Proton-directed redox control of O-O bond activation by heme hydroperoxidase models

Article abstract of DOI:10.1021/ja0683032

Hangman metalloporphyrin complexes poise an acid-base group over a redox-active metal center and in doing so allow the pull effect of the secondary coordination environment of the heme cofactor of hydroperoxidase enzymes to be modeled. Stopped-flow investigations have been performed to decipher the influence of a proton-donor group on O-O bond activation. Low-temperature reactions of tetramesitylporphyrin (TMP) and Hangman iron complexes containing acid (HPX-CO₂H) and methyl ester (HPX-CO ₂Me) functional groups with peroxyacids generate high-valent Fe=O active sites. Reactions of peroxyacids with (TMP)Fe^III(OH) and methyl ester Hangman (HPX-CO₂Me)Fe^III(OH) give both O-O heterolysis and homolysis products, Compound I (Cpd I) and Compound II (Cpd II), respectively. However, only the former is observed when the hanging group is the acid, (HPX-CO₂H)Fe^III(OH), because odd-electron homolytic O-O bond cleavage is inhibited. This proton-controlled, 2e^- (heterolysis) vs 1e^- (homolysis) redox specificity sheds light on the exceptional catalytic performance of the Hangman metalloporphyrin complexes and provides tangible benchmarks for using proton-coupled multielectron reactions to catalyze O-O bond-breaking and bond-making reactions.

Full text of DOI:10.1021/ja0683032

Published on Web 03/31/2007
Proton-Directed Redox Control of O-O Bond Activation by
Heme Hydroperoxidase Models
Jake D. Soper,† Sergey V. Kryatov,‡ Elena V. Rybak-Akimova,‡
and Daniel G. Nocera*,†
Contribution from the Department of Chemistry, 6-335, Massachusetts Institute of Technology,
77 Massachusetts AVenue, Cambridge Massachusetts 02139-4207, and Department of Chemistry,
Tufts UniVersity, 62 Talbot AVenue, Medford, Massachusetts 02155
Received November 20, 2006; E-mail: nocera@mit.edu
Abstract: Hangman metalloporphyrin complexes poise an acid-base group over a redox-active metal center
and in doing so allow the “pull” effect of the secondary coordination environment of the heme cofactor of
hydroperoxidase enzymes to be modeled. Stopped-flow investigations have been performed to decipher
the influence of a proton-donor group on O-O bond activation. Low-temperature reactions of tetramesi-
tylporphyrin (TMP) and Hangman iron complexes containing acid (HPX-CO2H) and methyl ester (HPX-
CO2Me) functional groups with peroxyacids generate high-valent FedO active sites. Reactions of
peroxyacids with (TMP)FeIII(OH) and methyl ester Hangman (HPX-CO2Me)FeIII(OH) give both O-O
heterolysis and homolysis products, Compound I (Cpd I) and Compound II (Cpd II), respectively. However,
only the former is observed when the hanging group is the acid, (HPX-CO2H)FeIII(OH), because odd-
electron homolytic O-O bond cleavage is inhibited. This proton-controlled, 2e- (heterolysis) vs 1e-
(homolysis) redox specificity sheds light on the exceptional catalytic performance of the Hangman
metalloporphyrin complexes and provides tangible benchmarks for using proton-coupled multielectron
reactions to catalyze O-O bond-breaking and bond-making reactions.
peroxidase [CcP]).9-10,12 It is generated by heterolysis of an
O-O bond in H2O2 or O2. Such heterolytic cleavage to release
Introduction
Heme hydroperoxidase enzymes are divided into subclasses
of peroxidases, catalases, and cytochrome P450 monooxyge-
nases.1 These enzymes are responsible for a wide array of
biological redox processes, which originate from an FeIII active
site with protoporphyrin IX prosthetic groups.2 Peroxidase
enzymes are important for plant cell wall biosynthesis and lignin
formation,3 removal of xenobiotics, and signaling during oxida-
tive stress.4 Catalases work to eliminate cytotoxic hydrogen
peroxide in vivo while avoiding the formation of harmful
hydroxyl radical species,5 and cytochrome P450 is responsible
for a variety of metabolic oxidation reactions.6,7 The diversity
of biological redox processes performed by these enzymes is
apparently achieved via a remarkably similar active oxidant,
commonly called Compound I (Cpd I).2,8-11 Cpd I is two redox
levels above FeIII with a ferryl FeIVdO and associated radical
(e.g., a porphyrin π-radical cation, P•+, in horseradish peroxidase
[HRP] and catalase, or an oxidized tryptophan in cytochrome c
H2O is accomplished by an internal redox disproportionation
coupled to the delivery of a H+, from a precisely positioned
acid/base residue in the active-site cavity to the distal O-atom
in an FeIII-OOH complex.1,2,4,8-10,13-16 In peroxidases such as
HRP, it is generally accepted that two residues in the distal
cavity of HRP (His42 and Arg38) assist this reaction by shuttling
protons to facilitate H2O release (see Scheme 1).16-20 Experi-
(6) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem. ReV. 1996,
96, 2841-2887.
