Efficient photodissociation of O2 from synthetic heme and heme/M (M = Fe, Cu) complexes

Article abstract of DOI:10.1021/ja045195f

Single wavelength excitation (λ_ex = 355 or 532 nm) of low-temperature stabilized (198 K) synthetic heme-dioxygen and heme-dioxygen/M complexes, where M = copper or iron in a non-heme environment, results in the dissociation of dioxygen as indicated by the generation of the ferrous heme (Soret band, 427 nm) and the bleaching of the ferric-superoxide (Fe^III(O₂-)) 410-nm Soret band in the transient absorption difference spectrum. Dioxygen rebinds to the four heme complexes studied with comparable rate constants (_～6-9 × 10⁵ M^-1 s^-1). However, the quantum yield for complete dissociation of O2 from our simplest heme-O₂ complex (F₈)Fe^III(O₂-) (φ = 0.60) is higher than the other complexes measured (φ = 0.2-0.3) as well as that for oxy-myoglobin (φ = 0.3). Copyright

Full text of DOI:10.1021/ja045195f

Published on Web 12/04/2004
Efficient Photodissociation of O2 from Synthetic Heme and Heme/M
(M ) Fe, Cu) Complexes
H. Christopher Fry, Paul G. Hoertz, Ian M. Wasser, Kenneth D. Karlin,* and Gerald J. Meyer*
Johns Hopkins UniVersity, Department of Chemistry, 3400 North Charles Street Baltimore, Maryland 21218
Received August 9, 2004; E-mail: meyer@jhu.edu; karlin@jhu.edu
Hemes play a central role in the activation and transport of
dioxygen (O2) in living organisms.1-3 In this regard, study of the
photodissociation of O2 from oxy-hemes has been an important
experimental tool for probing active site structure and reactivity.4
Pioneering investigations by Gibson showed that light excitation
of oxy-myoglobin or hemoglobin resulted in the release of O2 with
low quantum yields.5 Traylor and co-workers later concluded that
upon heme-O2 photolysis the initial quantum yield is unity and
that geminate recombination of O2 with the ferrous heme accounted
for the low yield measured on long time scales.6 However, recent
ultrafast studies on oxy-myoglobin cast serious doubt on this
conclusion7,8 and reveal a low intrinsic quantum yield of 0.3 at the
earliest observation times.7,9 The elucidation of factors that control
O2 photodissociation and rebinding remains of great interest.
Different O2-binding modes10,11 and/or a distribution of protein
Figure 1. Time-resolved absorption spectra recorded after pulsed 532-nm
light excitation of (F8)FeIII(O2-) in THF at 198 K. The data are shown at
confirmations12,13 have been discussed in these regards.
delay times of 10 ns (9), 100 µs (b), 200 µs (2), and 300 µs ([). A
In principle, spectroscopic studies of well-characterized synthetic
spectrum simulated as (Abs[(F8)FeII(thf)2] - Abs[(F8)FeIII(O2-)]) is overlaid
oxy-hemes in fluid solution could provide valuable insight into the
factors that govern O2-binding and dissociation. In the past,
photogeneration of O2 from synthetic “picket-fence” oxy-hemes has
focused upon axial ligand effects on the bimolecular O2-binding
step14,15 and geminate recombination of O2.16 We find that pho-
tolysis of synthetic heme-O2 adducts (formally FeIII-superoxo
(O2-) complexes) in tetrahydrofuran (THF) at 198 K produces free
O2 with quantum yields comparable to that known for oxy-
myoglobin. Furthermore, utilizing a simple, sterically unhindered
tetraphenyl porphyrin leads us to a complex possessing the highest
quantum yield measured (for complete O2 dissociation), φ ) 0.60.
The compound (F8)FeII (Chart 1), as a bis-THF adduct, (F8)FeII-
(thf)2, (F8 ) tetrakis(2,6-difluorophenyl)-porphyrinate-(2-)), has
previously been synthesized and structurally characterized.17-19 It
reacts with dioxygen below -40 °C in tetrahydrofuran (THF) to
form an adduct, well described as a heme-superoxo compound,
(F8)FeIII(O2-), ν(O-O) ) 1178 cm-1 (∆(18O2) ) -64 cm-1),
having electronic absorptions at 416 nm (Soret band) and 536 nm
(Q-band).18,20 Closely related heme-O2 adducts can be generated
in alternative environments. Thus, low-temperature oxygenation was
also used to generate (6L)FeIII(O2-) where 6L is 5-(ortho-O-[N,N,-
bis(2-pyridymethyl)-2-(6-methoxyl)pyridinemethanamine]-phenyl)-
10,15,20-tris(2-(2,6-di-fluorophenyl))porphine21 as well as bimetallic
compounds with a superstructured neighboring iron(II)22 or copper-
(I)23 ion coordinated to the tether arm of (6L)FeII (Chart 1).24
Pulsed 355- or 532-nm light (fwhm 8-10 ns; 5 mJ/pulse)
excitation of (F8)FeIII(O2-) under 1 atm of O2 in THF at 198 K
resulted in the immediate appearance of the absorption difference
spectrum shown in Figure 1.25 This reveals a bleach of the
absorption bands associated with the heme-superoxo compound
and the appearance of new absorption bands with λmax ) 427 and
560 nm (Figure 1). This transient spectrum corresponds well with
that calculated for (Abs[(F8)FeII(thf)2] - Abs[(F8)FeIII(O2-)]),
indicating that the new intermediate formed is the ferrous solvento
in red. The inset shows the observed rate constant as a function of the
dioxygen concentration leading to kO ) 5.7 × 105 M-1 s-1
.
