The first structurally characterized nitrosyl heme thiolate model complex

Article abstract of DOI:10.1039/b602611g

The NO ligand in the formally {FeNO}⁶ compound [Fe(oep)(NO)(thiolate)] is bent, and does not impart a significant structural trans effect to the Fe-S bond. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b602611g

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The first structurally characterized nitrosyl heme thiolate model
complex
Nan Xu, Douglas R. Powell, Lin Cheng and George B. Richter-Addo*
Received (in Austin, TX, USA) 21st February 2006, Accepted 22nd March 2006
First published as an Advance Article on the web 11th April 2006
DOI: 10.1039/b602611g
The NO ligand in the formally {FeNO}6 compound
We were thus interested in structurally characterizing a ferric
heme model complex containing mutually trans NO and thiolate
ligands. Our intent was to examine, for the first time, (i) the FeNO
bond geometry in the absence of distal amino acid residues that
could influence this geometry, and (ii) the effect of the bound NO
group on the trans Fe–S bond.
[Fe(oep)(NO)(thiolate)] is bent, and does not impart
a
significant structural trans effect to the Fe–S bond.
Heme thiolate proteins contain the [Fe(por)(SR)] moiety (por =
porphyrinato dianion; SR = cysteinate) and display rich and
diverse reactivities that include nitric oxide (NO) biosynthesis, NO
reduction, hydroxylation, and detoxification of xenobiotics.1,2 The
thiolate groups in these proteins serve as axial ligands that
influence the reactivities of the iron centers. Crystal structures of
the cytochromes P450 (P450) reveal stabilization of the Fe–SR
linkage via hydrogen bonding between the sulfur atom and
neighboring peptide NH groups; in NO synthase (NOS), one of
these hydrogen bonds is provided by the indole NH group of a
conserved Trp residue.3
The reaction of the five-coordinate iron porphyrin thiolate
complex [FeIII(oep){S-2,6-(CF3CONH)2C6H3}] (1)13 (oep
=
octaethylporphyrinato dianion) with NO in CH2Cl2 was moni-
tored by IR spectroscopy. The resulting spectra recorded at
different times after NO addition are shown in Fig. 1. A new band
at y1850 cm21 forms immediately after NO addition assigned
to uNO (cf., free NO at 1875 cm21).14 However, this band is
quickly consumed with the resulting formation of the uNO band
at 1667 cm21 due to the final five-coordinate [Fe(oep)(NO)]
compound.
NO binds to the iron center in P450 and inhibits the enzyme.4
Interestingly, the NO adduct of ferric NOS is an observable
intermediate in the catalytic cycle of NOS.5 In the case of fungal
P450nor (NO reductase), NO binds to the ferric center and is
reduced by NADH.2 NO reacts with the ferric heme thiolate
protein Cimex nitrophorin not only by nitrosylation of the iron
center, but also by nitrosation of the axial thiolate sulfur atom.6 A
related nitrosylation–nitrosation double reaction has been pro-
posed as a mechanism of deactivation of liver microsomal P450 by
NO;4 such processes occur with some ferric thiolate porphyrins.7
NO has been shown to bind reversibly to synthetic ferric thiolate
porphyrins.7,8 but no X-ray structural studies of synthetic nitrosyl
heme thiolate compounds have been reported to date.
Surprisingly, the Fe–N–O angles in the two structural reports on
the NO adducts of ferric heme thiolate proteins range from 100u to
165u.2,9 Distal pocket residues are known to affect the FeNO bond
geometry in ‘‘ferrous’’ {FeNO}7 nitrosyl heme proteins, but it is
not clear to what extent distal residues influence the FeNO
geometry in the formally ‘‘ferric’’ {FeNO}6 derivatives.10 For
example, Hu and Kincaid have shown that the FeNO moiety in
the NO adduct of ferric P450cam (camphor monooxygenase) is
linear in the absence of substrates, whereas it is slightly bent in the
presence of substrates.11 Further, Scherlis et al. have reported their
results of Density-Functional Theoretical calculations on a
[Fe(porphine)(NO)(cysteinate)] model that show that the ground
state structure possesses a linear FeNO geometry (178.7u), and that
higher energy states of this model may contain bent FeNO
geometries.12
Also observed is the reduction in intensity of the band at
1720 cm21 due to the uCO (of the thiolate ligand) of 1 and the
generation of a new band at 1743 cm21 coincident with the uCO of
the organic disulfide. We thus assign the new band at y1850 cm21
to the uNO of the target six-coordinate compound [Fe(oep)(NO){S-
2,6-(CF3CONH)2C6H3}] (2). All our attempts, to date, at crystal-
lizing 2 from this reaction mixture have not been successful; only
the final [Fe(oep)(NO)] derivative was obtained, consistent with
the ready loss of the thiolate ligand from the six-coordinate
compound in solution.
