Laser photolysis of iron(III) tetraphenylporphyrin in methanol. A kinetic study on the formation of the superoxide anion radical from the dioxygen adduct of iron(II) tetraphenylporphyrin

Full text of DOI:10.1021/ja9806783

6820
J. Am. Chem. Soc. 1998, 120, 6820-6821
Laser Photolysis of Iron(III) Tetraphenylporphyrin
in Methanol. A Kinetic Study on the Formation of
the Superoxide Anion Radical from the Dioxygen
Adduct of Iron(II) Tetraphenylporphyrin
Mikio Hoshino* and Tomoya Baba
The Institute of Physical and Chemical Research
Wako, Saitama 351, Japan
ReceiVed March 2, 1998
Chemical reactions of synthetic iron(II) porphyrins and dioxy-
gen have attracted much attention because of their importance as
model systems for understanding the roles and functions of natural
hemeproteins in vivo.1-9 Earlier studies of these reactions have
mainly focused on the reversible binding of dioxygen by iron(II)
porphyrins,10-15 and less attention has been paid to the formation
Figure 1. Transient absorption spectra observed for the methanol solution
of 1.0 × 10-5 M ClFeIIITPP saturated with oxygen gas at 1 atm, at 50 ns
(open circle) and 10 µs (closed circle) after the 355-nm laser pulse. The
decay of the transient monitored at 425 nm is shown in the inset.
-•
of O2 from the reaction of synthetic iron(II) porphyrins and
dioxygen.
-• 28-30
In 1933, neutrophils were found to exhibit a “respiratory burst”
slowly, reduce dioxygen to yield O2
.
Thus, studies on the
upon exposure to certain stimuli, producing the potentially
O2-• production by iron(II) porphyrins are important for elucida-
tion of the reduction mechanism of dioxygen by ferro-hemepro-
teins in vivo. The present kinetic study demonstrates that FeIITPP
(TPP ) tetraphenylporphyrin) produced by laser photolysis of
ClFeIIITPP in methanol reacts with dioxygen to give the dioxygen
-• 16,17
microbicidal reagent, O2
.
Subsequent studies revealed that
the O2-• producing enzyme in neutrophils contains the hemepro-
tein, cytochrome b558, which plays an essential role in the catalytic
reduction of dioxygen at the expense of NADPH.17-27 The
-•
-•
formation of O2 has also been observed in the autoxidation
adduct, (O2)FeIITPP, which dissociates O2 to regenerate
processes of ferro-myoglobin and -hemoglobin, which, although
FeIIITPP+.
Continuous photolysis of ClFeIIITPP in methanol was carried
out by a 250 W mercury lamp with a cutoff filter (λ > 330 nm).
In the aerated methanol solution, ClFeIIITPP was slightly photo-
decomposed. However, the degassed methanol solution quanti-
tatively yields (CH3OH)2FeIITPP upon irradiation.31
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The spectrum of (CH3OH)2FeIITPP in methanol exhibits an
absorption peak at 425 nm (ꢀ ) 2.26 × 105 M-1 cm-1). When
the methanol solution was exposed to air, the spectrum of
(CH3OH)2FeIITPP instantaneously returned to that of ClFeIIITPP
in methanol measured before irradiation.
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The photochemical formation of FeIITPP was again confirmed
by the 355-nm laser photolysis of ClFeIIITPP in degassed
methanol, i.e., the transient spectrum measured at 50 ns after the
pulse has a positive peak at 425 nm and a negative one at 405
nm, in good agreement with the difference spectrum (FeIITPP
minus ClFeIIITPP). The quantum yield for the formation of
FeIITPP is ca. 0.02. FeIITPP produced in degassed methanol was
found to exhibit neither a decay nor a rise over a few milliseconds
after the laser pulse.
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an oxygen-saturated methanol solution of 1.0 × 10-5
M
ClFeIIITPP at 50 ns and 10 µs delay after a 355-nm laser pulse.
The 50-ns spectrum is identical with that observed for the
degassed methanol solution, indicating the formation of FeIITPP.
The decay of FeIITPP in oxygen-saturated methanol was inves-
tigated by monitoring the absorbance change at 425 nm. As
shown in the inset of Figure 1, the absorbance change, ∆D, at
425 nm decreases with time and levels off within several
microseconds after the laser pulse. This result is best interpreted
in terms of the reaction of FeIITPP and O2, i.e., the FeIITPP reacts
with oxygen to produce the dioxygen adduct of FeIITPP,
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S0002-7863(98)00678-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/26/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 27, 1998 6821
The formation mechanism of the dioxygen adduct described
above is again supported kinetically. From eq 1, the initial
absorbance change, ∆D(t)0), is expressed as ∆D(t)0) ) ∆D0
+ ∆D∞. The ratio, Γ(O2) ) ∆D∞/∆D(t)0), is a function of the
oxygen concentration. Equations 2 and 3 give an expression for
Γ(O2), i.e.,
Γ(O2) ) (1 + γK[O2])/(1 + K[O2])
(6)
where γ is the value of Γ(O2) at an “infinite” concentration of
oxygen.
Figure 2. The plots of the decay rate constants (open circle), k, and the
Γ(O2) values (closed circle) represented as a function of the oxygen
concentration (see text). The solid curves of k and Γ(O2) are, respectively,
calculated with the use of eqs 8 and 10: parameters used are kf ) 1.25
× 106 s-1, kd ) 1.0 × 105 s-1, and kc/kb ) 3.6 × 102 M-1 for k and γ )
0.35 and K ) 4.5 × 103 M-1 for Γ(O2).
