Novel iron porphyrin-alkanethiolate complex with intramolecular NH···S hydrogen bond: Synthesis, spectroscopy, and reactivity [3]

Full text of DOI:10.1021/ja992511v

J. Am. Chem. Soc. 1999, 121, 11571-11572
11571
Novel Iron Porphyrin-Alkanethiolate Complex with
Intramolecular NH‚‚‚S Hydrogen Bond: Synthesis,
Spectroscopy, and Reactivity
Noriyuki Suzuki,† Tsunehiko Higuchi,*,† Yasuteru Urano,†
Kazuya Kikuchi,† Hidehiro Uekusa,‡ Yuji Ohashi,‡
Takeshi Uchida,§ Teizo Kitagawa,§ and Tetsuo Nagano*,†
Graduate School of Pharmaceutical Sciences, The UniVersity
of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Department of Chemistry, Faculty of Science, Tokyo Institute
of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan
Institute for Molecular Science, Okazaki National Research
Institutes, Myodaiji, Okazaki 444-8585, Japan
Figure 1. Structures of complexes 1-3 and SR.
Table 1. λmax of Absorption Spectrum and FeIII/FeII Redox Couple
(V) of 1-3 and SR
FeIII/FeII
λ
max (log ꢀ) of absorption spectrum (nm)
ReceiVed July 16, 1999
FeIII a
FeII-COb
redox couple (V)c
Among heme enzymes, cytochrome P450 and NO synthase
(NOS) have strong oxidizing ability and unusual structure, in that
their heme irons have thiolate coordination. Consequently, much
interest has been focused on their structure-function relationship.1
We have synthesized the first synthetic heme thiolate (SR
complex2) which retains thiolate coordination during catalytic
oxidation and have found several remarkable thiolate axial ligand
effects.2
Recently, the presence of an NH‚‚‚S hydrogen bond in the
active site of P450 and NOS has been suggested, based on the
analysis of their crystal structure.3 Such a bond should markedly
affect the chemistry of the heme thiolate. Ueyama and co-workers
have reported synthetic structural models of heme arenethiolate
with an NH‚‚‚S hydrogen bond,4 but their paper did not include
data about the influence of the NH‚‚‚S hydrogen bond on the
catalytic activity of the heme thiolate.
1
430 (5.04)
539 (4.06)
429 (4.96)
537 (3.95)
425 (5.10)
537 (4.04)
428 (5.03)
538 (3.98)
380 (4.77), 456 (5.08)
556 (4.01)
391 (4.73), 465 (4.82)
558 (3.77)
388 (4.78), 460 (4.94)
557 (3.86)
385 (4.76), 460 (4.94)
562 (3.77)
-0.41
-0.56
-0.53
-0.52
2
3
SR
a In dimethyl sulfoxide. b 30 min after the addition of NaBH4 under
a CO atmosphere in dimethyl sulfoxide. c In 0.1 M n-tetrabutylammo-
nium perchlorate/CH2Cl2, at Pt electrode vs SCE reference.
We report here a novel iron porphyrin-alkanethiolate complex
with an intramolecular NH‚‚‚S hydrogen bond that we synthesized
in order to examine the influence of the NH‚‚‚S hydrogen bond
on catalytic oxidation. Complex 1 was designed to form an
NH‚‚‚S hydrogen bond by introducing amide NH in the vicinity
of the thiolate, while complexes 2 and 3 were designed not to
form an NH‚‚‚S hydrogen bond by replacing amide NH with
N-methyl or by introducing acetamide in a position apart from
the sulfur atom (Figure 1).5
Figure 2. Molecular structure (ORTEP drawing; 25% probability level)
of 1. Fe and S atoms are shown as octant shading ellipsoids, N and O as
octant ellipsoids.
copy, and X-ray crystal structure analysis. The absorption spectra
of the ferrous-CO complexes of 1-3 exhibited typical hyper-
porphyrin spectra for a thiolate-ligated iron(II) porphyrin-CO
complex (Table 1). The Soret band of the ferrous-CO complex
of 1 (456 nm, which arises from a transition between the lone
pair p orbital of the thiolate and the eg orbital of heme) was
considerably blue-shifted compared to that of the other complexes,
indicating electron deficiency of thiolate in complex 1 arising
from the NH‚‚‚S hydrogen bond.