(7) Ortiz de Montellano, P. R. Cytochrome P450: Structure Mechanism, and
Biochemistry, 2nd ed.; Plenum: New York, 1995.
(8) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem. ReV. 1996,
96, 2841-2887.
(9) Dunford, H. B. Heme Peroxidases; Wiley: New York, 1999.
(10) Watanabe, Y. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 4, pp 97-117.
(11) Newcomb, M.; Zhang, R., Chandrasena, R. E. P.; Halgrimson, J. A.; Horner,
J. H.; Makris, T. M.; Sligar, S. G. J. Am. Chem. Soc. 2006, 128, 4580-
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245, 738-740.
† Massachusetts Institute of Technology.
(13) Veitch, N. C.; Smith, A. T. AdVances in Inorganic Chemistry; Academic
Press: New York, 2001; Vol. 51, pp 107-162.
‡ Tufts University.
(1) Suslick, K. S. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 4, pp 41-63.
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J. AM. CHEM. SOC. 2007, 129, 5069-5075
5069

A R T I C L E S
Scheme 1
Soper et al.
mental and computational studies support the hypothesis that
His42 likely acts first as a base and then an acid.4,17,20 In this
way, the histidine does the “pull” part of the “push-pull
mechanism” for peroxide activation.8,10,13,21,22 The “push” part
of the mechanism is performed by an electron-rich proximal
imidazolate-like histidine fifth ligand.8,10,23 The distal Arg38
residue likely aligns H2O2 in the active site and contributes to
polarization of the O-O bond, also a “pull effect”.4 A similar
active site environment exists in CcP.9 “Push-pull” effects have
been also implicated in O-O heterolysis leading to Cpd I
formation in catalases and cytochromes P450.1,2,9 Here a
proximal tyrosinate likely provides the “push” for O-O
heterolysis in catalase, while a distal histidine again “pulls”.24
It is proposed that cytochrome P450 enzymes compensate for
the lack of acid-base “pull” residues in the distal cavity with
a “strong push” from an electron-rich proximal cysteinate.8,22,25-27
Investigations of “push-pull effects” on Cpd I formation have
been performed using heme model systems10,17,28-31 and re-
engineered myoglobin.17-19,32 O-O heterolysis in acylperoxo
porphyrin complexes is enhanced when electron-rich axial
ligands are introduced or electron-releasing groups are attached
to the meso-positions of the metalloporphyrin ring, simulating
the “push”.10,28 “Pull effects” have been indirectly simulated in
model complexes by using substituted peroxybenzoic ac-
ids.10,29,30 Selective mutations in the distal cavity of myoglobin
mutants have demonstrated the importance of a precisely
positioned histidine as a proton shuttlesan H+ “pull” residues
to the redox selectivity of peroxide O-O bond activation.17-19,32
The advantage of re-engineered proteins, as opposed to typical
model systems, is that the H+ pull residue is structurally
established by the secondary and tertiary structure of the protein
environment. Conversely, the secondary coordination environ-
ment of model compounds is difficult to control. Because H+
residues are not oriented, “pull” effects cannot be systematically
investigated. It is on this count that “Hangman” metallo-active
sites are distinguished from typical model compounds. The
Hangman architecture suspends an acid-base functional group
over the face of redox-active macrocycles.33-36 Crystal structures
of the ferric Hangman complex containing a xanthene pillar
and a pendent carboxylic acid (HPX-CO2H)FeIII(OH) show that
a structured water molecule is oriented between the distal acid
group and hydroxide ligand of the heme via hydrogen bonding
interactions.33 The water molecule remains bound in solution,
and titrations with base and H2O have shown this binding to be
chemically reversible. In this regard, the Hangman heme
porphyrin is as a minimalist model for the secondary coordina-
tion environment of heme hydroperoxidase enzymes.37,38 The
hanging acid group mimics the amino acid residues that orient
water in the distal cavities of heme peroxidases.