2
Chart 1
compound, (F8)FeII(thf)2, and O2 has been released (Scheme 1). If
superoxide (O2-) had been photoreleased, the absorption difference
spectra of the resulting ferric heme would be present, contrary to
what was observed.26
The coordination of O2 to the reduced heme takes place on a
time scale amenable to nanosecond flash photolysis measurements.
Observed rate constants for O2 rebinding (Scheme 1), kobs, under
pseudo first-order conditions (excess O2), were independent of the
monitoring wavelength, the [(F8)FeIII(O2-)] concentration (0.2-
6.0 µM), and the laser excitation energy (5-10 mJ cm-2) under
9
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J. AM. CHEM. SOC. 2004, 126, 16712-16713
10.1021/ja045195f CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
Acknowledgment. This work was supported by the National
Science Foundation (CRAEMS, K.D.K. and G.J.M.) and the
National Institutes of Health (K.D.K.).
Supporting Information Available: Figures of wavelength-
independent O2-rebinding and difference spectra. Description of
comparative actinometry calculations. This material is available free
of charge via the Internet at http://pubs.acs.org.
Table 1. O2-Rebinding Rate Constants (kO and Corresponding
Quantum Yields (φ) for the Various Synthet2ic Heme Complexes in
THF at 198 Ka
1
kO2 (M-1 s-
)
φ
References
complex
-
(F8)FeIII(O2-
)
)
5.7 × 105
6.4 × 105
6.8 × 105
9.0 × 105
0.60 ( 0.02
0.22 ( 0.03
0.34 ( 0.04
0.18 ( 0.02
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(6L)FeIII(O2
[(6L)FeIII(O2-)CuI]+
[(6L)FeIII(O2-)FeII(Cl)]+
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a The viscosity of THF at 198 K is 2.1 centipoise.32
all conditions studied (Figure S1).27 At low [O2] concentrations
(1-5 mM), the process was also shown to be first-order, and a
plot of kobs as a function of the dioxygen concentration (Figure 1
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rebinding to (F8)FeII(thf)2 of kO ) 5.7 ( 0.5 × 105 M-1 s-1 at 198
2
(9) Related femtosecond analysis of MbNO photolytic processes affords a
greater yield (0.8-1.0) than previously measured on a picosecond time
scale (0.5); ref 7. Zemojtel, T.; Rini, M.; Heyne, K.; Dandekar, T.;
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K.28
The other heme dioxygen complexes, (6L)FeIII(O2-),21 [(6L)FeIII-
(O2-)FeII(Cl)]+,22 and [(6L)FeIII(O2-)CuI]+ 23, were also studied to
examine whether the presence of a Lewis base or a second metal
center affects the O2-photodissociation quantum yield or rebinding
kinetics.29 The photogenerated transient spectra were within
experimental error, the same as that observed after excitation of
(F8)FeIII(O2-) (Figure S2).27 Small, but measurable, changes in the
observed rate constant for dioxygen coordination were observed
(Table 1).
(10) Watanabe, T.; Ama, T.; Nakamoto, K. J. Phys. Chem. 1984, 88, 440-
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(11) De Angelis, F.; Car, R.; Spiro, T. G. J. Am. Chem. Soc. 2003, 125, 15710-
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The most striking data were the differences in the quantum yield
(φ) for photorelease of O2, determined by comparative actinometry
(Table 1).27 In all cases, the quantum yields were measured on a
10 ns, µs, and ms (10-9 - 10-3 s) time scale. To our knowledge,
the φ ) 0.60 ( 0.02 after light absorption by (F8)FeIII(O2-) is the
highest ever measured by these methods. Such a high yield can be
attributed to the sterically unhindered nature of the porphyrin
allowing solvent to easily coordinate to the heme and thus efficiently
displace O2. The φ ) 0.22 ( 0.03 for (6L)FeIII(O2-) is remarkably
similar to that measured for oxy-myoglobin, which also has a ligated
strong (i.e., pyridine or imidazole) base axial base.7 The O2 quantum
yield of the heme/non-heme compound was not dramatically
different (Table 1), perhaps indicating that O2 was released from
the porphyrin face opposite that of the non-heme FeII moiety (Chart
1).30 The CuI containing analogue [(6L)FeIICuI]+ is of direct
relevance to cytochrome c oxidase model chemistry where the role
of the copper center in dioxygen activation is of considerable
interest.3,31 It is tempting to suggest that the improved quantum
yield for photodissociation in [(6L)FeIII(O2-)CuI]+ compared to
(6L)FeIII(O2-) stems from O2-coordination to copper; however,
additional spectroscopic studies on shorter time scales are necessary
to fully elucidate the mechanism(s).
