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Multimodal Spectroscopic Analysis of the Fe-S Clusters of the as-Isolated Escherichia coli SufBC 2D Complex. Inorg Chem 2024; 63:8730-8738. [PMID: 38687645 DOI: 10.1021/acs.inorgchem.4c00304] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/02/2024]
Abstract
Iron-sulfur (Fe-S) clusters are essential inorganic cofactors dedicated to a wide range of biological functions, including electron transfer and catalysis. Specialized multiprotein machineries present in all types of organisms support their biosynthesis. These machineries encompass a scaffold protein, on which Fe-S clusters are assembled before being transferred to cellular targets. Here, we describe the first characterization of the native Fe-S cluster of the anaerobically purified SufBC2D scaffold from Escherichia coli by XAS and Mössbauer, UV-visible absorption, and EPR spectroscopies. Interestingly, we propose that SufBC2D harbors two iron-sulfur-containing species, a [2Fe-2S] cluster and an as-yet unidentified species. Mutagenesis and biochemistry were used to propose amino acid ligands for the [2Fe-2S] cluster, supporting the hypothesis that both SufB and SufD are involved in the Fe-S cluster ligation. The [2Fe-2S] cluster can be transferred to ferredoxin in agreement with the SufBC2D scaffold function. These results are discussed in the context of Fe-S cluster biogenesis.
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2
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X-ray absorption and emission spectroscopy of N 2S 2 Cu(II)/(III) complexes. Dalton Trans 2024; 53:7828-7838. [PMID: 38624161 DOI: 10.1039/d4dt00085d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/17/2024]
Abstract
This study investigates the influence of ligand charge on transition energies in a series of CuN2S2 complexes based on dithiocarbazate Schiff base ligands using Cu K-edge X-ray absorption spectroscopy (XAS) and Kβ valence-to-core (VtC) X-ray emission spectroscopy (XES). By comparing the formally Cu(II) complexes [CuII(HL1)] (HL12- = dimethyl pentane-2,4-diylidenebis[carbonodithiohydrazonate]) and [CuII(HL2)] (HL22- = dibenzyl pentane-2,4-diylidenebis[carbonodithiohydrazonate]) and the formally Cu(III) complex [CuIII(L2)], distinct changes in transition energies are observed, primarily attributed to the metal oxidation state. Density functional theory (DFT) calculations demonstrate how an increased negative charge on the deprotonated L23- ligand stabilizes the Cu(III) center through enhanced charge donation, modulating the core transition energies. Overall, significant shifts to higher energies are noted upon metal oxidation, emphasizing the importance of scrutinizing ligand structure in XAS/VtC XES analysis. The data further support the redox-innocent role of the Schiff base ligands and underscore the criticality of ligand protonation levels in future spectroscopic studies, particularly for catalytic intermediates. The combined XAS-VtC XES methodology validates the Cu(III) oxidation state assignment while offering insights into ligand protonation effects on core-level spectroscopic transitions.
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3
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Three-Coordinate Nickel and Metal-Metal Interactions in a Heterometallic Iron-Sulfur Cluster. J Am Chem Soc 2024; 146:4013-4025. [PMID: 38308743 PMCID: PMC10993082 DOI: 10.1021/jacs.3c12157] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2024]
Abstract
Biological multielectron reactions often are performed by metalloenzymes with heterometallic sites, such as anaerobic carbon monoxide dehydrogenase (CODH), which has a nickel-iron-sulfide cubane with a possible three-coordinate nickel site. Here, we isolate the first synthetic iron-sulfur clusters having a nickel atom with only three donors, showing that this structural feature is feasible. These have a core with two tetrahedral irons, one octahedral tungsten, and a three-coordinate nickel connected by sulfide and thiolate bridges. Electron paramagnetic resonance (EPR), Mössbauer, and superconducting quantum interference device (SQUID) data are combined with density functional theory (DFT) computations to show how the electronic structure of the cluster arises from strong magnetic coupling between the Ni, Fe, and W sites. X-ray absorption spectroscopy, together with spectroscopically validated DFT analysis, suggests that the electronic structure can be described with a formal Ni1+ atom participating in a nonpolar Ni-W σ-bond. This metal-metal bond, which minimizes spin density at Ni1+, is conserved in two cluster oxidation states. Fe-W bonding is found in all clusters, in one case stabilizing a local non-Hund state at tungsten. Based on these results, we compare different M-M interactions and speculate that other heterometallic clusters, including metalloenzyme active sites, could likewise store redox equivalents and stabilize low-valent metal centers through metal-metal bonding.
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4
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High-Spin and Reactive Fe 13 Cluster with Exposed Metal Sites. Angew Chem Int Ed Engl 2023; 62:e202313880. [PMID: 37871234 PMCID: PMC10962695 DOI: 10.1002/anie.202313880] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2023] [Revised: 10/23/2023] [Accepted: 10/23/2023] [Indexed: 10/25/2023]
Abstract
Atomically defined large metal clusters have applications in new reaction development and preparation of materials with tailored properties. Expanding the synthetic toolbox for reactive high nuclearity metal complexes, we report a new class of Fe clusters, Tp*4 W4 Fe13 S12 , displaying a Fe13 core with M-M bonds that has precedent only in main group and late metal chemistry. M13 clusters with closed shell electron configurations can show significant stability and have been classified as superatoms. In contrast, Tp*4 W4 Fe13 S12 displays a large spin ground state of S=13. This compound performs small molecule activations involving the transfer of up to 12 electrons resulting in significant cluster rearrangements.