Reaction of the iron porphyrin thiolate compound 1 as a
powder with NO gas in a solvent-free environment produced a
derivative with a uNO band of 1839 cm21 (Nujol mull). This solid
product was stable under an atmosphere of NO for several days in
Department of Chemistry and Biochemistry, University of Oklahoma,
620 Parrington Oval, Norman, Oklahoma, USA.
E-mail: grichteraddo@ou.edu; Fax: +1 405 325 6111;
Tel: +1 405 325 6401
Fig. 1 Infrared spectral monitoring of the reaction of 1 with NO in
CH2Cl2; intervals are 0 s, 10 s, 20 s, 35 s, 60 s, and 120 s.
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˚
remarkable y0.46 A apical movement of the Fe atom in this solid-
state reaction of 1 with NO gas! Even more striking is that the
trans Fe–S bond length does not change in going from the five-
coordinate 1 to the six-coordinate 2, implying that there is no
significant structural trans effect of NO in this six-coordinate iron
nitrosyl thiolate porphyrin when prepared from the five-coordinate
crystals. This may be compared with the structural trans effect
observed in the {FeNO}7 compounds [Fe(por)(NO)(N-base)]
˚
which show a y0.2 A increase in the Fe–N(ax) distance upon
NO binding.15
Third, the FeNO and FeSC(thiolate) planes are essentially
mutually perpendicular, and these two planes straddle the Fe–N1
bond; C37–S1–Fe1–N1 torsion angle = 255.6(4)u, and O1–N5–
Fe1–N1 = 43(2)u. In addition, there is a slight opening of the Fe1–
S1–C37 angle (from 104u to 111u) upon NO binding and
subsequent movement of the S-atom toward the mean porphyrin
plane.
Fig. 2 Molecular structure of [Fe(oep)(NO){S-2,6-(CF3CONH)2C6H3}]
(2) with thermal ellipsoids drawn at 35%.
Whenplacedinabroadercontext,thecrystalstructureof2reveals
an intrinsic tilting (of the nitrosyl N-atom from the por-
phyrin normal) and bending of the NO group in this formally
{FeNO}6 species. We have previously reported that such a tilting
and bending feature represents a low energy conformation in the
related {FeNO}6 compound [Fe(oep)(NO)(C6H4F)].16 Our results
suggest that such a tilting and bending of the NO group may be a
common feature in the NO adducts of ferric heme thiolate proteins.
We are grateful to the US National Institutes of Health (GM
64476) for funding for this work.
the dark. However, attempted dissolution and crystallization of the
nitrosyl product 2 resulted only in the generation of the precursor
compound 1 and [Fe(oep)(NO)].
We then explored the possibility of a heterogeneous reaction of
crystals of 1 with NO gas with the hope that product formation
would not be accompanied by extensive crystal fragmentation.
The crystals used for the reaction were grown from CH2Cl2–
hexane (2 : 1) at room temperature.{{13 Several crystals of purple 1
were hand-picked and exposed to NO gas at room temperature for
several hours to allow for NO diffusion into the crystal lattice. The
molecular structure of the dark red nitrosyl product 2 is shown in
Fig. 2. Selected structural data of 2 are presented in Table 1, and
are compared with the related data from 1.
Notes and references
{ The crystal structure of 1 was reported previously by Ueyama et al.13
Their crystals were grown from toluene, and the Fe–S and Fe–Npor bond
˚
lengths in their structure were 2.356(3) and 2.048(avg) A, respectively.
{ Crystal data. Compound 1?1/2(hexane): C49H56F6FeN6O2S, M = 962.91,
There are several interesting features about the structure of 2.