(17) Obias, H. V.; van Strijdonck, G. P. F.; Lee, D.-H.; Ralle, M.; Blackburn,
N. J.; Karlin, K. D. J. Am. Chem. Soc. 1998, 120, 9696-9697.
(18) Ghiladi, R. A.; Kretzer, R. M.; Guzei, I.; Rheingold, A. L.; Neuhold, Y.-
M.; Hatwell, K. R.; Zuberbu¨hler, A. D.; Karlin, K. D. Inorg. Chem. 2001,
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R. A.; Lam, K.-C.; Rheingold, A. L.; Karlin, K. D.; Meyer, G. J. Inorg.
Chem. 2003, 42, 5211-5218.
(20) Kim, E.; Helton, M. E.; Wasser, I. M.; Karlin, K. D.; Lu, S.; Huang, H.
W.; Moenne-Loccoz, P.; Incarvito, C. D.; Rheingold, A. L.; Honecker,
M.; Kaderli, S.; Zuberbu¨hler, A. D. Proc. Natl. Acad. Sci. U.S.A. 2003,
100, 3623-3628.
(21) Ghiladi, R. A.; Karlin, K. D. Inorg. Chem. 2002, 41, 2400-2407.
(22) Wasser, I. M.; Huang, H.-w.; Moenne-Loccoz, P.; Karlin, K. D. Submitted
for publication, 2004.
(23) Ghiladi, R. A.; Ju, T. D.; Lee, D.-H.; Moe¨nne-Loccoz, P.; Kaderli, S.;
Neuhold, Y.-M.; Zuberbu¨hler, A. D.; Woods, A. S.; Cotter, R. J.; Karlin,
K. D. J. Am. Chem. Soc. 1999, 121, 9885-9886.
(24) An alternative photochemical method was sometimes used for in situ
generation of the heme-superoxo/non-heme compound. Prolonged pho-
tolysis of the heme/non-heme oxo-bridged compound under 1 atm of
dioxygen in THF at 198 K produced UV-visible absorption spectra
consistent with complete generation of the heme-superoxo/non-heme
compound.
(25) Steady-state UV-vis absorption measurements before and after laser
excitation showed no evidence for permanent photochemistry.
(26) Hoshino, M.; Baba, T. J. Am. Chem. Soc. 1998, 120, 6820-6821.
(27) See Supporting Information.
In conclusion, we have shown that synthetic heme-O2 adducts
photorelease O2 with quantum yields very close to the value known
for oxy-myoglobin.7 At a minimum, this indicates that the protein
matrix and structure of the natural heme pocket are not strict
requirements for modeling oxy-myoglobin photoreactivity. In
addition, we report the highest quantum yield for complete dioxygen
release for any oxy-heme ever measured. This finding opens the
door toward the use of other synthetic hemes, with tailored
electronic and steric properties, for further fundamental studies and
for possible applications, e.g., in therapeutic controlled photorelease
of dioxygen.33,34
(28) At 298 K other synthetic hemes have been shown to bind dioxygen with
rate constants of 108 M-1 s-1; see refs 14 and 15.
(29) The superoxide species, [(6L)FeIII(O2-)CuI]+, is stable at 198 K but
converts to the peroxo species, [(6L)FeIII(O22-)CuII]+, upon warming.
(30) Dioxygen is not known to bind to the non-heme portion of this complex.
(31) Kim, E.; Chufan, E. E.; Kamaraj, K.; Karlin, K. D. Chem. ReV. 2004,
104, 1077-1133.
(32) Yaws, C. L. Handbook of Transport Property Data; Gulf Publishing:
Houston, TX, 1995.
(33) MacArthur, R.; Sucheta, A.; Chong, F. F. S.; Einarsdo´ttir, O. Proc. Natl.
Acad. Sci. U.S.A. 1995, 92, 8105-8109.
(34) Van Eps, N.; Szundi, I.; Einarsdottir, O. Biochemistry 2000, 39,
14576-14582.
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