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5
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Sulfur-Ligated [2Fe-2C] Clusters as Synthetic Model Systems for Nitrogenase. Inorg Chem 2023; 62:2663-2671. [PMID: 36715662 PMCID: PMC9930126 DOI: 10.1021/acs.inorgchem.2c03693] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2022] [Indexed: 01/31/2023]
Abstract
Metal clusters featuring carbon and sulfur donors have coordination environments comparable to the active site of nitrogenase enzymes. Here, we report a series of di-iron clusters supported by the dianionic yldiide ligands, in which the Fe sites are bridged by two μ2-C atoms and four pendant S donors.The [L2Fe2] (L = {[Ph2P(S)]2C}2-) cluster is isolable in two oxidation levels, all-ferrous Fe2II and mixed-valence FeIIFeIII. The mixed-valence cluster displays two peaks in the Mössbauer spectra, indicating slow electron transfer between the two sites. The addition of the Lewis base 4-dimethylaminopyridine to the Fe2II cluster results in coordination with only one of the two Fe sites, even in the presence of an excess base. Conversely, the cluster reacts with 8 equiv of isocyanide tBuNC to give a monometallic complex featuring a new C-C bond between the ligand backbone and the isocyanide. The electronic structure descriptions of these complexes are further supported by X-ray absorption and resonant X-ray emission spectroscopies.
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6
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Abstract
The biological process of dinitrogen reduction to ammonium occurs at the cofactors of nitrogenases, the only enzymes that catalyze this challenging chemical reaction. Three types of nitrogenases have been described, named according to the heterometal in their cofactor: molybdenum, vanadium or iron nitrogenases. Spectroscopic and structural characterization allowed the unambiguous identification of the cofactors of molybdenum and vanadium nitrogenases and revealed a central μ6 -carbide in both of them. Although genetic studies suggested that the cofactor of the iron nitrogenase contains a similar Fe6 C core, this has not been experimentally demonstrated. Here we report Valence-to-Core X-ray Emission Spectroscopy providing experimental evidence that this cofactor contains a carbide, thereby making the Fe6 C core a feature of all nitrogenase cofactors.
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7
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Iron Insertion at the Assembly Site of the ISCU Scaffold Protein Is a Conserved Process Initiating Fe-S Cluster Biosynthesis. J Am Chem Soc 2022; 144:17496-17515. [PMID: 36121382 PMCID: PMC10163866 DOI: 10.1021/jacs.2c06338] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Iron-sulfur (Fe-S) clusters are prosthetic groups of proteins biosynthesized on scaffold proteins by highly conserved multi-protein machineries. Biosynthesis of Fe-S clusters into the ISCU scaffold protein is initiated by ferrous iron insertion, followed by sulfur acquisition, via a still elusive mechanism. Notably, whether iron initially binds to the ISCU cysteine-rich assembly site or to a cysteine-less auxiliary site via N/O ligands remains unclear. We show here by SEC, circular dichroism (CD), and Mössbauer spectroscopies that iron binds to the assembly site of the monomeric form of prokaryotic and eukaryotic ISCU proteins via either one or two cysteines, referred to the 1-Cys and 2-Cys forms, respectively. The latter predominated at pH 8.0 and correlated with the Fe-S cluster assembly activity, whereas the former increased at a more acidic pH, together with free iron, suggesting that it constitutes an intermediate of the iron insertion process. Iron not binding to the assembly site was non-specifically bound to the aggregated ISCU, ruling out the existence of a structurally defined auxiliary site in ISCU. Characterization of the 2-Cys form by site-directed mutagenesis, CD, NMR, X-ray absorption, Mössbauer, and electron paramagnetic resonance spectroscopies showed that the iron center is coordinated by four strictly conserved amino acids of the assembly site, Cys35, Asp37, Cys61, and His103, in a tetrahedral geometry. The sulfur receptor Cys104 was at a very close distance and apparently bound to the iron center when His103 was missing, which may enable iron-dependent sulfur acquisition. Altogether, these data provide the structural basis to elucidate the Fe-S cluster assembly process and establish that the initiation of Fe-S cluster biosynthesis by insertion of a ferrous iron in the assembly site of ISCU is a conserved mechanism.
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8
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Determination of the iron(IV) local spin states of the Q intermediate of soluble methane monooxygenase by Kβ X-ray emission spectroscopy. J Biol Inorg Chem 2022; 27:573-582. [PMID: 35988092 PMCID: PMC9470658 DOI: 10.1007/s00775-022-01953-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2022] [Accepted: 08/07/2022] [Indexed: 11/29/2022]
Abstract
Soluble methane monooxygenase (sMMO) facilitates the conversion of methane to methanol at a non-heme FeIV2 intermediate MMOHQ, which is formed in the active site of the sMMO hydroxylase component (MMOH) during the catalytic cycle. Other biological systems also employ high-valent FeIV sites in catalysis; however, MMOHQ is unique as Nature’s only identified FeIV2 intermediate. Previous 57Fe Mössbauer spectroscopic studies have shown that MMOHQ employs antiferromagnetic coupling of the two FeIV sites to yield a diamagnetic cluster. Unfortunately, this lack of net spin prevents the determination of the local spin state (Sloc) of each of the irons by most spectroscopic techniques. Here, we use Fe Kβ X-ray emission spectroscopy (XES) to characterize the local spin states of the key intermediates of the sMMO catalytic cycle, including MMOHQ trapped by rapid-freeze-quench techniques. A pure XES spectrum of MMOHQ is obtained by subtraction of the contributions from other reaction cycle intermediates with the aid of Mössbauer quantification. Comparisons of the MMOHQ spectrum with those of known Sloc = 1 and Sloc = 2 FeIV sites in chemical and biological models reveal that MMOHQ possesses Sloc = 2 iron sites. This experimental determination of the local spin state will help guide future computational and mechanistic studies of sMMO catalysis.
