M.; Cherapanov, D.; B€ ogershausen, O.;
Biochim. Biophys. Acta 2004, 1655, 195.
(3) (a) Fang, J. Y.; Hammes-Schiffer, S. J. Chem. Phys. 1997,
107, 5727. (b) Turro, C.; Chang, C. K.; Leroi, G. E.; Cukier, R. I.;
R. I. J. Phys. Chem. 1995, 99, 945. (d) Rappaport, F.; Boussac, A.; Force,
D. A.; Peloquin, J.; Brynda, M.; Sugiura, M.; Britt, R. W.; Diner, B. A.
J. Am. Chem. Soc. 2009, 131, 4425.
(4) (a)Sj€ odin,M.;Styring,S.;Åkermark,B.;Sun,L.;Hammarstr€ om,
L. J. Am. Chem. Soc. 2000, 122, 3932. (b) Sj€ odin, M.; Styring, S.;
Wolpher, H.; Xu, Y.; Sun, L.; Hammarstr€ om, L. J. Am. Chem. Soc. 2005,
127, 3855. (c) Irebo, T.; Reece, S. Y.; Sj€ odin, M.; Norcera, D. G.;
Hammarstr€ om, L. J. Am. Chem. Soc. 2007, 129, 15462.
(5) (a) Carra, C.; Iordanova, N.; Hammes-Schiffer, S. J. Am. Chem.
Soc. 2003, 125, 10429. (b) Fecenko, C. J.; Meyer, T. J; Thorp, H. H.
J. Am. Chem. Soc. 2006, 128, 11020. (c) Costentin, C.; Robert, M.;
Sav? eant, J.-M. J. Am. Chem. Soc. 2007, 129, 5870. (d) Costentin, C.;
Louault, C.; Robert, M.; Sav? eant, J.-M. J. Am. Chem. Soc. 2008,
130, 15817. (e) Song, N.; Stanbury, D. M. Inorg. Chem. 2008,
47, 11458. (f) Bonin, J.; Costentin, C.; Louault, C.; Robert, M.; Routier,
M.; Sav? eant, J.-M. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 3367.
(6) (a) Krishtalik, L. I. Biochim. Biophys. Acta 2003, 13, 1604. (b)
Krishtalik, L. I. Biofizika 1989, 34, 883.
(7) (a) Aubert, C.; Vos, M. H.; Mathis, P.; Eker, A. P; Brettel, K.
Nature 2000, 405, 586. (b) Stubbe, J.; Nocera, D. G.; Yee, C. S.; Chang,
M. C. Y. Chem. Rev. 2003, 103, 2167–2201.
(8) Harriman, A. J. Phys. Chem. 1987, 91, 6102.
(9) Veryslow (τ≈2ms)recoveryofthebleachat450nmobserved
with [CoIII(NH3)5Cl]Cl as sacrificial quencher was attributed to an
irreversible reaction of the radical fraction in equilibrium with RuIII.
(10) Remers, W. A.; Brown, R. K. In Indoles, Part 1; Chemistry of
(11) The deprotonation is pH-independent and slower than that
previously reported (130 ns), which was measured under higher buffer
concentration (10 mM). According to our buffer experiment (Figure
S5), the basic form of the buffer is a significant proton acceptor at high