The precise positioning of the acid/base residue in Hangman
complexes allows for the functional modeling of enzymatic
“push-pull effects.” (HPX-CO2H)FeIIICl exhibits exceptional
catalytic activity upon peroxide activation for the catalase-like
disproportionation of H2O2 and the cytochrome P450-like
oxygenation of substrates.33-38 It has been proposed that the
hanging acid group facilitates O-O bond heterolysis to generate
a Cpd I oxidant by shuttling H+, similar to histidine “pull”
residues in the active sites of peroxidase and catalase enzymes.
Reported here are mechanistic experiments designed to probe
directly the effects of a positioned acid group on internal redox
partitioning in peroxide O-O bond activation to form Hangman
Cpd I and II active sites.
Results
The reactions of ferric Hangman porphyrin complexes with
peroxides were examined by stopped-flow techniques at cryo-
genic temperatures. In all cases, reactions were performed with
Hangman metalloporphyrin complexes (HPX-CO2R; R ) Me,
H) and the parent tetramesitylporphyrin (TMP) analogue for
side-by-side comparison. This allowed for direct examination
of the effects of a positioned acid-base group by providing a
benchmark for comparative reactivity to the TMP system, which
(21) Poulos, T. L. In AdVances in Inorganic Biochemistry; Eichhorn, G. L.,
Marzilli, L. G., Eds.; Elsevier: New York, 1988; Vol. 7, pp 1-36.
(22) Dawson, J. H. Science 1988, 240, 433-439.
(23) Heering, H. A.; Indiani, C.; Regelsberger, G.; Jakopitsch, C.; Obinger, C.;
Smulevich, G. Biochemistry 2002, 41, 9237-9247.
(24) Fita, I.; Rossmann, M. G. J. Mol. Biol. 1985, 185, 21-37.
(25) Green, M. T.; Dawson, J. H.; Gray, H. B. Science 2004, 304, 1653-1656.
(26) Green, M. T. J. Am. Chem. Soc. 2006, 128, 1902-1906.
(27) Behan, R. K.; Green, M. T. J. Biol. Inorg. Chem. 2006, 100, 448-459.
(28) Groves, J. T.; Watanabe, Y. J. Am. Chem. Soc. 1988, 110, 8443-8452.
(29) Yamaguchi, K.; Watanabe, Y.; Morishima, I. J. Am. Chem. Soc. 1993, 115,
4058-4065.
(30) Traylor, T. G.; Ciccone, J. P. J. Am. Chem. Soc. 1989, 111, 8413-8420.
(31) Nam, W.; Lim, M. H.; Oh, S.-Y. Inorg. Chem. 2000, 39, 5572-5575.
(32) Watanabe, Y.; Ueno, T. Bull. Chem. Soc. Jpn. 2003, 76, 1309-1322.
(33) Yeh, C.-Y.; Chang, C. J.; Nocera, D. G. J. Am. Chem. Soc. 2001, 123,
1513-1514.
(34) Chang, C. J.; Yeh, C.-Y.; Nocera, D. G. J. Org. Chem. 2002, 67, 1403-
1406.
(35) Liu, S.-Y.; Nocera, D. G. J. Am. Chem. Soc. 2005, 127, 5278-5279.
(36) Liu, S.-Y.; Soper, J. D.; Yang, J. Y.; Rybak-Akimova, E. V.; Nocera,
D. G. Inorg. Chem. 2006, 45, 7572-7574.
(37) Chang, C. J.; Chng, L. L.; Nocera, D. G. J. Am. Chem. Soc. 2003, 125,
1866-1876.
(38) Chng, L. L.; Chang, C. J.; Nocera, D. G. Org. Lett. 2003, 5, 2421-2424.
9
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Proton-Directed Redox Control of O−O Bond Activation
A R T I C L E S
Figure 2. Selected time-resolved data for the disappearance of (P)FeIII-
(OH) (9.0 × 10-6 M; λmax ) 575 nm) in reactions with 3.5 × 10-4
M
Figure 1. Stopped-flow UV-vis spectra from reaction of m-CPBA
(3.5 × 10-4 M) with (HPX-CO2Me)FeIII(OH) (9.0 × 10-6 M) in CH2Cl2
at -40 °C to generate (HPX-CO2Me)FeIII(O2COAr) at 0.1 s (black) and
50.0 s (red).
m-CPBA at -40 °C in CH2Cl2 to make (P)FeIII(O2COAr) and subsequent
O-O bond heterolysis [P ) TMP (O), HPX-CO2Me (0), HPX-CO2H
(4)].
comprises an extensive literature for reactions of peroxides and
peroxyacids with (TMP)FeIII(OH).10,28,29,31,39,40
site in Hangman metalloporphyrins is positioned to participate
in reactions at the ferric heme center.