The structure of 1 determined by X-ray crystal structure
analysis8 (Figure 2) shows that the hydrogen atom of the amide
group is directed toward the thiolate sulfur atom, and the distances
of N- - -S and NH‚‚‚S and the angle of NsH‚‚‚S are 3.423(4) Å,
2.800 Å, and 130.73°, respectively. These values indicate that 1
possesses an intramolecular NH‚‚‚S hydrogen bond. The Fe-S
and Fe-N bond distances are listed in Table 2 and are compared
with those reported for some synthetic heme thiolates and P450
Complexes 1-3 were characterized by FAB MS, IR, EPR,
electronic absorption spectroscopy, resonance Raman spectros-
† The University of Tokyo.
‡ Tokyo Institute of Technology.
§ Okazaki National Research Institutes.
(1) (a) Ortiz de Montellano, P., Ed. Cytochrome P-450; Plenum: New York,
1986. (b) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem. ReV.
1996, 96, 2841. (c) Dawson, J. H. Science 1988, 240, 433.
(2) (a) Higuchi, T.; Uzu, S.; Hirobe, M. J. Am. Chem. Soc. 1990, 112,
7051. (b) Higuchi, T.; Shimada, K.; Maruyama, N.; Hirobe, M. J. Am. Chem.
Soc. 1993, 115, 7551. (c) Urano, Y.; Higuchi, T.; Hirobe, M.; Nagano, T. J.
Am. Chem. Soc. 1997, 119, 12008. Recently, Wagenknecht and Woggon
reported on the synthesis and catalytic reactivity of synthetic heme arenethi-
olates as chemical models of chloroperoxidase. Wagenknecht, H. A.; Woggon,
W. D. Angew. Chem., Int. Ed. Engl. 1997, 36, 390.
(3) (a) Poulos, T. L.; Finzel, B. C.; Howard, A. J. J. Mol. Biol. 1987, 195,
687. (b) Sundaramoorthy, M.; Terner, J.; Poulos, T. L. Structure 1995, 3,
1367. (c) Li, H.; Poulos, T. L. Acta Crystallogr. 1995, D51, 21. (d) Hasemann,
C. A.; Ravichandran, K. G.; Peterson, J. A.; Deisenhofer, J. J. Mol. Biol.
1994, 236, 1169. (e) Cupp-Vickery, J. R.; Poulos, T. L. Nature Struct. Biol.
1995, 2, 144. (f) Crane, B. R.; Arvai, A. S.; Gachhui, R.; Wu, C.; Ghosh, D.
K.; Getzoff, E. D.; Stuehr, D. J.; Tainer, J. A. Science 1997, 278, 425.
(4) (a) Ueyama, N.; Nishikawa, N.; Yamada, Y.; Okamura, T.; Nakamura,
A. J. Am. Chem. Soc. 1996, 118, 12826. (b) Ueyama, N.; Nishikawa, N.;
Yamada, Y.; Okamura, T.; Oka, S.; Sakurai, H.; Nakamura, A. Inorg. Chem.
1998, 37, 2415.
(7) The EPR spectra of complexes 1-3 showed low-spin signals of a single
species, of which the g values indicate that the axial ligand is thiolate anion
since their V/∆ values are 1.14-1.30, as obtained by the method of Taylor.
Taylor, C. P. S. Biochim. Biophys. Acta 1977, 491, 137.
(8) Crystal data for 1: monoclinic, P21/n, black needles, 0.025 × 0.020 ×
0.3 mm, Cu KR (λ ) 1.541 84 Å), Rigaku R-AXIS RAPID Weissenberg IP
System, a ) 26.387(2) Å b ) 17.436(1) Å c ) 13.802(1) Å R ) 90°, â )
101.650(2)°, γ ) 90°, V ) 6219.3(8) Å3, Z ) 4, 10 850 unique reflections
(2θ max ) 136°, completeness ) 0.953), R1 ) 0.0545 for 3058 reflections.
