Imidazole and p-nitrophenolate complexes of oxoiron(IV) porphyrin π-cation radicals as models for compounds I of peroxidase and catalase

Full text of DOI:10.1021/ic970271j

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,
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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

Communications
Inorganic Chemistry, Vol. 36, No. 27, 1997 6143
The temperature dependence of NMR signals of (P•)FeIVO-
(Im) showed normal Curie law behavior from -100 to -40
°C, suggesting that the axial imidazole binds tightly with the
porphyrin iron. From the Curie plots for (P•)FeIVO(Im), we
estimated the pyrrole methyl signals to be 71 ppm at 25 °C,
which is close to the methyl signals (average; 64 ppm at 25
°C) of compound I of HRP.12
The small paramagnetic shift of the meso proton signal of
(P•)FeIVO(Im) indicates an a1u radical state, as has been observed
for (P•)FeIVO(ClO4) and (P•)FeIVO(mCB).7 The NMR spectral
change with imidazole coordination may be explained by a
minor change in spin distribution in the a1u orbital in (P•)FeIVO-
(Im) resulting from vibrational mixing7,13 and/or a withdrawal
of iron(IV) into the porphyrin plane with imidazole coordination.
Although it has been proposed that the HOMO of the compound
I species is altered with coordination of strong donor imidazole
and oxo ligands on the basis of the a2u assignment of compound
I of HRP,14 the present study demonstrates that the HOMO
remains unchanged on coordination with imidazole. Further-
more, all of the present results for (P•)FeIVO(Im) indicate the
a1u radical state of compound I of HRP.7
We also examined the oxidation of (P)FeIII(OAr) with ozone
in dichloromethane at -95 °C. The absorption spectrum of (P)-
FeIII(OAr) (405, 493, 617 nm) is similar to that of the resting
form of catalase (405, 505 and 625 nm).1a,15 On oxidation by
ozone, the absorption spectrum of (P)FeIII(OAr) quickly changed
to a spectrum which had absorption peaks at 392 and 655 nm.
The absorption spectrum was different from those of (P•)FeIVO-
(ClO4), (P•)FeIVO(mCB), and (P•)FeIVO(Im) but closely re-
sembled that of compound I of catalase, which has absorption
peaks at 400 and 662 nm.1a,15 While (P•)FeIVO(Im) was stable
at -80 °C for 1 h, the oxidized complex of (P)FeIII(OAr) was
quite unstable even at -95 °C and decomposed to a ferric
complex within a few minutes showing a new spectrum with
absorption peaks at 387 and 640 nm. We attempted to obtain
an 1H-NMR spectrum of the oxidized complex but failed
because of its short lifetime. Although further work will be
needed to define the structure of the oxidized complex of (P)-
FeIII(OAr), the absorption spectral features imply the formation
of the p-nitrophenolate complex of the oxoiron(IV) porphyrin
π-cation radical, (P)FeIVO(OAr).
Figure 1. 1H-NMR spectra for the titration of (P•)FeIVO(ClO4) with
imidazole at -80 °C in dichloromethane-d2. Concentration of iron
porphyrin: 5.3 mM. Imidazole added: (a) 0.0 equiv; (b) 0.5 equiv; (c)
1.0 equiv; (d) meso-d4 (P•)FeIVO(ClO4) with 1.0 equiv of imidazole;
(e) (P•)FeIVO(ClO4) with 1.0 equiv of imidazole-d3.
and perchlorate anion ligands.11 With the addition of imidazole,
the signals for (P•)FeIVO(ClO4) decreased in intensity and new
signals appeared (Figure 1b,c). The signals for (P•)FeIVO(ClO4)
were nearly displaced by the new signals when 1 equiv of
imidazole was added, and further addition of imidazole caused
reduction to an oxoiron(IV) porphyrin complex. As shown in
Figure 1c,d, the meso proton signal of (P•)FeIVO(Im) is observed
at -1 ppm. The pyrrole â-methyl proton was assigned the
signal at 111 ppm. The m-proton and o-methyl signals of the
pyrrole â-mesityl group were observed at 12 and 7 ppm,
respectively. The splitting of meta proton and o-methyl signals
is also indicative of two different axial ligands: oxo and
imidazole coordination. More unambiguous evidence of (P•)-
FeIVO(Im) is the iron-bound-imidazole signals at -3 and -14
ppm, which was confirmed by using imidazole-d3 as the titrant
(Figure 1e). The assignment of these imidazole signals is also
supported by 2H-NMR measurements. All of these results
indicate the formation of (P•)FeIVO(Im) with the addition of 1
equiv of imidazole to (P•)FeIVO(ClO4). As observed for
We have prepared imidazole and p-nitrophenolate complexes
of oxoiron(IV) porphyrin π-cation radicals and characterized
them using absorption and NMR spectroscopies. Further work,
including stabilization of the phenolate complex and the
reactivity of these complexes, is currently in progress.
Acknowledgment. This work was supported by Grants
09235236 (Molecular Biometallics) and 09740504 from the
Ministry of Education, Science, Sports, and Culture of Japan.
Supporting Information Available: Figure S1, showing absorption
spectra of (P•)FeIVO(Im) and (P•)FeIVO(OAr), Figure S2, showing
absorption spectral changes during the titration of (P)FeIIIClO4 with
imidazole in dichloromethane, and Figure S3, showing 1H-NMR spectra
for the titration of (P)FeIIIClO4 with imidazole in CD2Cl2 at 24 °C (3
pages). Ordering information is given on any current masthead page.
IC970271J
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absorption spectral measurements, the H-NMR spectrum of
(P•)FeIVO(Im) was also observed when (P)FeIII(Im) was oxidized
(12) La Mar, G.; de Ropp, J. S.; Smith, K. M.; Langry, K. C. J. Biol. Chem.
1981, 256, 237-243.
by ozone in dichloromethane-d2 at -80 °C.
(13) Czernuszewicz, R. S.; Macor, K. A.; Li, X.-Y.; Kincaid, J. R.; Spiro,
T. G. J. Am. Chem. Soc. 1989, 111, 3860-3869.
(11) (a) Balch, A. L.; Latos-Grazynski, L.; Renner, M. W. J. Am. Chem.
Soc. 1985, 107, 2983-2985. (b) Eaton, S. S.; Eaton, G. R. J. Am.
Chem. Soc. 1975, 97, 3660-3666.
(14) Dolphin, D.; Forman, D. C.; Fajer, B. J.; Felton, R. H. Proc. Natl.
Acad. Sci. U.S.A. 1971, 3, 614-618.
(15) Brill, A. S.; Williams, R. J. P. Biochem. J. 1961, 78, 253-262.