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9
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An Fe6C Core in All Nitrogenase Cofactors. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202209190] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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10
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Challenges and Opportunities for Applications of Advanced X-ray Spectroscopy in Catalysis Research. ACS Catal 2022. [DOI: 10.1021/acscatal.2c01016] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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11
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Abstract
The ability to observe the changes that occur at an enzyme active site during electrocatalysis can provide very valuable information for understanding the mechanism and ultimately aid in catalyst design. Herein, we discuss the development of X-ray absorption spectroscopy (XAS) in combination with electrochemistry for operando studies of enzymatic systems. XAS has had a long history of enabling geometric and electronic structural insights into the catalytic active sites of enzymes, however, XAS combined with electrochemistry (XA-SEC) has been exceedingly rare in bioinorganic applications. Herein, we discuss the challenges and opportunities of applying operando XAS to enzymatic electrocatalysts. The challenges due to the low concentration of the photoabsorber and the instability of the protein in the X-ray beam are discussed. Methods for immobilizing enzymes on the electrodes, while maintaining full redox control are highlighted. A case study of combined XAS and electrochemistry applied to a [NiFe] hydrogenase is presented. By entrapping the [NiFe] hydrogenase in a redox polymer, relatively high protein concentrations can be achieved on the electrode surface, while maintaining redox control. Overall, it is demonstrated that the experiments are feasible, but require precise redox control over the majority of the absorber atoms and careful controls to discriminate between electrochemically-driven changes and beam damage. Opportunities for future applications are discussed.
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A Combined Spectroscopic and Computational Study on the Mechanism of Iron-Catalyzed Aminofunctionalization of Olefins Using Hydroxylamine Derived N-O Reagent as the "Amino" Source and "Oxidant". J Am Chem Soc 2022; 144:2637-2656. [PMID: 35119853 PMCID: PMC8855425 DOI: 10.1021/jacs.1c11083] [Citation(s) in RCA: 14] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
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Herein, we study
the mechanism of iron-catalyzed direct synthesis
of unprotected aminoethers from olefins by a hydroxyl amine derived
reagent using a wide range of analytical and spectroscopic techniques
(Mössbauer, Electron Paramagnetic Resonance, Ultra-Violet Visible
Spectroscopy, X-ray Absorption, Nuclear Resonance Vibrational Spectroscopy,
and resonance Raman) along with high-level quantum chemical calculations.
The hydroxyl amine derived triflic acid salt acts as the “oxidant”
as well as “amino” group donor. It activates the high-spin
Fe(II) (St = 2) catalyst [Fe(acac)2(H2O)2] (1) to generate
a high-spin (St = 5/2) intermediate (Int I), which decays to a second intermediate (Int II) with St = 2. The analysis of spectroscopic
and computational data leads to the formulation of Int I as [Fe(III)(acac)2-N-acyloxy] (an alkyl-peroxo-Fe(III)
analogue). Furthermore, Int II is formed by N–O
bond homolysis. However, it does not generate a high-valent
Fe(IV)(NH) species (a Fe(IV)(O) analogue), but instead a high-spin
Fe(III) center which is strongly antiferromagnetically coupled (J = −524 cm–1) to an iminyl radical,
[Fe(III)(acac)2-NH·], giving St = 2. Though Fe(NH) complexes as isoelectronic surrogates
to Fe(O) functionalities are known, detection of a high-spin Fe(III)-N-acyloxy intermediate (Int I), which undergoes
N–O bond cleavage to generate the active iron–nitrogen
intermediate (Int II), is unprecedented. Relative to
Fe(IV)(O) centers, Int II features a weak elongated Fe–N
bond which, together with the unpaired electron density along the
Fe–N bond vector, helps to rationalize its propensity for N-transfer reactions onto styrenyl olefins, resulting in
the overall formation of aminoethers. This study thus demonstrates
the potential of utilizing the iron-coordinated nitrogen-centered
radicals as powerful reactive intermediates in catalysis.
