Cytochrome P-450 model (porphinato)(thiolato)iron(III) complexes with single and double NH···S hydrogen bonds at the thiolate site

Full text of DOI:10.1021/ja9622970

12826
J. Am. Chem. Soc. 1996, 118, 12826-12827
Communications to the Editor
Cytochrome P-450 Model
(Porphinato)(thiolato)iron(III) Complexes with
Single and Double NH‚‚‚S Hydrogen Bonds at the
Thiolate Site
Norikazu Ueyama,* Nami Nishikawa, Yusuke Yamada,
Taka-aki Okamura, and Akira Nakamura*
Department of Macromolecular Science
Graduate School of Science, Osaka UniVersity
Toyonaka, Osaka 560, Japan
ReceiVed July 5, 1996
Cytochrome P-450 contains a cysteine thiolate ligand at the
axial position of (porphinato)iron(III) core. The axial thiolate
has been shown to promote the heterolysis of the coordinated
dioxygen upon sequential donation of electron and proton.1-3
The crystallographic structure of cytochrome P-450cam reported
by Poulos et al.4 has suggested the presence of weak double
NH‚‚‚S hydrogen bonds between S of Cys 357 and two amide
NHs of Gly 359 and Gln 360, as shown in Figure 1a. Recently,
the presence of two clear NH‚‚‚S hydrogen bonds was reported
for the crystal structure of chloroperoxidase.5 Similar hydrogen
bonds are found in the active site of P-450BM-3,6 P-450terp,7 and
8
P-450eryF
.
For P-450cam, a molecular dynamics simulation from
the reported crystallographic structure results in the shortening
of the NH‚‚‚S hydrogen bond distances.9 Furthermore, com-
parison of the Protein Data Bank crystallographical structural
parameters at the active sites of substrate-bound P-450cam with
substrate-free P-450cam10 indicates shorter N‚‚‚S distance at the
active site of the substrate-bound P-450cam accompanied by a
positive shift of the reported redox potential.11 A remarkable
effect on the electrochemical properties by NH‚‚‚S hydrogen
bond have already been revealed by us for various metal thiolate
complexes12-14 as well as iron-sulfur proteins.15-17 In order
(1) Higuchi, T.; Uzu, S.; Hirobe, M. J. Am. Chem. Soc. 1990, 112, 7051-
7053.
(2) Higuchi, T.; Shimada, K.; Maruyama, N.; Hirobe, M. J. Am. Chem.
Soc. 1993, 115, 7551-7552.
(3) Dawson, J. H. Science 1988, 240, 435-439.
(4) Poulos, T. L.; Finzel, B. C.; Howard, A. J. J. Mol. Biol. 1987, 195,
687-700.
(5) Sundaramoorthy, M.; Terner, J.; Poulos, T. L. Structure 1995, 3,
1367-1377.
(6) Li, H.; Poulos, T. L. Acta Crystallogr. 1995, D51, 21-32.
(7) Hasemann, C. A.; Ravichandran, K. G.; Peterson, J. A.; Deisenhofer,
J. J. Mol. Biol. 1994, 236, 1169-1185.
Figure 1. Crystal structures of (a) the active site of P-450cam,4 (b)
doubly NH‚‚‚S hydrogen-bonded [FeIII(OEP){S-2,6-(CF3CONH)2C6H3}]
(1), and (c) singly NH‚‚‚S hydrogen-bonded [FeIII(OEP)(S-2-
CF3CONHC6H4)] (3).
(8) Cupp-Vickery, J. R.; Poulos, T. L. Nat. Struct. Biol. 1995, 2, 144-
153.
(9) A BIOGRAF soft ware (Molecular Simulation Inc.) was used for
the molecular mechanics and dynamics calculations that were carried out
under ꢀ ) 7 and 38 at 300 K.
(10) Poulos, T. L.; Finzel, B. C.; Howard, A. J. Biochemistry 1986, 25,
5314-5322.
to investigate the chemical function of NH‚‚‚S hydrogen bonds
in cytochrome P-450, we synthesized novel P-450 iron(III)
porphyrin model complexes having intramolecular NH‚‚‚S
hydrogen bonds. This paper communicates the synthesis and
properties of novel P-450 model complexes electrochemically
modified and remarkably stabilized by single or double NH‚‚‚S
hydrogen bonds.
Doubly ortho-acylamino-substituted arenethiolates were em-
ployed as a Cys thiolate model ligand having two amide dipoles
similar to a helix dipole. Thus, [FeIII(OEP){S-2,6-(RCONH)2-
C6H3}] (R ) CF3 (1), CH3 (2); OEP ) octaethylporphinato)
was synthesized with a ligand exchange reaction (eq 1) between
[FeIII(OEP)(SPh)] and the corresponding disulfide {S-2,6-
(11) Fisher, M. T.; Sligar, S. G. J. Am. Chem. Soc. 1985, 107, 5018-
5019.