First, the FeNO moiety in 2 is bent with an angle of 159.6(8)u in
this formally {FeNO}6 compound, and the nitrosyl N-atom is
tilted 9.1u from the normal to the 4N (and 24 atom) porphyrin
plane. The bent NO geometry is not the result of any close
intermolecular contacts; the shortest intermolecular distances are
between the nitrosyl O-atom and a thiolate F-atom of another
molecule, and between the nitrosyl N-atom and another porphyrin
¯
˚
˚
˚
triclinic, space group P1, a = 8.9881(14) A, b = 13.208(2) A, c = 19.879(3) A,
3
˚
a = 100.889(5)u, b = 91.761(5)u, c = 101.373(5)u, V = 2266.3(6) A , T =
100(2) K, Z = 2, Z9 = 1, Dc = 1.411 g cm23, m = 0.451 mm21, 18050
reflections measured, 8813 unique (Rint = 0.0260), F(000) = 1008, R1 (obs.
data) = 0.0563, wR2 (all data) = 0.1645. Compound 2?1/2(hexane):
¯
˚
C49H56F6FeN7O3S, M = 992.92, triclinic, space group P1, a = 9.181(5) A,
˚
˚
b = 12.455(7) A, c = 21.261(12) A, a = 98.573(9)u, b = 94.080(9)u,
3
˚
˚
ethyl carbon atom (both distances are ¢3.3 A).
Second, the Fe atom in 2 is situated almost in the 4N plane of
c = 99.425(9)u, V = 2360(2) A , T = 100(2) K, Z = 2, Z9 = 1, Dc
=
1.397 g cm23, m = 0.438 mm21, 9889 reflections measured, 4873 unique
(Rint = 0.0680), F(000) = 1038, R1 (obs. data) = 0.0833, wR2 (all data) =
0.2035. CCDC 298938 (2) and CCDC 298939 (1). For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b602611g
˚
the porphyrin, with an upward displacement of 0.05 A towards the
NO ligand. Concurrent with this upward movement of the Fe
atom upon formation of 2 is the shortening of the Fe–Npor bond
˚
lengths by y0.05 A. Considering that the Fe atom was displaced
1 D. Mansuy and P. Battioni, in The Porphyrin Handbook, ed. K. M.
Kadish, K. M. Smith and R. Guilard, Academic Press, San Diego, 2000,
vol. 4, ch. 26, pp. 1–15.
˚
0.41 A towards the S atom in the precursor 1, this represents a
2 H. Shimizu, E. Obayashi, Y. Gomi, H. Arakawat, S.-Y. Park,
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Mechanism, and Biochemistry, ed. P. R. Ortiz de Montellano, Plenum,
New York, 3rd edn, 2005, ch. 3, pp. 87–114.
˚
Table 1 Selected structural data (in A and u) for compounds 1 and 2
1
2
Fe1–N5
N5–O1
—
—
1.671(9)
1.187(9)
4 Y. Minamiyama, S. Takemura, S. Imaoka, Y. Funae, Y. Tanimoto and
M. Inoue, J. Pharmacol. Exp. Ther., 1997, 283, 1479–1485.
5 H. M. Abu-Soud, K. Ichimori, H. Nakazawa and D. J. Stuehr,
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6 A. Weichsel, E. M. Maes, J. F. Andersen, J. G. Valenzuela, T. K.
Shokhireva, F. A. Walker and W. R. Montfort, Proc. Natl. Acad. Sci.
USA, 2005, 102, 594–599.
Fe1–Npor
Fe1–S1
/Fe1–N5–O1
/S1–Fe1–N5
/C37–S1–Fe1
S tilta
2.054(2)–2.064(2)
2.359(1)
—
—
104.22(11)
3.4 [4.3]
2.003(8)–2.017(7)
2.356(3)
159.6(8)
165.5(3)
110.6(3)
6.3 [6.7]
a
Tilt of the S atom from the normal to the porphyrin 4N [24 atom]
7 A. Franke, G. Stochel, N. Suzuki, T. Higuchi, K. Okuzono and R. van
Eldik, J. Am. Chem. Soc., 2005, 127, 5360–5375.
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