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13
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Stabilization of intermediate spin states in mixed-valent diiron dichalcogenide complexes. Nat Chem 2022; 14:328-333. [PMID: 35058610 PMCID: PMC8898764 DOI: 10.1038/s41557-021-00853-5] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2021] [Accepted: 11/03/2021] [Indexed: 11/24/2022]
Abstract
The electronic structure and ground spin states, S, observed for mixed-valent iron–sulfur dimers (FeII-FeIII) are typically determined by the Heisenberg exchange interaction, J, that couples the magnetic interaction of the two metal centres either ferromagnetically (J > 0, S = 9/2) or antiferromagnetically (J < 0, S = 1/2). In the case of antiferromagnetically coupled iron centres, stabilization of the high-spin S = 9/2 ground state is also feasible through a Heisenberg double-exchange interaction, B, which lifts the degeneracy of the Heisenberg spin states. This theorem also predicts intermediate spin states for mixed-valent dimers, but those have so far remained elusive. Herein, we describe the structural, electron paramagnetic resonance and Mössbauer spectroscopic, and magnetic characterization of a series of mixed-valent complexes featuring [Fe2Q2]+ (Q = S2–, Se2–, Te2–), where the Se and Te complexes favour S = 3/2 spin states. The incorporation of heavier chalcogenides in this series reveals a delicate balance of antiferromagnetic coupling, Heisenberg double-exchange and vibronic coupling. ![]()
Despite extensive investigations of mixed-valence complexes, molecules with intermediate spin states have remained elusive. Now, selenium- and tellurium-bridged mixed-valent iron dimers have been prepared in which a balance of Heisenberg exchange and double-exchange coupling of the unpaired electron, combined with moderate vibronic contributions, stabilizes S = 3/2 ground spin states.
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14
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Combining Valence-to-Core X-ray Emission and Cu K-edge X-ray Absorption Spectroscopies to Experimentally Assess Oxidation State in Organometallic Cu(I)/(II)/(III) Complexes. J Am Chem Soc 2022; 144:2520-2534. [PMID: 35050605 PMCID: PMC8855422 DOI: 10.1021/jacs.1c09505] [Citation(s) in RCA: 25] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
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A series of organometallic
copper complexes in formal oxidation
states ranging from +1 to +3 have been characterized by a combination
of Cu K-edge X-ray absorption (XAS) and Cu Kβ valence-to-core
X-ray emission spectroscopies (VtC XES). Each formal oxidation state
exhibits distinctly different XAS and VtC XES transition energies
due to the differences in the Cu Zeff, concomitant with
changes in physical oxidation state from +1 to +2 to +3. Herein, we
demonstrate the sensitivity of XAS and VtC XES to the physical oxidation
states of a series of N-heterocyclic carbene (NHC) ligated organocopper
complexes. We then extend these methods to the study of the [Cu(CF3)4]− ion. Complemented by computational
methods, the observed spectral transitions are correlated with the
electronic structure of the complexes and the Cu Zeff.
These calculations demonstrate that a contraction of the Cu 1s orbitals
to deeper binding energy upon oxidation of the Cu center manifests
spectroscopically as a stepped increase in the energy of both XAS
and Kβ2,5 emission features with increasing formal
oxidation state within the [Cun+(NHC2)]n+ series. The newly synthesized Cu(III) cation
[CuIII(NHC4)]3+ exhibits spectroscopic
features and an electronic structure remarkably similar to [Cu(CF3)4]−, supporting a physical oxidation
state assignment of low-spin d8 Cu(III) for [Cu(CF3)4]−. Combining XAS and VtC XES
further demonstrates the necessity of combining multiple spectroscopies
when investigating the electronic structures of highly covalent copper
complexes, providing a template for future investigations into both
synthetic and biological metal centers.
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A Conformational Role for NifW in the Maturation of Molybdenum Nitrogenase P-cluster. Chem Sci 2022; 13:3489-3500. [PMID: 35432878 PMCID: PMC8943848 DOI: 10.1039/d1sc06418e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2021] [Accepted: 02/28/2022] [Indexed: 12/03/2022] Open
Abstract
Reduction of dinitrogen by molybdenum nitrogenase relies on complex metalloclusters: the [8Fe:7S] P-cluster and the [7Fe:9S:Mo:C:homocitrate] FeMo-cofactor. Although both clusters bear topological similarities and require the reductive fusion of [4Fe:4S] sub-clusters to achieve their respective assemblies, P-clusters are assembled directly on the NifD2K2 polypeptide prior to the insertion of FeMo-co, which is fully assembled separately from NifD2K2. P-cluster maturation involves the iron protein NifH2 as well as several accessory proteins, whose role has not been elucidated. In the present work, two NifD2K2 species bearing immature P-clusters were isolated from an Azotobacter vinelandii strain in which the genes encoding NifH and the accessory protein NifZ were deleted, and characterized by X-ray absorption spectroscopy and EPR. These analyses showed that both NifD2K2 complexes harbor clusters that are electronically and structurally similar, with each NifDK unit containing two [4Fe:4S]2+/+ clusters. Binding of the accessory protein NifW parallels a decrease in the distance between these clusters, as well as a subtle change in their coordination. These results support a conformational role for NifW in P-cluster biosynthesis, bringing the two [4Fe:4S] precursors closer prior to their fusion, which may be crucial in challenging cellular contexts. Upon binding of NifW, a subtle conformation change occurs in NifD2K2, decreasing the distance between the two [4Fe:4S] clusters precursors of the P-cluster in nitrogenase.![]()
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[2Fe-2S] Cluster Supported by Redox-Active o-Phenylenediamide Ligands and Its Application toward Dinitrogen Reduction. Inorg Chem 2021; 60:13811-13820. [PMID: 34043353 DOI: 10.1021/acs.inorgchem.1c00683] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
As prevalent cofactors in living organisms, iron-sulfur clusters participate in not only the electron-transfer processes but also the biosynthesis of other cofactors. Many synthetic iron-sulfur clusters have been used in model studies, aiming to mimic their biological functions and to gain mechanistic insight into the related biological systems. The smallest [2Fe-2S] clusters are typically used for one-electron processes because of their limited capacity. Our group is interested in functionalizing small iron-sulfur clusters with redox-active ligands to enhance their electron storage capacity, because such functionalized clusters can potentially mediate multielectron chemical transformations. Herein we report the synthesis, structural characterization, and catalytic activity of a diferric [2Fe-2S] cluster functionalized with two o-phenylenediamide ligands. The electrochemical and chemical reductions of such a cluster revealed rich redox chemistry. The functionalized diferric cluster can store up to four electrons reversibly, where the first two reduction events are ligand-based and the remainder metal-based. The diferric [2Fe-2S] cluster displays catalytic activity toward silylation of dinitrogen, affording up to 88 equiv of the amine product per iron center.