(HPX)FeIII Substitution Chemistry. On mixing dichloro-
methane or toluene solutions of 3-chloroperoxybenzoic acid (m-
CPBA) and (P)FeIII(OH) at -40 °C (P ) TMP, HPX-CO2H,
HPX-CO2Me), a decrease is observed in the UV-vis absorp-
tion intensity of the 580 nm band of (P)FeIII(OH) with a
concomitant appearance of a Soret absorption band at 416 nm
and a Q-band at 506 nm (Figures 1, S1). These new features
are identical to previously reported spectra for the ferric acyl-
peroxo complex (P)FeIII(O2COAr) (eq 1) that is generated from
low-temperature substitution of hydroxide with 3-chloror-
perbenzoate when P ) TMP.28,29
OsO Bond Heterolysis. In CH2Cl2 solutions, formation of
(P)FeIII(O2COAr) complexes at -40 °C is immediately followed
by a second set of spectral changes that are characterized by a
blue shift and decrease in the Soret band intensity (to 406 nm
and ꢀ ≈ 70 000 M-1 cm-1) and formation of tailing features
between 600 and 700 nm (Figures 3 and 4). These exactly match
the well-established spectra of iron(IV) oxoporphyrin cation
radicals.42,43 The observed changes can therefore be assigned
to heterolytic scission of the O-O bond to furnish Cpd I (P•+)-
FeIVdO (eq 2).
As illustrated in Figure 3b, the decay of the Soret band for
(TMP)FeIII(O2COAr) at 416 nm occurs with concomitant growth
of a Cpd I feature at 678 nm. Cpd I yields and rate constants
for the heterolytic cleavage were obtained by global analysis
of the full spectral window (380-700 nm). Both the growth
and decay are simultaneously fit well by a first-order exponential
equation with kobsd ) (6.5 ( 1.0) × 10-3 s-1 (at [m-CPBA] )
3.4 × 10-5 M), indicating that heterolytic cleavage to generate
(TMP•+)FeIVdO occurs without intermediates. The observed
rate constant concurs with a previously reported rate constant
for the same reaction.28
In contrast to TMP Cpd I formation, the decrease in the
intensity of the Soret band for the Hangman (HPX)FeIII(O2-
COAr) complexes at 416 nm is not solely due to Cpd I
formation, as indicated by the biphasic nature of the time-
resolved intensity at 678 nm (Figure 4b). The full-spectral data
are best fit by an A f B f C kinetic model with two sequential
exponential equations corresponding to two first-order processes.
The spectral changes observed in the A f B phase are consistent
The similarity of these spectra indicates that analogous
acylperoxo products are formed on the Hangman metallopor-
phyrin complexes. However, contrary to previous reports,
substitution to make (TMP)FeIII(O2COAr) is not instantaneous.28
Complete reaction requires >115 s in CH2Cl2 (at [(TMP)-
FeIII(OH)] ) 9.0 × 10-6 M and [m-CPBA] ) 3.4 × 10-5 M)
at -40 °C.
Rigorous exclusion of ambient light is essential for reproduc-
ibility. The formation of (P)FeIII(O2COAr) is accelerated with
increased [m-CPBA] or with exposure to light in CH2Cl2,
presumably due to the in situ formation of trace HCl.41 Similar
axial substitution to form a Hangman analogue (HPX-
CO2Me)FeIII(O2COAr) is faster (∼60 s), whereas formation of
(HPX-CO2H)FeIII(O2COAr) is complete in <10 s under
identical reaction conditions (Figure 2). In sum, these results
indicate an acid-assisted substitution reaction. More importantly,
the trend in substitution HPX-CO2H > HPX-CO2Me > TMP
suggests that the protonic microenvironment about the active
(39) Bruice, T. C. Acc. Chem. Res. 1991, 24, 243-249.
(40) Lee, W. A.; Yuan, L.-C.; Bruice, T. C. J. Am. Chem. Soc. 1988, 110, 4277-
4283.
(42) Groves, J. T.; Haushalter, R. C.; Nakamura, M.; Nemo, T. E.; Evans, B. J.
