6142
Inorg. Chem. 1997, 36, 6142-6143
Imidazole and p-Nitrophenolate Complexes of Oxoiron(IV) Porphyrin π-Cation Radicals as Models for
Compounds I of Peroxidase and Catalase
Hiroshi Fujii,* Tetsuhiko Yoshimura, and Hitoshi Kamada
Institute for Life Support Technology, Yamagata Technopolis Foundation, Matsuei, Yamagata 990, Japan
ReceiVed March 6, 1997
The involvement of oxoiron(IV) porphyrin π-cation radical
species as intermediates in catalytic cycles of peroxidases,1
catalases,2 and cytochrome P-450s, is well-known.3 For the case
of peroxidases and catalases, an oxoiron(IV) porphyrin π-cation
radical, called compound I, has been identified as a reactive
intermediate.1 A compound I species has also been proposed
as the reactive oxygenating intermediate of cytochrome P-450.3
Interestingly, in spite of the fact that compound I is common
to a series of enzymes, the reactivity of the compound I species
differs from one enzyme to another. In cytochrome P-450, the
compound I species catalyzes the direct transfer of a single
oxygen atom to a variety of substrates,3,4 while for the case of
peroxidase and catalase, it catalyzes the oxidation of the organic
compounds and hydrogen peroxide, respectively.1,2,5 These
diverse functions are generally thought to depend on heme
structures, such as, for example, porphyrin peripheral structures
and the heme proximal ligand, as well as protein structures in
the immediate vicinity of the heme. Indeed, different proximal
ligands in these enzymes (e.g., imidazole in peroxidase,
phenolate in catalase, and thiolate in cytochrome P-450) suggest
that the proximal ligand structure controls the reactivity of
compound I species.
While a few reports on the effect of the axial ligand of
oxoiron(IV) porphyrin π-cation radicals have appeared,6 there
are no reported oxoiron(IV) porphyrin π-cation radical com-
plexes having imidazole and phenolate as axial ligands. We
have prepared imidazole, (P•)FeIVO(Im), and p-nitrophenolate,
(P•)FeIVO(OAr), complexes of an oxoiron(IV) 2,7,12,17-tet-
ramethyl-3,8,13,18-tetramesitylporphyrin π-cation radical.7 These
radical complexes may be more desirable than those of meso-
tetraarylporphyrins as spectroscopic models for biological heme
enzymes because of similarities in the porphyrin structure to
those of naturally occurring compounds.7a
Scheme 1
ligand field.8 Ozone was used as an oxidant, since ozone
oxidation of an iron(III) porphyrin forms only an oxoiron(IV)
porphyrin π-cation radical complex and dioxygen, which is
easily removed by bubbling with nitrogen gas.6,7e
A ferric mono(imidazole) complex, (P)FeIII(Im), was prepared
by addition of 1 equiv of imidazole to (P)FeIII(ClO4) in
dichloromethane.8,9 The absorption spectrum of (P)FeIII(Im),
which has peaks at 385, 503, and 626 nm, closely resembled
that of the resting form of horseradish peroxidase (HRP).10
When (P)FeIII(Im) was oxidized in dichloromethane at -80 °C
by ozone, a green complex was formed. The absorption
spectrum of the green complex showed the characteristic features
of the oxoiron(IV) porphyrin π-cation radical, namely a Soret
band at 390 nm with decreased intensity and a broad band
around 640 nm. The spectral features of the green complex
were not identical with those of the previously characterized
(P•)FeIVO(mCB) (389 and 624 nm) but were close to those of
compound I (402 and 650 nm) of HRP.10 The absorption
spectrum of the green complex was also formed with isosbestic
points when 1 equiv of imidazole was titrated into a perchlorate
oxoiron(IV) porphyrin π-cation radical complex, (P•)FeIVO-
(ClO4), prepared from the oxidation of (P)FeIII(ClO4) by ozone
in dichloromethane at -80 °C. These findings suggest that the
green complex is (P•)FeIVO(Im).