In the crystal form, the O atom of the acetamide group ligates to the Fe atom
of an adjacent molecule as a sixth ligand.
(5) To prepare 1-3, [(2-acetylthiomethyl-3-acetamido)phenoxy]acetic acid
and other corresponding acids were combined with meso-R,R,R,R-tetrakis(o-
aminophenyl)porphyrin,6 and then complexes 1-3 were synthesized according
to the same procedure as used for SR in our previous report.2a
(6) Collman, J. P.; Gagne, R. R.; Reed, C. A.; Harbert, T. R.; Lang, G.;
Robinson, W. T. J. Am. Chem. Soc. 1975, 97, 1427.
10.1021/ja992511v CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/24/1999

11572 J. Am. Chem. Soc., Vol. 121, No. 49, 1999
Communications to the Editor
Table 2. Selected Bond Distances and Fe-S Modes of Synthetic
Heme Thiolates and Native P450s
Table 3. Competitive Oxidation of Cyclooctane/Cyclooctene
Catalyzed by Iron Porphyrinsa
Fe-S
(Å)
Fe-N
(mean, Å)
νFe-S
(cm-1
)
1a
2.18
2.20b
2.30
2.20d
2.15
1.98
1.99b
2.06
2.05d
1.96
394
SR
366a
FeOEP(S-C6H5)c
P450cam (substrate-bound)
P450terp (substrate-free)f
products yield (%)b
350e
total yield alkane/alkene,
iron porphyrin
4
5
6
(%)
(4 + 5)/6
none
ndc
nd
6.64
6.64
24.96
11.97
16.20
13.58
52.89
a This work. b The bond distances of SR were obtained by extended
X-ray absorption fine structure spectroscopy.2a c Reference 9. d Ref-
erence 3a. e Reference 10. f Reference 3d.
1
5.66 0.34 18.96
0.32
0.80
0.63
0.60
0.05
2
3
5.25 0.07
4.90 1.39
5.01 0.04
6.65
9.91
8.52
SR
SR-Imd
0.93 1.42 50.54
enzymes. Complex 1 is the first example of a synthetic alkanethi-
olate-ligated heme to which the crystal structure analysis has been
applied. Bond distances of 1 are much closer to the corresponding
ones of P450 enzymes than those of arenethiolate-ligated hemes,
as shown in Table 2.
a Reaction conditions: [iron porphyrins] ) [mCPBA] ) 1.0 mM,
[cyclooctane] ) 200 mM, [cyclooctene] ) 20 mM. All reactions were
carried out in CH2Cl2 under Ar at -15 °C for 5 min. b Yields were
based on mCPBA. c Not detected. d Reference 16.
The results from the crystal structure analysis of 1 and EXAFS
study of SR suggest that the Fe-S bond distance in 1 is shortened
by the NH‚‚‚S hydrogen bond. Therefore, we carried out a
resonance Raman investigation to compare the bond distances
between 1 and SR under the same solution conditions. To identify
the Fe-S stretching modes in SR and 1, we incorporated 34S
isotope into their thiolate sulfur. The νFe-S signal was examined
at λex ) 363.8 nm using the same conditions as in the case of
P450cam.10 As shown in Table 2, the Fe-S mode in 1 is shifted
to 394 cm-1, compared with 366 cm-1 in SR. Therefore, it is
confirmed that hydrogen bonding shortens the Fe-S bond distance
in the case of alkanethiolate-ligated heme, just as previously
reported for most of the thiolate non-heme complexes,11 contrary
to the result of the heme-arenethiolates reported by Ueyama et
al.4 These results can be explained as follows: NH‚‚‚S hydrogen
bonding shortens the metal-S bond length when the HOMO
involves antibonding metal-S interaction.4b,11 In the case of hemes
in which the fifth ligand is a relatively weak electron donor such
as Cl- or ClO4-, the HOMO is on the porphyrin ligand.12
However, when the fifth ligand is a strongly basic one, such as
OH- or CH3O-, the HOMO is not on the porphyrin ligand but
on the central metal.12 From these results, it is highly probable
that the HOMO of 1 is on the central metal, owing to the highly
basic and electron-donating alkanethiolate axial ligand, in contrast
to the heme arenethiolates. Therefore, in the case of our complex
1, NH‚‚‚S hydrogen bonding shortens the Fe-S bond length,
similar to the case of the non-heme complexes.