J. Am. Chem. Soc. 1981, 103, 2884-2886.
(43) Groves, J. T.; Han, Y. In Cytochrome P-450 Structure, Mechanism and
Biochemistry; Ortiz de Montellano, P. R., Ed.; Plenum Press: New York,
1986; pp 3-48.
(41) Exposure of (TMP)FeIII(OH) to ambient light in anhydrous CH2Cl2 gives
quantitative conversion to (TMP)FeIIICl.
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A R T I C L E S
Soper et al.
Figure 3. (a) Formation of Cpd I (TMP•+)FeIVdO (green line) from Os
O heterolysis on 9.0 × 10-6 M (TMP)FeIII(O2COAr) (red line) in CH2Cl2
at -40 °C. (b) Selected stopped-flow data for the disappearance of (TMP)-
FeIII(O2COAr) (red O, λmax ) 416 nm) and concomitant appearance of Cpd
I (TMP•+)FeIVdO (green O, λmax ) 678) at [m-CPBA] ) 3.5 × 10-5 M.
Figure 4. (a) Formation of Cpd I (HP•+XsCO2H)FeIVdO (green line)
from OsO heterolysis on 9.0 × 10-6 M (HPXsCO2H)FeIII(O2COAr) (red
line) in CH2Cl2 at -40 °C. (b) Selected stopped-flow data for the
disappearance of (TMP)FeIII(O2COAr) (red O, λmax ) 416 nm) and
concomitant appearance of Cpd I (TMP•+)FeIVdO (green O, λmax ) 678)
at [m-CPBA] ) 1.0 × 10-4 M. The biphasic data are best fit globally with
two sequential exponential equations where k1 ) kheterolysis ) (1.0 ( 0.2) ×
Global data analysis gives kobsd ) (6.5 ( 1.0) × 10-3 s-1
.
10-2 s-1
.
with heterolytic OsO bond cleavage to generate Cpd I (HP•+X)-
FeIVdO transients, while the B f C phase of the reaction occurs
with a full-spectrum bleach of the Cpd I species. The sequential
nature of the spectral evolution suggests that the decomposition
originates from the high-valent Cpd I intermediate and occurs
at a rate comparable to that for O-O bond heterolysis. Both
heterolysis and subsequent decomposition are accelerated by
increased [m-CPBA] (Table S1). Perhaps surprisingly, the rates
of Cpd I formation are not significantly perturbed by the
substitution of an ester for the acid functional groups on the
Hangman scaffold, with kobsd ) (1.0 ( 0.3) × 10-2 s-1 for
HPX-CO2Me and kobsd ) (6 ( 3) × 10-3 s-1 for HPX-CO2H
at [m-CPBA] ) 3.4 × 10-5 M.
2.7 × 10-5 M), with P ) HPXsCO2Me the ferric N-oxide is
formed in only 24% relative yield (Figure S2); no ferric N-oxide
product is observed in reactions when P ) HPX-CO2H (Figure
6). These results indicate that porphyrin N-oxide formation (an
indirect measure of homolysis and Cpd II yield)28 is significantly
depressed by the Hangman scaffold. For this reason, the N-oxide
spectrum is most easily observed for TMP (Figure 5) as opposed
to Hangman platforms (Figure S2 for HPX-CO2Me and no
spectral change on time scale shown in Figure 6 for HPX-
CO2H).
OsO Bond Homolysis. In toluene, generation of (TMP)-
FeIII(O2COAr) at -20 °C is immediately followed by the
formation of a new species with a red-shifted Soret band at 441
nm (Figure 5a) that is identical to the previously reported
spectrum of a ferric porphyrin N-oxide complex.28,43 Formation
of the ferric porphyrin N-oxide product apparently results from
an OsO bond homolysis process where the initially formed
Cpd II (TMP)FeIVdO is rapidly consumed in a subsequent
reaction with excess m-CPBA.28 However, under the same
reaction conditions ([FeIII] ) 9.0 × 10-6 M and [m-CPBA] )
A more direct measure of Cpd II yield is obtained from
reactions of (P)FeIII(OH) with 1.2 equiv of phenylperoxyacetic
acid (PPAA) at -40 °C ([FeIII] ) 9.0 × 10-6 M and
[PPAA] ) 1.1 × 10-5 M). Hydroxide exchange by peracetate
to make (P)FeIII(O2COCH2Ph) is complete in seconds. When
P ) TMP or HPXsCO2Me, the substitution is immediately
followed by the formation of a new compound with a Soret
band at 416 nm and a Q-band at 540 nm that match exactly the
(44) Cheng, R.; Latos-Grazynski, L.; Balch, A. L. Inorg. Chem. 1982, 21, 2412-
2418.
