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Iron
Sulfur Clusters
We have been fortunate to be involved in characterizing novel
structures such as the nitrogenase
M and P-centers,1 the coupled heme-[4Fe-4S] chromophore
of E. coli. sulfite reductase,2 [3Fe-4S]
clusters,3 the clusters of carbon monoxide dehydrogenase,4
the key FeIVFeIV intermediate (compound
Q) in the catalytic cycle of methane
monooxygenase,5 the H-cluster of [Fe]-hydrogenases,6
and the oxygen sensor of E. coli.7 For
well established structures, we have been interested in attaining
new oxidation states, e.g. the all-ferrous [4Fe-4S] cluster.8
In general terms, our interests are described in Ref 9. We
continue to study a variety of iron-sulfur proteins.
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Our idea of the
nitrogenase P-cluster shortly
before crystallographic refinement by Rees et al. (1992)
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(1)
Surerus, K. K.; Hendrich, M.
P.; Christie, P.; Rottgardt, D.;
Orme-Johnson, W. H.; Münck,
E. J. Am. Chem. Soc. 1992, 114,
8579-8590.
(2) Bominaar,
E. L.; Hu, Z.; Münck, E; Girerd, J.-J.; Borshch, S. J.
Am. Chem. Soc. 1995, 117, 6976-89.
(3) Emptage,
M. H.; Kent, T. A.; Huynh, B. H.; Rawlings, J.; Orme-Johnson,
W.H.; Münck, E. J. Biol. Chem.
1980, 255, 1793-1796.
(4) Xia,
J.; Hu, Z.; Popescu, C. V.; Lindahl, P. A.; Münck, E. J.
Am. Chem. Soc . 1997, 119, 8301-12.
(5) Shu,
L.; Nesheim, J. C.; Kauffmann, K.; Münck,
E.; Lipscomb, J. D.; Que Jr., L.
Science 1997, 275, 515-518.
(6) Popescu,
C. V.; Münck, E. J. Am.
Chem. Soc. 1999, 121, 7877-7884.
(7) Popescu,
C.; Bates, D. M.; Beinert, H.; Münck,
E.; Kiley, P. J. Proc. Natl.
Acad. Sci. USA 1998, 95, 13431-13435.
(8) Yoo,
S. J.; Angove H. C.; Burgess, B.K.; Hendrich,
M.P.; Münck, E. J. Am. Chem. Soc. 1999,
121, 2534-2545.
(9) Beinert,
H.; Holm, R.H.; Münck, E. "Iron-Sulfur
Clusters: Nature’s Modular Multipurpose Structures"
Science 1997, 277, 653-659.
(10) Chakrabati,
M.; Deng, L.; Holm, R. H.; Munck, E.; Bominaar, E. L. Inorg.
Chem. 2009, 48, 2735-2727 (Cover
page Inorg. Chem. Apr. 6, 2009).
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Iron
Proteins with Mononuclear or Binuclear Centers
We are interested in the catalytic
cycles of dioxygenases and diiron
proteins, and in the past we have studied a substantial number
of proteins from each class. Our current emphasis is to characterize,
in collaboration with various groups, short-lived catalytic
intermediates by rapid-quench techniques, or to extend the life-time
of such intermediates by preparing samples in cryogenic fluids,
such as water/glycerol. Presently we are studying methane monooxygenase,
fatty acid desaturase, benzoate dioxygenase
and α-ketoglutarate-dependent enzymes. The studies of the diiron protein intermediates are complemented with investigations
of suitable model complexes; e.g. FeIIIFeIV
compounds. |
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High-Valent
complexes of Biological Relevance
Together with the group of Lawrence Que,
Jr., at the University
of Minnesota, we
are studying the electronic structure the electronic structure
of high-valent iron complexes relevant to oxygen activation.
Our joint projects have produced a larger number of interesting
compounds. Among these is the first nonheme FeIV-oxo complex,
1 [FeIV(O)(TMC)(NCMe)]2+. This complex
has electronic spin S = 1. For nonheme proteins the combination
of carboxylate, histidine, H2O, OH- ligands
generally produces high-spin (S = 2) FeIV sites,
an environment not easily produced in a synthetic complex. However,
we reported in 2009 the first high-spin FeIV=O complex,
[FeIV(O)(TMG3tren)]2+.2 The
Que group has recently modified the TMG3tren ligand to TMG2dien,
which allows introduction of carboxylate ligands.
We have recently studied a group of
fascinating high-valent diiron complexes based on the TPA ligand
(traded by insiders as the Castro Brothers). The FeIVFeIV
complex has two local S = 1 sites which are ferromagnetically
coupled to yield an S = 2 system state. One site, Feb,
has a terminal oxo group; Fea has a hydroxo ligand.
Given that the Fe-O-Fe angle is 130°, the observation of
ferromagnetic coupling was puzzling, but could be explained
quite well after realizing that the two sites have different
ligand fields that produce a crucial pair of orthogonal “magnetic”
orbitals.3 Reduction to FeIVFeIII
(spin S = ½) renders the Feb site high-spin
FeIII. Concomitantly the FeIV=O site undergoes
a transition to high-spin FeIV=O, a transition
that is driven by superexchange interactions between Fea
and Feb.4
The family of dinuclear TPA complexes
contains a closed-core FeIV(µ-O)2FeIII
complex. H-bond cleaving reactivity increases 1000-fold upon
opening the core to O=FeIV-O-FeIII-OH.
The spin transition at the oxo site yields an additional 1000-fold
increase;5 see Figure 4 of ref (5).