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Probing Physical Oxidation State by Resonant X-ray Emission Spectroscopy: Applications to Iron Model Complexes and Nitrogenase. Angew Chem Int Ed Engl 2021; 60:10112-10121. [PMID: 33497500 PMCID: PMC8252016 DOI: 10.1002/anie.202015669] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2020] [Indexed: 11/07/2022]
Abstract
The ability of resonant X-ray emission spectroscopy (XES) to recover physical oxidation state information, which may often be ambiguous in conventional X-ray spectroscopy, is demonstrated. By combining Kβ XES with resonant excitation in the XAS pre-edge region, resonant Kβ XES (or 1s3p RXES) data are obtained, which probe the 3dn+1 final-state configuration. Comparison of the non-resonant and resonant XES for a series of high-spin ferrous and ferric complexes shows that oxidation state assignments that were previously unclear are now easily made. The present study spans iron tetrachlorides, iron sulfur clusters, and the MoFe protein of nitrogenase. While 1s3p RXES studies have previously been reported, to our knowledge, 1s3p RXES has not been previously utilized to resolve questions of metal valency in highly covalent systems. As such, the approach presented herein provides chemists with means to more rigorously and quantitatively address challenging electronic-structure questions.
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Probing Physical Oxidation State by Resonant X‐ray Emission Spectroscopy: Applications to Iron Model Complexes and Nitrogenase. Angew Chem Int Ed Engl 2021. [DOI: 10.1002/ange.202015669] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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19
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The Asp1 pyrophosphatase from S. pombe hosts a [2Fe-2S] 2+ cluster in vivo. J Biol Inorg Chem 2021; 26:93-108. [PMID: 33544225 PMCID: PMC8038993 DOI: 10.1007/s00775-020-01840-w] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2020] [Accepted: 11/29/2020] [Indexed: 11/25/2022]
Abstract
The Schizosaccharomyces pombe Asp1 protein is a bifunctional kinase/pyrophosphatase that belongs to the highly conserved eukaryotic diphosphoinositol pentakisphosphate kinase PPIP5K/Vip1 family. The N-terminal Asp1 kinase domain generates specific high-energy inositol pyrophosphate (IPP) molecules, which are hydrolyzed by the C-terminal Asp1 pyrophosphatase domain (Asp1365-920). Thus, Asp1 activities regulate the intracellular level of a specific class of IPP molecules, which control a wide number of biological processes ranging from cell morphogenesis to chromosome transmission. Recently, it was shown that chemical reconstitution of Asp1371-920 leads to the formation of a [2Fe-2S] cluster; however, the biological relevance of the cofactor remained under debate. In this study, we provide evidence for the presence of the Fe-S cluster in Asp1365-920 inside the cell. However, we show that the Fe-S cluster does not influence Asp1 pyrophosphatase activity in vitro or in vivo. Characterization of the as-isolated protein by electronic absorption spectroscopy, mass spectrometry, and X-ray absorption spectroscopy is consistent with the presence of a [2Fe-2S]2+ cluster in the enzyme. Furthermore, we have identified the cysteine ligands of the cluster. Overall, our work reveals that Asp1 contains an Fe-S cluster in vivo that is not involved in its pyrophosphatase activity.
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Abstract
A series of β-diketiminate Ni-NO complexes with a range of NO binding modes and oxidation states were studied by X-ray emission spectroscopy (XES). The results demonstrate that XES can directly probe and distinguish end-on vs side-on NO coordination modes as well as one-electron NO reduction. Density functional theory (DFT) calculations show that the transition from the NO 2s2s σ* orbital has higher intensity for end-on NO coordination than for side-on NO coordination, whereas the 2s2s σ orbital has lower intensity. XES calculations in which the Ni-N-O bond angle was fixed over the range from 80° to 176° suggest that differences in NO coordination angles of ∼10° could be experimentally distinguished. Calculations of Cu nitrite reductase (NiR) demonstrate the utility of XES for characterizing NO intermediates in metalloenzymes. This work shows the capability of XES to distinguish NO coordination modes and oxidation states at Ni and highlights applications in quantifying small molecule activation in enzymes.
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21
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Unravelling the local structure of catalytic Fe-oxo clusters stabilized on the MOF-808 metal organic-framework. Chem Commun (Camb) 2020; 56:15615-15618. [PMID: 33290455 DOI: 10.1039/d0cc06134d] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
Stabilizing catalytic iron-oxo-clusters within nanoporous metal-organic frameworks (MOFs) is a powerful strategy to prepare new active materials for the degradation of toxic chemicals, such as bisphenol A. Herein, we combine pair distribution function analysis of total X-ray scattering data and X-ray absorption spectroscopy, with computational modelling to understand the local structural nature of added redox-active iron-oxo clusters bridging neighbouring zirconia-nodes within MOF-808.