(45) Calderwood, T. S.; Bruice, T. C. Inorg. Chem. 1985, 25, 3722-3724.
9
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Proton-Directed Redox Control of O−O Bond Activation
A R T I C L E S
Figure 5. (a) Stopped-flow UV-vis spectra from reaction of m-CPBA (2.7 × 10-5 M) with (TMP)FeIII(OH) (9.0 × 10-6 M) (red line) in toluene at
-20 °C to generate a ferric N-oxide, homolysis product (blue line). (b) Selected data for the disappearance of (TMP)FeIII(OH) (red O, λmax ) 416 nm) and
simultaneous appearance of the N-oxide product (blue O, λmax ) 441 nm). As indicated in heading, formation of the N-oxide is greatly suppressed when
mesityl is replaced by a Hangman scaffold.
bands in the well-known spectrum of Cpd II (P)FeIVdO, the
product of OsO bond homolysis (eq 3).28,29,44,45
materials with m-CPBA give exclusively the expected even-
electron heterolysis Cpd I product, as shown in pathway b.
However, generation of Cpd I in these conditions is not
kinetically more facile for the (HPX-CO2H)FeIII(OH) reactant
as compared to (P)FeIII(OH) (P ) TMP or HPX-CO2Me). The
pseudo first-order rate constants for O-O bond heterolysis of
the bound peroxide are the same within error for all porphyrin
macrocycles investigated here. In contrast, formation of Cpd II
products by a single-electron homolysis reaction, pathway c, is
strongly affected by a proximal H+ donor. Under conditions
that are known to favor peroxide O-O bond homolysis,
generation of Cpd II products is strongly disfavored in Hangman
metalloporphyrin systems vs the parent TMP complex. No O-O
homolysis products are observed in reaction with m-CPBA or
PPAA in toluene when P ) HPX-CO2H. While the ultimate
fate of the Hangman products in these reactions has not been
established, the presence of an H+ donor pendent group clearly
introduces a kinetic barrier to the odd-electron reaction pathway.
Full-spectral fitting indicates the Cpd II material is generated
in ∼70% yield at 60 s, and isosbestic features at 523 and 560
nm indicate that OsO homolysis occurs without the intervention
of intermediates on the time scale of the stopped-flow experi-
ment (Figure 7). Treatment of the acid Hangman complex
(HPXsCO2H)FeIII(OH) with PPAA shows only a very slow
full-spectral bleach. No Cpd II homolysis products are observed.
This is not due to an inherent instability of the homolysis
product, as Cpd II (HPX-CO2H)FeIVdO has been previously
generated under different reaction conditions.37 In sum, these
results suggest that the acid-base residue of the Hangman
system is preventing Cpd II formation by inhibition of 1e-
homolytic OsO bond activation.
Discussion
Analysis of cryogenic stopped-flow reactions of (P)FeIII(OH)
with peroxyacids in aprotic solvents shows the influence of an
intramolecular proton donor on reactions at the ferric porphyrin
center in Hangman metalloporphyrin complexes. The acid-base
group is manifest in two ways: (1) Acid-assisted axial ligand
substitution of hydroxide to form the ferric acylperoxo products
(P)FeIII(O2COAr) is accelerated by the acidic environment
proximate to the iron center in the HPX-CO2H complex; (2)
the stopped-flow studies demonstrate the ability of the acidic
“pull” functionality to effect selective redox chemistry on the
heme iron in subsequent O-O bond activation of peroxide.
As summarized in Scheme 2, the effects of a properly oriented
H+-directing group favor heterolytic (2e-) O-O bond cleavage
over competing homolysis (1e-) pathways. This selectivity
appears to have a kinetic basis. Reactions of all the (P)FeIII(OH)
In this context, the high catalytic efficiencies of Hangman
metalloporphyrins as measured by increased TON relative to
the parent TMP analogues in mono-oxygenase and catalase-
type reactions merit evaluation.37,38 Three factors may rationalize
this superior activity of Hangman porphyrin systems: (A) A
properly positioned Hangman carboxylic acid residue may
increase the rate of OsO heterolysis and Cpd I formation in a
rate-determining peroxide activation. (B) The Hangman por-
phyrin catalyst is inherently more stabile toward destructive self-
oxidation by the high-valent (P•+)FeIVdO transients and in-
creased catalyst lifetime leads to increased TON. (C) The
Hangman architecture impacts the ratio of productive heterolytic
OsO activation to homolytic OsO scission that generates (P)-
FeIVdO materials. In scenario C, the Cpd II products are poor
9
J. AM. CHEM. SOC. VOL. 129, NO. 16, 2007 5073

A R T I C L E S
Scheme 2a
Soper et al.