As shown in Figure 2, the value of Γ(O2) measured in the range
0 < [O2] < 8.5 × 10-3 M decreases with an increase in [O2] to
attain a constant value at the oxygen concentration higher than
ca. 4 × 10-3 M. The plot of Γ(O2) vs [O2] is well reproduced
by eq 6 with the use of γ ) 0.35 and K ) 4.5 × 103 M-1. The
fact that the equilibrium constant obtained from eq 6 is identical
with that from eq 5 supports the mechanism of the reversible
formation of (CH3OH)(O2)FeIITPP mentioned above.
(CH3OH)(O2)FeIITPP, which gives the 10-µs spectrum. The 10-
µs spectrum having a positive peak at 420 nm eventually
disappeared according to first-order kinetics over a few hundred
Continuous photolysis of ClFeIIITPP in methanol reveals that
(CH3OH)2FeIITPP reacts with oxygen and eventually returns to
the iron(III) porphyrin. This result suggests that the dissociation
milliseconds after the pulse: the rate constant is 14 s-1
.
The decay of the absorbance change, ∆D, at 425 nm is
expressed by
-•
of O2 occurs from (O2)FeIITPP.
∆D ) ∆D0 exp(-kt) + ∆D∞
(1)
(CH3OH)(O2)FeIITPP + CH3OH f
The plot of ln(∆D - ∆D∞) vs t gives a straight line with a slope
k. The rate constants, k, were measured as a function of the
oxygen concentration, [O2].
Figure 2 shows the plot of k vs [O2]. The value of k
asymptotically approaches to a limiting value with an increase
in [O2]. We, therefore, consider that the dioxygen adduct,
(CH3OH)(O2)FeIITPP, is produced according to the following
reaction mechanism.
(CH3OH)(CH3O-)FeIIITPP + O2-• + H+ (7)
-•
Detection of O2 produced by autodissociation from the
dioxygen adduct, (CH3OH)(O2)FeIITPP, was made by the spin
trapping method. The aerated methanol solution of 1.0 × 10-5
M (Cl)FeIIITPP and 1.0 × 10-2 M DMPO (5,5′-dimethyl-1-
pyroline N-oxide) was irradiated by the mercury lamp with the
cutoff filter (λ > 330 nm). The ESR spectrum of the DMPO
R
adduct thus obtained exhibits AN ) 13.5 ( 0.3 and AH ) 10.0
-•
( 0.3, in good agreement with those reported for the O2 (or
(2)
(3)
HOO•) adduct of DMPO in methanol.33
The laser photolysis studies of ClFeIIITPP in oxygen-saturated
methanol revealed that the dioxygen adduct,(CH3OH)(O2)FeIITPP,
slowly decays with the first-order rate constant of 14 s-1. The
rate constant was found to be almost independent of [O2] in the
concentration range [O2] > 1.0 × 10-3 M. This finding implies
that the rate for the dissociation of O2-• is much slower than that
for achieving the equilibrium shown by eqs 4 and 5. The rate
constant is four or 5 orders of magnitude larger than those
measured for the dioxygen adducts of native, mutant, and
tetrazorium-modified myoglobins in aqueous solution.28-30,34
When the steady-state approximation is taken with regard to
(CH3OH)FeIITPP, the rate constant, k, for the decay of
(CH3OH)2FeIITPP is formulated as
An outer-sphere electron-transfer reaction from reduced hemes
to dioxygen has been assumed for the oxidation of cytochrome
35
b558
.
The oxidation mechanism of myoglobin by dioxygen has
kfkc[O2] + kbkd
been explained by a combination of an outer-sphere electron-
transfer reaction from myoglobin to dioxygen and a unimolecular
ionic dissociation of O2-• from oxymyoglobin.29,34 In the present
study, we conclude that the outer-sphere electron transfer does
not take place from both (CH3OH)2FeIITPP and (CH3OH)FeIITPP
to dioxygen because of the facts that (1) the rate constant k
increases and asymptotically levels off with an increase in [O2]
and (2), unlike the oxidation of myoglobin, the rate constant k
does not exhibit a “bell-shaped” O2 concentration dependence.29
k )
(4)
kb + kc[O2]
The plot of k vs [O2] in Figure 2 gives kf ) 1.25 × 106 s-1, kd )
1.0 × 105 s-1, and kc/kb ) 3.6 × 102 M-1. The equilibrium
constant, K, for the formation of (CH3OH)(O2)FeIITPP in methanol
is given by
K ) kf kc/kbkd ) 4.5 × 103 M-1
(5)
Further studies are in progress.
This value is in moderate agreement with those obtained for the
formation of the dioxygen adducts of manganese(II) tetra-
phenylporphyrin32 and iron(II) picket fence porphyrins2 in toluene
solutions.
JA9806783
(33) Harbor, J. R.; Hair, M. L. J. Phys. Chem. 1978, 82, 1397-1399.
(34) Wallace, W. J.; Houtchens, R. A.; Maxwell, J. C.; Caughey, W. S. J.
Biol. Chem. 1982, 257, 4966-4977.
(35) Isogai, Y.; Iizuka, T.; Shiro, Y. J. Biol. Chem. 1995, 270, 7853-
7857.
(32) Hoshino, M.; Nagashima, Y.; Seki, H.; DeLeo, M. D.; Ford, P. C.
Inorg. Chem. Submitted for publication.