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Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase. J Am Chem Soc 2020; 142:18795-18813. [PMID: 32976708 DOI: 10.1021/jacs.0c05920] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Binding of N2 by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 e- and 4 H+ equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(μ-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron μ-carbyne complex (P6ArC)Fe2(μ-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(μ-C)(μ-H)Fe model complexes linked by a 2 e- transfer which bear some resemblance to a pair of En and En+2 states of nitrogenase. Both species populate S = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the μ-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe2(μ-H)}2 state of FeMoco (E4(4H)), the data and analysis presented herein suggest that the hydride ligands in E4(4H) bridge isovalent (most probably FeIII) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(P6ArC)Fe2(μ-H)]+1 and [(P6ArC)Fe2(μ-H)]-1 by pulse EPR revealed that redox chemistry induces significant changes in Fe-C covalency (-50% upon 2 e- reduction), a conclusion further supported by X-ray absorption spectroscopy, 57Fe Mössbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.
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Abstract
Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation. Synthetic model chemistry and theory have also played significant roles in developing our present understanding of these systems and are discussed in the context of their contributions to interpreting the nature of nitrogenases. Despite years of significant progress, there is still much to be learned from these enzymes through spectroscopic means, and we highlight where further spectroscopic investigations are needed.
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25
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Multielectron, Cation and Anion Redox in Lithium-Rich Iron Sulfide Cathodes. J Am Chem Soc 2020; 142:6737-6749. [DOI: 10.1021/jacs.0c00909] [Citation(s) in RCA: 29] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
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Theoretical Analysis of Fe K-Edge XANES on Iron Pentacarbonyl. ACS OMEGA 2020; 5:4991-5000. [PMID: 32201785 PMCID: PMC7081404 DOI: 10.1021/acsomega.9b03887] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/14/2019] [Accepted: 02/21/2020] [Indexed: 05/21/2023]
Abstract
Iron pentacarbonyl (Fe(CO)5) is a versatile material that is utilized as an inhibitor of flame, shows soot suppressibility, and is used as a precursor for focused electron-beam-induced deposition (FEBID). X-ray absorption near-edge structure (XANES) of the K edge, which is a powerful technique for monitoring the oxidation states and coordination environment of metal sites, can be used to gain insight into Fe(CO)5-related reaction mechanisms in in situ experiments. We use a finite difference method (FDM) and molecular-orbital-based time-dependent density functional theory (TDDFT) calculations to clarify the Fe K-edge XANES features of Fe(CO)5. The two pre-edge peaks P1 and P2 are mainly the Fe(1s) → Fe-C(σ*) and Fe(1s) → Fe-C(π*) transitions, respectively. When the geometry transformed from D 3h to C 4v symmetry, a ∼30% decrease of the pre-edge P2 intensity was observed in the simulated spectra. This implies that the π bonding of Fe and CO is sensitive to changes in geometry. The following rising edge and white line regions are assigned to the Fe(1s) → Fe(4p)(mixing C(2p)) transitions. Our results may provide useful information to interpret XANES spectra variations of in situ reactions of metal-CO or similar compounds with π acceptor ligandlike metal-CN complexes.
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The influences of carbon donor ligands on biomimetic multi-iron complexes for N 2 reduction. Chem Sci 2020; 11:12710-12720. [PMID: 34094466 PMCID: PMC8163302 DOI: 10.1039/d0sc03447a] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023] Open
Abstract
The active site clusters of nitrogenase enzymes possess the only examples of carbides in biology. These are the only biological FeS clusters that are capable of reducing N2 to NH4+, implicating the central carbon and its interaction with Fe as important in the mechanism of N2 reduction. This biological question motivates study of the influence of carbon donors on the electronic structure and reactivity of unsaturated, high-spin iron centers. Here, we present functional and structural models that test the impacts of carbon donors and sulfide donors in simpler iron compounds. We report the first example of a diiron complex that is bridged by an alkylidene and a sulfide, which serves as a high-fidelity structural and spectroscopic model of a two-iron portion of the active-site cluster (FeMoco) in the resting state of Mo-nitrogenase. The model complexes have antiferromagnetically coupled pairs of high-spin iron centers, and sulfur K-edge X-ray absorption spectroscopy shows comparable covalency of the sulfide for C and S bridged species. The sulfur-bridged compound does not interact with N2 even upon reduction, but upon removal of the sulfide it becomes capable of reducing N2 to NH4+ with the addition of protons and electrons. This provides synthetic support for sulfide extrusion in the activation of nitrogenase cofactors. High-spin diiron alkylidenes give insight into the electronic structure and functional relevance of carbon in the FeMoco active site of nitrogenase.![]()
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Evaluation of cobalt complexes with tripod ligands for zinc finger targeting. Dalton Trans 2020; 49:16143-16153. [DOI: 10.1039/d0dt00067a] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
We report the ability of CoII and CoIII complexes of tri(2-pyridylmethyl)amine and N,N-di(2-pyridylmethyl)glycinate to disrupt zinc fingers.