(a) HO2COR’ [R’ = CH2Ph (PPAA), C6H4Cl (m-CPBA)], CH2Cl2 or toluene, -40 °C. (b) R’ = C6H4Cl, CH2Cl2, -40 °C. (c) R’ = CH2Ph, toluene,
-40 °C.
Scheme 3
Figure 6. Formation of ferric N-oxide products (with λmax ) 441 nm) for
P ) TMP (O), HPX-CO2Me (0), and HPX-CO2H (4) at [FeIII] ) 9.0 ×
10-6 M and [m-CPBA] ) 2.7 × 10-5 M at -20 °C in toluene.
re-engineer myoglobin perturb the ratios of heterolytic to
homolytic O-O bond cleavage in peroxide activation.17,19,32
Their work suggests that precise positioning of a distal histidine
residue near the iron active site is essential for enzymatic O-O
heterolysis to make a peroxidase-like Cpd I species. Similarly,
we show here that the positioned acidic functional group near
a ferric heme center effects redox partitioning in O-O bond
activation by favoring a proton-coupled 2e- reaction over a
competing 1e- pathway. The demonstration of such proton-
directed 2e- vs 1e- redox control in a small-molecule model
system is key to future advances in the basic science of
multielectron redox chemistry, and especially reactivity pertain-
ing to O-O bond formation and breaking reactions. To this
end, the work has consequence to bioenergy conversion
schemes46-51 and points the way to the design of O-O bond
breaking mimics (cytochrome c oxidase) and O-O bond making
mimics (artificial photosynthetic systems).
oxidants and are outside the catalytic cycle. The stopped-flow
kinetics results described here strongly support scenario C over
A and B and accordingly lead to the cycle shown in Scheme 3
where Cpd II formation represents a catalyst deactivation
pathway. The stopped-flow experiments establish that OsO
heterolysis of the peroxide, and Cpd I formation is not
accelerated by the acid-base hanging group, disfavoring A. The
Hangman system is in fact less stable toward decomposition
under the reaction conditions, suggesting that B is unlikely.
However, the stopped-flow studies do show that the positioned
acid group in HPX-CO2H disfavors a homolytic OsO cleavage
pathway and Cpd II formation. This implies that redox selectiv-
ity in OsO bond activation is in fact more important that the
stability of the catalyst itself. By removing a competing pathway
for OsO bond activation, the Hangman catalysts maintain a
lower concentration of catalytically inactive Cpd II oxidant. The
HPXsCO2H complex thus exhibits high turnover numbers and
consequently is a more efficient catalyst, despite its proclivity
toward self-oxidation.
(46) Dempsey, J. L.; Esswein, A. J.; Manke, D. R.; Rosenthal, J.; Soper, J. D.;
Nocera, D. G. Inorg. Chem. 2005, 44, 6879-6892.
(47) Eisenberg, R.; Nocera, D. G. Inorg. Chem. 2005, 44, 6799-6801.
(48) Chang, C. J.; Chang, M. C. Y.; Damrauer, N. H.; Nocera, D. G. Biophys.
Biochim. Acta 2004, 1655, 13-28.
(49) Reece, S. Y.; Hodgkiss, J. M.; Stubbe, J.; Nocera, D. G. Philos. Trans. R.
Soc. London, Ser. B 2006, 361, 1351-1364.
(50) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729-
15735.
Proton-directed redox specificity for multielectron reactivity
of the Hangman metalloporphyrins nicely complements the
experiments of Watanabe and co-workers where mutations to
(51) Nocera, D. G. Dædalus 2006, 135, 112-115.
(52) Kadish, K. M; Smith, K. M.; Guilard, R. The Porphyrin Handbook;
Academic Press: San Diego, CA, 2000; and references therein.
(53) Silbert, L. S.; Siegel, E.; Swern, D. J. Org. Chem. 1962, 27, 1336-1342.