The structure of (P•)FeIVO(Im) was further confirmed by 1H-
and 2H-NMR measurements. Figure 1 shows 1H-NMR spectral
changes on titration of (P•)FeIVO(ClO4) with imidazole in
dichloromethane-d2 at -80 °C. The assignments are based on
selectively deuterated (meso-d4 and imidazole-d3) samples and
the intensities of the signals. As shown in Figure 1a, the pyrrole
â-methyl and meso proton signals of (P•)FeIVO(ClO4) are
observed at 132 and 54 ppm, respectively. The splitting of the
meta proton (15 ppm) and o-methyl (11 ppm) signals is
indicative of two different axial ligands in (P•)FeIVO(ClO4): oxo
We examined the preparation of (P•)FeIVO(Im) by two
alternative routes, shown in Scheme 1. We employed perchlo-
ratoiron(III) 2,7,12,17-tetramethyl-3,8,13,18-tetramesityporphy-
rin, (P)FeIII(ClO4), as the starting complex because the perchlo-
rate ligand is easily displaced by imidazole due to its weak
(1) (a) Dunford, H. B. AdV. Inorg. Biochem. 1982, 4, 41-68. (b) Poulos,
T. L. AdV. Inorg. Biochem. 1988, 7, 1-36.
(2) Schonbaum, G. R.; Chance, B. In The Enzymes; Boyer, P. D., Ed.;
Academic Press: New York, 1976; Vol. 13, pp 363-408.
(3) (a) Oritz de Montellano, P. R. In Cytochrome P-450; Oritz de
Montellano, P. R., Ed.; Plenum Press: New York, 1986; pp 217-
271. (b) Watanabe, Y.; Groves, J. T. In The Enzymes; Sigman, D. S.,
Ed.; Academic Press: San Diego, CA, 1992; Vol. 20, pp 405-452.
(4) Griffin, B. W.; Peterson, J. A.; Estabrook, R. W. In The Porphyrins;
Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. 7, pp 333-
375.
(5) Hewson, W. D.; Harger, L. P. In The Porphyrins; Dolphin, D., Ed.;
Academic Press: New York, 1979; Vol. 7, pp 295-332.
(6) (a) Gross, Z.; Nimri, S. Inorg. Chem. 1994, 33, 1731-1732. (b)
Czarnecki, K.; Nimri, S.; Gross, Z.; Proniewicz, L. M.; Kincaid, J. R.
J. Am. Chem. Soc. 1996, 118, 2929-2935.
(7) (a) Fujii, H. J. Am. Chem. Soc. 1993, 115, 4641-4641. (b) Fujii, H.;
Ichikawa, K. Inorg. Chem. 1992, 31, 1110-1112. (c) Fujii, H. Chem.
Lett. 1994, 1491-1494. (d) Fujii, H.; Yoshimura, T.; Kamada, H.
Inorg. Chem. 1996, 35, 2373-2377. (e) Czarnecki, K.; Proniewicz,
L. M.; Fujii, H.; Kincaid, J. R. J. Am. Chem. Soc. 1996, 118, 4680-
4685.
(8) (a) Quinn, R.; Nappa, M.; Valentain, J. S. J. Am. Chem. Soc. 1982,
104, 2588-2595. (b) Scheidt, W. R.; Geiger, D. K.; Lee, Y. J.; Reed,
C. A.; Lang, G. J. Am. Chem. Soc. 1985, 107, 5693-5699.
(9) Although the formation of (P)FeIII(Im) was confirmed by absorption
1
and H-NMR measurements, the isolation of (P)FeIII(Im) resulted in
a mixture of (P)FeIII(Im)2 and (P)FeIIIClO4. The absorption and 1H-
NMR spectra of (P)FeIII(Im) are shown in the Supporting Information.
(10) Blumberg, W. E.; Peisach, J.; Wittenberg, B. A.; Wittenberg, J. B. J.
Biol. Chem. 1968, 243, 1854-1862.
S0020-1669(97)00271-1 CCC: $14.00 © 1997 American Chemical Society