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Electronic characterization of redox (non)-innocent Fe 2S 2 reference systems: a multi K-edge X-ray spectroscopic study. RSC Adv 2020; 10:729-738. [PMID: 35494446 PMCID: PMC9048190 DOI: 10.1039/c9ra08903a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2019] [Accepted: 12/19/2019] [Indexed: 01/03/2023] Open
Abstract
Di-iron dithiolate hydrogenase model complexes are promising systems for electrocatalytic production of dihydrogen and have therefore been spectroscopically and theoretically investigated in this study. The direct effect of ligand substitution on the redox activity of the complex is examined. In order to understand and eventually optimize such systems, we characterised both metal and ligand in detail, using element specific X-ray absorption Fe- and S-K edge XAS. The (electronic) structure of three different [Fe2S2] hydrogenase systems in their non-reduced state was investigated. The effect of one- and two-electron reduction on the (electronic) structure was subsequently investigated. The S K-edge XAS spectra proved to be sensitive to delocalization of the electron density into the aromatic ring. The earlier postulated charge and spin localization in these complexes could now be measured directly using XANES. Moreover, the electron density (from S K-edge XANES) could be directly correlated to the Fe–CO bond length (from Fe K-edge EXAFS), which are in turn both related to the reported catalytic activity of these complexes. The delocalization of the electron density into the conjugated π-system of the aromatic moieties lowers the basicity of the diiron core and since protonation occurs at the diiron (as a rate determining step), lowering the basicity decreases the extent of protonation and consequently the catalytic activity. Di-iron dithiolate hydrogenase model complexes are promising systems for electrocatalytic production of dihydrogen and have therefore been spectroscopically and theoretically investigated in this study.![]()
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Towards theoretical spectroscopy with error bars: systematic quantification of the structural sensitivity of calculated spectra. Chem Sci 2019; 11:1862-1877. [PMID: 34123280 PMCID: PMC8148348 DOI: 10.1039/c9sc05103a] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Molecular spectra calculated with quantum-chemical methods are subject to a number of uncertainties (e.g., errors introduced by the computational methodology) that hamper the direct comparison of experiment and computation. Judging these uncertainties is crucial for drawing reliable conclusions from the interplay of experimental and theoretical spectroscopy, but largely relies on subjective judgment. Here, we explore the application of methods from uncertainty quantification to theoretical spectroscopy, with the ultimate goal of providing systematic error bars for calculated spectra. As a first target, we consider distortions of the underlying molecular structure as one important source of uncertainty. We show that by performing a principal component analysis, the most influential collective distortions can be identified, which allows for the construction of surrogate models that are amenable to a statistical analysis of the propagation of uncertainties in the molecular structure to uncertainties in the calculated spectrum. This is applied to the calculation of X-ray emission spectra of iron carbonyl complexes, of the electronic excitation spectrum of a coumarin dye, and of the infrared spectrum of alanine. We show that with our approach it becomes possible to obtain error bars for calculated spectra that account for uncertainties in the molecular structure. This is an important first step towards systematically quantifying other relevant sources of uncertainty in theoretical spectroscopy. Uncertainty quantification is applied in theoretical spectroscopy to obtain error bars accounting for the structural sensitivity of calculated spectra.![]()
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Spectroscopic X-ray and Mössbauer Characterization of M 6 and M 5 Iron(Molybdenum)-Carbonyl Carbide Clusters: High Carbide-Iron Covalency Enhances Local Iron Site Electron Density Despite Cluster Oxidation. Inorg Chem 2019; 58:12918-12932. [PMID: 31553598 PMCID: PMC6784818 DOI: 10.1021/acs.inorgchem.9b01870] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
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The present study
employs a suite of spectroscopic techniques to
evaluate the electronic and bonding characteristics of the interstitial
carbide in a set of iron-carbonyl-carbide clusters, one of which is
substituted with a molybdenum atom. The M6C and M5C clusters are the dianions (Et4N)2[Fe6(μ6-C)(μ2-CO)2(CO)14] (1),
[K(benzo-18-crown-6)]2[Fe5(μ5-C)(μ2-CO)1(CO)13] (2), and [K(benzo-18-crown-6)]2[Fe5Mo(μ6-C)(μ2-CO)2(CO)15] (3). Because 1 and 2 have the same overall cluster charge (2−) but different numbers
of iron sites (1: 6 sites → 2: 5
sites), the metal atoms of 2 are formally oxidized compared
to those in 1. Despite this, Mössbauer studies
indicate that the iron sites in 2 possess significantly
greater electron density (lower spectroscopic oxidation state)
compared with those in 1. Iron K-edge X-ray absorption
and valence-to-core X-ray emission spectroscopy measurements, paired
with density functional theory spectral calculations, revealed the
presence of significant metal-to-metal and carbide 2p-based character
in the filled valence and low-lying unfilled electronic manifolds.
In all of the above experiments, the presence of the molybdenum atom
in 3 (Fe5Mo) results in somewhat unremarkable
spectroscopic properties that are essentially a “hybrid”
of 1 (Fe6) and 2 (Fe5). The overall electronic portrait that emerges illustrates that
the central inorganic carbide ligand is essential for distributing
charge and maximizing electronic communication throughout the cluster.