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5074 J. AM. CHEM. SOC. VOL. 129, NO. 16, 2007

Proton-Directed Redox Control of O−O Bond Activation
A R T I C L E S
FeIII(OH),52 (HPX-CO2H)FeIII(OH),37 and (HPX-CO2Me)FeIII(OH)37
were prepared according to published methods. 3-Chloroperoxybenzoic
acid (m-CPBA) was purchased from Aldrich (77%) and purified by
washing with pH 7.40 phosphate buffer and recrystallization from
pentane to remove 3-chlorobenzoic acid. Purity (>95%) was determined
1
by H NMR.
Phenylperoxyacetic acid (PPAA) was prepared by adapting a
literature procedure.53 Dropwise addition of 50% H2O2 (3.0 mL, 90
mmol) to a vigorously stirring solution of 0.31 M phenylacetic acid
(4.08 g, 30 mmol) in methanesulfonic acid (9.7 mL, 150 mmol) gave
a white precipitate. The temperature was maintained below 28 °C in a
water bath during H2O2 addition and then heated to 30 °C for 40 min.
The slurry was cooled to 10 °C, and ∼15 mg of ice were added. The
solids were collected by filtration and washed with 40 mL of ice-cold
water to obtain phenylperoxyacetic acid (PPAA) (4.18 g, 91%). The
crude material was purified by washing with phosphate buffer and
recrystallized from pentane to remove residual acid. Purity (>95%)
Figure 7. UV-vis spectra of (TMP)FeIII(O2COCH2Ph) (red line) and Cpd
II (TMP)FeIVdO (blue line) obtained from global analysis (350-700 nm)
of a stopped-flow reaction of 9.0 × 10-6 M (TMP)FeIII(OH) with 1.1 ×
10-5 M PPAA in toluene at -40 °C. No Cpd II homolysis products are
observed for (HPX-CO2H)FeIII(OH).
1
was checked by H NMR.
Stopped-Flow Kinetics. In a representative procedure, a gastight
syringe was charged with 1.8 × 10-5 M (TMP)FeIII(OH) in toluene
(25 mL, 0.45 µmol). A second gastight syringe was charged with 7.0
× 10-5 M m-CPBA in toluene (25 mL, 1.75 µmol). Precautions were
taken to exclude ambient light rigorously. The solutions were loaded
into the stopped-flow spectrophotometer and triggered by simultaneous
injection of 0.1 mL of each solution to make the reactant concentrations
9.0 × 10-6 M and 3.5 × 10-5 M in iron and m-CPBA, respectively.
The solutions were cooled to -40 °C prior to mixing and maintained
at that temperature in a chilled heptane bath throughout the reaction.
The reaction was monitored by UV-vis spectroscopy. Spectra (350-
700 nm) were acquired for 100 s at 0.5 s intervals. Data were analyzed
as described in Results and Discussion.
Experimental Details
General Considerations. All manipulations were performed under
an inert atmosphere, in nitrogen- or argon-filled glove boxes or using
standard Schlenk techniques, unless otherwise noted. Routine UV-
vis spectra were obtained with a Spectral Instruments SI400 Series diode
array spectrophotometer (350 to 700 nm) at ambient temperature.
Kinetics measurements were made using a Hi-Tech Scientific SF-43
multimixing anaerobic cryogenic stopped-flow instrument equipped
with a Hi-Tech Scientific Kinetascan diode array rapid scanning unit.
Full-spectral kinetics data were fit globally with the commercially
available software Specfit32 from Spectrum Software Associates or at
single wavelengths or with IS-2 Rapid Scanning Kinetic Software (Hi-
Tech Scientific). In all kinetic global analyses, rate constants were
obtained by fitting to an appropriate kinetic model and minimization
without constraints. Multiple starting points for the iterations were
chosen to ensure that minima were not localized. Global fits were
evaluated by rms deviations and comparison of calculated spectra with
experimental data.
Acknowledgment. Financial support for this research was
provided by the National Institutes of Health (GM 47274) and
the U.S. Department of Energy (DOE DE-FG02-05ER15745
(DGN) and NSF CHE-0111202 (EVRA). J.D.S. gratefully
acknowledges support from an NIH Ruth L. Kirschstein
postdoctoral fellowship (GM 69244).
Supporting Information Available: Table S1 and Figures S1
and S2. This material is available free of charge via the Internet
at http://pubs.acs.org
Materials. Solvents were purchased from VWR Scientific Products,
passed through an M. Braun purification system and sparged with N2
to remove trace O2 prior to use. The porphyrin compounds (TMP)-
JA0683032
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