(12) Ueyama, N.; Okamura, T.; Nakamura, A. J. Chem. Soc., Chem.
Commun. 1992, 1019-1020.
(13) Ueyama, N.; Okamura, T.; Nakamura, A. J. Am. Chem. Soc. 1992,
114, 8129-8137.
(14) Ueyama, N.; Terakawa, T.; Nakata, M.; Nakamura, A. J. Am. Chem.
Soc. 1983, 105, 7098.
(15) Watenpaugh, K. D.; Sieker, L. C.; Jensen, L. H. J. Mol. Biol. 1979,
131, 509.
(16) Tsukihara, T.; Fukuyama, K.; Nakamura, M.; Katsube, Y.; Kanaka,
N.; Kakudo, M.; Hase, T.; Wada, K.; Matsubara, H. J. Biol. Chem. 1981,
90, 1763.
(17) Adman, E.; Watenpaugh, K. D.; Jensen, L. H. Proc. Natl. Acad.
Sci. U.S.A. 1975, 72, 4854.
S0002-7863(96)02297-4 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12827
1
(e.g., -92 cm-1 for 1 and -123 cm-1 for 3). In the H NMR
Table 1. Crystal Parameters, ν(NH) Shifts, and Redox Potentials
of Doubly NH‚‚‚S Hydrogen-Bonded
spectra of 1-5 in benzene-d6, the paramagnetically shifted
aromatic proton signals appear in the range from -100 to 70
ppm, which are characteristic for various high-spin iron(III)
porphyrin complexes.24 The contact-shifted amide NH signals
of 1 and 3 are observed at -24.8 and -38.1 ppm due to the
direct NH‚‚‚S hydrogen bond formation. [FeIII(OEP)(S-2-
CH3C6H4)] exhibits methyl protons at 87.1 ppm with the
opposite contact-shifted spin sign resulting from the π-conjuga-
tion. Similarly, the amide NH signal of [FeIII(OEP)(S-4-
CF3CONHC6H4)] was observed at 30.4 ppm in C6D6.
The Fe-N4 plane distances of 1 and 3 are shorter compared
with that of [FeIII(OEP)(SPh)] reported in Table 1. If the
elongation was caused by steric hindrance, the distance betweeen
Fe atom and N4 plane must be elongated as well as the Fe-S
bond distance, as found in [Co(TPP)(1-MeIm)] and [Co(TPP)(1,2-
MeIm)].25 Actually, [FeIII(OEP)(S-2-CH3C6H4)] gradually de-
composes and is less stable against dioxygen than 1 and 3. Two
bulky thiolate Fe(III) complexes, [FeIII(OEP)(S-2,4,6-Me3C6H2)]
and [FeIII(OEP){S-2,4,6-(i-Pr)3C6H2}], are readily reduced to
Fe(II) species by their dissociative thiolates to form Fe(II)
species. Thus, the steric hindrance of bulky thiolates is not
crucial for the stabilization of the Fe(III) state.
[FeIII(OEP){S-2,6-(CF3CONH)2C6H3}] (1), Singly NH‚‚‚S
Hydrogen-Bonded [FeIII(OEP)(S-2-CF3CONHC6H4)] (3) and
[FeIII(OEP)(SPh)
1
3
[FeIII(OEP)(SPh)]a
Fe-Sb
2.356(3)
0.4142
104.5(3)
2.327(4)
0.3862
104.0(5)
2.299(3)
0.466(1)
102.5(3)
Fe-N4 planeb
Fe-S-Cc
N‚‚‚Sb
2.962(9), 2.956(9) 2.93(1)
ν(NH) shiftd
-92e
-123f
-0.52
FeIII/FeII redox -0.35
-0.68
coupleg
a The crystal parameters were quoted from the literature.22 b In
angstroms. c In degrees. d In centimeters-1 (KBr disk). e Shift from free
ν(NH) at 3370 cm-1 of {S-2,6-(CF3CONH)2C6H3}2 in dichloromethane.
f Shift from free ν(NH) at 3358 cm-1 of (S-2-CF3CONHC6H4)2 in
dichloromethane. g In volts vs SCE in dichloromethane.
18
(RCONH)2C6H3}2
(1:0.5), whereas [FeIII(OEP)(S-2-
RCONHC6H4)] (R ) CF3 (3), CH3 (4), t-Bu (5)) was prepared
by a reaction (eq 2) between [FeIII(OEP)]2O and the thiol, as
for [FeIII(OEP)(S-4-NO2C6H4)].19
The cyclic voltammograms of 1-5 in dichloromethane show
a positively shifted FeIII/FeII redox couple in the comparison
with that (-0.68 V vs SCE) of [Fe(OEP)(SPh)] without NH‚‚‚S
hydrogen bond by 0.10-0.33 V. For example, 1 exhibits the
redox couple at -0.35 V vs SCE in dichloromethane. These
results are consistent with the known fact that the substrate-
bound P-450cam has a more positively shifted redox potential
[FeIII(OEP)(SPh)] + 1/2ArSSAr f
[FeIII(OEP)(SAr)] + PhSSPh (1)
[FeIII(OEP)]2O + 2ArSH f 2[FeIII(OEP)(SAr)] + H2O (2)
(SAr ) S-2,6-(RCONH)2C6H3 or S-2-RCONHC6H4)
(-0.415 V) when compared with that (-0.572 V) of substrate-
11
free P-450cam
.