Together with O. Pestovsky
and Andreja
Bakac
of the Ames Laboratory at Iowa State University we have characterized
a short-lived FeIV=O intermediate in the reaction of aqueous
Fe2+ with ozone.6 The DFT-deduced structure,
(H2O)5FeIV=O, shown below,
nicely produces the parameters obtained from a Mössbauer analysis.
This complex has long been postulated by some researchers to
be the reactive intermediate in Fenton chemistry. However, as
shown be our collaborators, (H2O)5FeIV=O yields products quite different
from those observed in Fenton chemistry.
We are also studying a variety of
other FeIV complexes, including
highly reactive intermediates of Fe-TAML activators studied
in the lab of our CMU colleague T. C. Collins. |
[FeIV(O)(TMC)(NCMe)]2+ |
[FeIV(O)(TMG3tren)]2+ |
[(O)FeIV(µ-O)FeIV(OH)(L2)]3+
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[FeIV(O)(H2O)5]2+ |
(1)
Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel,
W. W.; Bukowsky, M. R.; Stubna,
A.; Münck, E.; Nam, W.; Que
Jr., L. Science 2003, 299, 1037-1039.
(2) England, J.; Martinho, M.; Farquhar,
E. R.; Frisch, J. R.; Bominaar,E. L.; Münck, E.; Que,
L., Jr. Angew. Chem., Int. Ed. 2009, 48,
3622–3626.
(3)
Martinho, M.; Xue, G.; Fiedler, A. T.; Que, L., Jr.; Bominaar,
E. L.; Munck, E. J. Am. Chem. Soc. 2009,
131, 5823-5830. (JACS Select
#9 Article)
(4)
De Hont, R. F.; Xue, G.; Hendrich, M. P.; Que, L. Jr.; Bominaar,
E. L.; Munck, E. Inorg. Chem. 2010,
49, 8310-8322.
(5)
Xue, G.; De Hont, R. F.; Munck, E.; Que, L. Jr. Nature
Chem. 2010, 2, 400-405.
(6) Pestovsky,
O.; Stoian, S.; Bominaar, E. L.;
Shan, X.; Münck, E.; Que,
L.; Bakac, A. Angew.
Chem. 2005, 44, 2-6.
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Magnetochemistry
Some of our metalloprotein
studies have revealed intriguing spectral features that required
digging deep into magnetochemistry.
For instance, our studies of the [3Fe-4S]0
cluster of D. gigas ferredoxin
II lead to the introduction of double exchange into the chemical
literature.1 Further, some puzzling Mössbauer observations
on the FeIIIFeIII
cluster of the hydroxylase of methane
monooxygenase let to the recognition
that antisymmetric exchange was at
the root of the problem.2
More recently we have been studying FeII
and FeI diketiminate
complexes. These systems have orbitally
degenerate ground states that yield exceptionally large magnetic
hyperfine fields (largest on record until 2003). The N2
bridged diketiminate dimer,
LFeIN2FeIL, exhibits unquenched
orbital angular momentum and what looked like unreasonably strong
ferromagnetic coupling. A DFT study revealed transfer of substantial
beta spin density to the bridging dinitrogen
(cartoon), suggesting that the complex is better described as
LFeIIN22-FeIIL for
which the spin ofeach iron (Sa = Sb
= 2) is antiferromagnetically
coupled by strong direct exchange to N22-
(Sc = 1) to yield a ground state with Stot
= 3. This model naturally yields parallel alignment of the iron
spins. |
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(1)
Papaefthymiou, V.; Girerd, J.
J.; Moura, I.; Moura,
J. J. G.; Münck, E. J. Am. Chem.
Soc. 1987, 109, 4703-4710.
(2) Kauffmann, K. E.; Popescu,
C.V.; Dong, Y.; Lipscomb, J. D.; Que,
L., Jr.; Münck, E. J. Am. Chem. Soc. 1998,
120, 8739-8746.
(3) Stoian,
S.; Vela, J.; Smith, J.; Holland, P. L.; Münck,
E.; Bominaar, E .L. J. Am. Chem. Soc., submitted
2006.
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| Electronic
Structure Analysis
The spectroscopic studies of our group
have often been complemented by DFT calculations. These computations
provide theoretical estimates of experimentally determined spin-Hamiltonian
parameters, such as zero-field splittings, exchange-coupling
constants, 57Fe isomer shifts, quadrupole splittings,
and magnetic hyperfine coupling constants, and give detailed
insights into the dependency of these parameters on molecular
geometry and electronic structure. These studies have clarified
the origin of the unquenched orbital momentum in the diketiminate
complexes of iron,1,2 the intrinsic mechanism for
the distortion of the Fe(SR)4 center in rubredoxins,3
the influence of excited spin triplet states on the zero-field
splitting in reduced form of these centers,4 and
the oxidation state of the cofactor in nitrogenase.5 |
(1)
Andres, H.; Bominaar, E.L.; Smith, J.M.; Eckert,
N.A.; Holland, P.L.; Münck, E. J. Am. Chem. Soc.
2002, 124, 3012-3025.
(2) Stoian, S.A.; Yu, Y.; Smith, J.M.; Holland,
P.L.; Bominaar, E.L.; Münck, E. Inorg. Chem. 2005,
44, 4915-4922.
(3) Vrajmasu, V.V.; Münck, E.; Bominaar, E.L.
Inorg. Chem. 2004, 43, 4862-4866;
ibid. 4867-4879.
(4) Vrajmasu, V.V.; Bominaar, E.L.; Meyer, J.;
Münck, E. Inorg. Chem. 2002,
41, 6358-6371.
(5) Vrajmasu, V.V.; Münck, E.; Bominaar,
E.L. Inorg. Chem. 2003, 42, 5974-5988.
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