Also in support of the presence of the hydrogen bond is that
the redox potential is positively shifted only in the case of 1 (Table
1). The cyclic voltammograms of 1 showed a positively shifted
FeIII/FeII redox couple (by ∼0.1 V), in comparison with those of
2, 3, and SR, which lack the hydrogen bond, and this value is
extremely close to those of native P450 forms (-0.42 V, P450cam
in high spin state13). Each complex showed a clear, reversible
redox couple. Since it has been reported that the second electron
transfer is the rate-determining step in P450 catalysis,14 this
positive shift indicates that the hydrogen bond present in P450
contributes to the catalytic efficiency.
the change of the EPR spectra. Therefore, we concluded that the
NH‚‚‚S hydrogen bond stabilizes the Fe(III) porphyrin-alkanethi-
olate structure both by increasing the Fe-S bond order and by
inducing a positive shift of the redox potential.
The most characteristic feature of the reaction of P450 is its
high activity as a monooxygenase. It is generally accepted that
alkane hydroxylation and alkene epoxidation proceed via different
mechanisms,15 so it is expected that alkane-alkene competitive
oxidation can be used as a probe for discrimination of differences
in chemical properties among active species derived from iron
porphyrins. Thus, competitive oxidation of cyclooctane-cyclo-
octene was studied in novel heme thiolate-mCPBA systems to
examine the effect of the NH‚‚‚S hydrogen bond. Reaction
conditions followed those in our previous report on the reactivity
of an active intermediate derived from SR.16 The thiolate ligation
of 1-3 and SR during oxidation reaction was confirmed by
measuring the EPR spectrum before and after the reaction. The
intermediate of each of 1-3 and SR was confirmed to be the
two-electron-oxidized form by using peroxyphenylacetic acid,
which has frequently been used as a probe for this purpose.17 As
shown in Table 3, every heme thiolate effectively oxidized alkane
and, like P450,15 showed a higher ratio of cyclooctanol to
cycloctene oxide compared with SR-Im,2b which is an axial
imidazole-ligated analogue of SR. However, significant differ-
ences in the ratio of oxidation products were observed between
1 and the other heme thiolates. In the case of 2, 3, and SR, the
ratios of cyclooctanol to cycloctene oxide were ∼0.6. On the other
hand, 1 showed a lower ratio than the other heme thiolates. These
results indicate that the electronic structure of the active inter-
mediate of 1 is somewhat more advantageous for electrophilic
reaction than are those of the other complexes.18
Acknowledgment. This work was supported in part by a Grant-in-
Aid for Scientific Research (No. 07407079, Biometallics No. 10129203,
and No. 11116207) from the Ministry of Education, Science, Sports and
Culture, Japan. N.S. gratefully acknowledges the Japan Society for the
Promotion of Science for the JSPS Research Fellowships for Young
Scientists. We thank Dr. Keiko Miura (JASRI) for her helpful discussions
about X-ray crystal structure analysis.
Although SR itself is an exceptionally stable alkanethiolate-
ligated heme with respect to O2, H2O, and other reactive chemical
species, it was found that 1 is even more stable than 2, 3, and
SR under air at room temperature in solution by investigation of
Supporting Information Available: X-ray crystallographic data, in
CIF format, are available. This material is available free of charge via
the Internet at http://pubs.acs.org.
JA992511V
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