When 1 and 3 were reduced by tetraethylammonium boro-
hydride, the isolated iron(II) species was found to be detected
as [FeII(OEP)] without axial thiolate. The direct oxidation of
1 by PhIO results in the formation of [FeIII(OEP)]2O, whereas,
H2O2 decomposes the OEP ligand itself.
Although all the known iron(III) porphyrin thiolato complexes
have been reported to be unstable to air and moisture,19,25 these
new complexes are stable against O2 or H2O. For example, 1
is extremely stable in benzene and not oxidized at least for 5
days under air. A polyethyene glycol lauryl ether (10%)
aqueous micellar solution (0.20 mM) of 1 is also stable even
under similar aerobic conditions. These results indicate that
the hydrogen bond plays an important role in the regulation of
the stability and reactivity of P-450cam in biological systems.
The molecular structures of 1 and 3 were determined by X-ray
analysis to indicate that 1 and 3 possess intramolecular double
and single NH‚‚‚S hydrogen bonds, respectively, as illustrated
in Figure 1b,c.20,21 The selected crystal parameters are listed
in Table 1 and compared with those reported for [FeIII(OEP)-
(SPh)], implying that the Fe-S bond distance for 1 is longer
than those of 3 or [FeIII(OEP)(SPh)].22 For Fe(II) and Mo(IV)
complexes, the NH‚‚‚S hydrogen bonding shortens the M-S
bond, mainly due to the charge transfer from the antibonding
M-S HOMO to amide LUMO.12,13,23 On the contrary, the
hydrogen bonding in 1 and 3 results in a bond elongation, as
shown in Table 1. The ν(NH) shift in the IR spectra of these
complexes also supports the presence of the hydrogen bonds
(18) Ueyama, N.; Yamada, Y.; Okamura, T.; Kimura, S.; Nakamura, A.
Inorg. Chem. in press.
Acknowledgment. Support of this work by a Grant-in-Aid for
Specially Promoted Research (no. 06101004) from the Ministry of
Education, Science, and Culture of Japan is gratefully acknowledged.
(19) Tang, S. C.; Koch, S.; Papaefthymiou, G. C.; Foner, S.; Frankel,
R. B.; Ibers, J. A.; Holm, R. H. J. Am. Chem. Soc. 1976, 98, 2414-2434.
(20) Crystal data for [Fe(OEP){S-2,6-(CF3CONH)2C6H3}]‚1/2C7H8: tri-
clinic, PO(1h), black plate, a ) 13.304(4) Å, b ) 20.151(5) Å, c ) 9.201(2)
Å, R ) 90.82(2)°, â ) 101.77(2)°, γ ) 100.65(2)°, V ) 2369(1) Å3, Z )
Supporting Information Available: Tables of non-hydrogen atom
anisotropic thermal parameters, complete geometric data and atomic
coordinates (17 pages). See any current masthead page for ordering
and Internet access instructions.
2, Dcalcd ) 1.354 g cm-1. The structure was refined to R ) 0.063, Rw
0.069 for 2965 reflections with GOF ) 1.81.
)
(21) Crystal data for [Fe(OEP) (S -2-CF3CONHC6H4)]‚1/2C7H8: triclinic,
PO(1), black needle, a ) 13.467(4) Å, b ) 14.797(6) Å, c ) 12.821(5) Å,
R ) 103.88(3)°, â ) 107.71(3)°, γ ) 65.94(2)°, V ) 2202(1) Å3, Z ) 2,
Dcalcd ) 1.289 g cm-1. The structure was refined to R ) 0.069, Rw ) 0.068
for 2329 reflections with GOF ) 1.83.
JA9622970
(24) Arasasingham, R. D.; Balch, A. L.; Cornman, C. R.; Ropp, J. S. d.;
Eguchi, K.; Mar, G. N. L. Inorg. Chem. 1990, 29, 1847-1850.
(25) Scheidt, W. R.; Geiger, D. K.; Lee, Y. J.; Reed, C. A.; Lang, G. J.
Am. Chem. Soc. 1985, 107, 5693-5699.
(26) Ogoshi, H.; Sugimoto, H.; Yoshida, Z. Tetrahedron Lett. 1975,
27, 2289-2292.
(22) Miller, K. M.; Strouse, C. E. Acta Crystallogr. 1984, C40, 1324-
1327.
(23) Chung, W. P.; Dewan, J. C.; Walters, M. A. J. Am. Chem. Soc.
1991, 113, 525-530.