It is evident that the carbide coordination environment is quite flexible
and adaptive: it can drastically modify the covalency of individual
Fe–C bonds based on local structural changes and redox manipulation
of the clusters. In light of these findings, our data and calculations
suggest a potential role for the central carbon atom in FeMoco, which
likely performs a similar function in order to maintain cluster integrity
through multiple redox and ligand binding events. An in-depth
spectroscopic investigation of a series of iron-carbonyl
carbide complexes: [Fe6C] (1), [Fe5C] (2), and [Fe5CMo] (3) is
described. Using Mössbauer spectroscopy, valence-to-core X-ray
emission spectroscopy, and high-energy-resolution fluorescence-detected
X-ray absorption spectroscopy, we detail the ability of the conserved
central carbon atom in maintaining cluster stability despite dramatic
geometric rearrangements. Our study suggests a potential role for
the interstitial carbide in FeMoco as an electronic modulator, allowing
for charge and ligand accumulation under turnover conditions.
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Localized Electronic Structure of Nitrogenase FeMoco Revealed by Selenium K-Edge High Resolution X-ray Absorption Spectroscopy. J Am Chem Soc 2019; 141:13676-13688. [PMID: 31356071 PMCID: PMC6716209 DOI: 10.1021/jacs.9b06988] [Citation(s) in RCA: 35] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2019] [Indexed: 11/28/2022]
Abstract
The size and complexity of Mo-dependent nitrogenase, a multicomponent enzyme capable of reducing dinitrogen to ammonia, have made a detailed understanding of the FeMo cofactor (FeMoco) active site electronic structure an ongoing challenge. Selective substitution of sulfur by selenium in FeMoco affords a unique probe wherein local Fe-Se interactions can be directly interrogated via high-energy resolution fluorescence detected X-ray absorption spectroscopic (HERFD XAS) and extended X-ray absorption fine structure (EXAFS) studies. These studies reveal a significant asymmetry in the electronic distribution of the FeMoco, suggesting a more localized electronic structure picture than is typically assumed for iron-sulfur clusters. Supported by experimental small molecule model data in combination with time dependent density functional theory (TDDFT) calculations, the HERFD XAS data is consistent with an assignment of Fe2/Fe6 as an antiferromagnetically coupled diferric pair. HERFD XAS and EXAFS have also been applied to Se-substituted CO-inhibited MoFe protein, demonstrating the ability of these methods to reveal electronic and structural changes that occur upon substrate binding. These results emphasize the utility of Se HERFD XAS and EXAFS for selectively probing the local electronic and geometric structure of FeMoco.
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Spectroscopic Description of the E 1 State of Mo Nitrogenase Based on Mo and Fe X-ray Absorption and Mössbauer Studies. Inorg Chem 2019; 58:12365-12376. [PMID: 31441651 PMCID: PMC6751781 DOI: 10.1021/acs.inorgchem.9b01951] [Citation(s) in RCA: 30] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Mo nitrogenase (N2ase) utilizes a two-component protein system, the catalytic MoFe and its electron-transfer partner FeP, to reduce atmospheric dinitrogen (N2) to ammonia (NH3). The FeMo cofactor contained in the MoFe protein serves as the catalytic center for this reaction and has long inspired model chemistry oriented toward activating N2. This field of chemistry has relied heavily on the detailed characterization of how Mo N2ase accomplishes this feat. Understanding the reaction mechanism of Mo N2ase itself has presented one of the most challenging problems in bioinorganic chemistry because of the ephemeral nature of its catalytic intermediates, which are difficult, if not impossible, to singly isolate. This is further exacerbated by the near necessity of FeP to reduce native MoFe, rendering most traditional means of selective reduction inept. We have now investigated the first fundamental intermediate of the MoFe catalytic cycle, E1, as prepared both by low-flux turnover and radiolytic cryoreduction, using a combination of Mo Kα high-energy-resolution fluorescence detection and Fe K-edge partial-fluorescence-yield X-ray absorption spectroscopy techniques. The results demonstrate that the formation of this state is the result of an Fe-centered reduction and that Mo remains redox-innocent. Furthermore, using Fe X-ray absorption and 57Fe Mössbauer spectroscopies, we correlate a previously reported unique species formed under cryoreducing conditions to the natively formed E1 state through annealing, demonstrating the viability of cryoreduction in studying the catalytic intermediates of MoFe.
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From Ylides to Doubly Yldiide-Bridged Iron(II) High Spin Dimers via Self-Protolysis. Inorg Chem 2019; 58:9358-9367. [PMID: 31260277 PMCID: PMC6750861 DOI: 10.1021/acs.inorgchem.9b01086] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/14/2019] [Indexed: 12/20/2022]
Abstract
A synthetic strategy for the preparation of novel doubly yldiide bridged iron(II) high spin dimers ([(μ2-C)FeL]2, L = N(SiMe3)2, Mesityl) has been developed. This includes the synthesis of ylide-iron(II) monomers [(Ylide)FeL2] via adduct formation. Subsequent self-protolysis at elevated temperatures by in situ deprotonation of the ylide ligands results in a dimerization reaction forming the desired bridging μ2-C yldiide ligands in [(μ2-C)FeL]2. The comprehensive structural and electronic analysis of dimers [(μ2-C)FeL]2, including NMR, Mössbauer, and X-ray spectroscopy, as well as X-ray crystallography, SQUID, and DFT calculations, confirm their high-spin FeII configurations. Interestingly, the Fe2C2 cores display very acute Fe-C-Fe angles (averaged: 78.6(2)°) resulting in short Fe···Fe distances (averaged: 2.588(2) Å). A remarkably strong antiferromagnetic coupling between the Fe centers has been identified. Strongly polarized Fe-C bonds are observed where the negative charge is mostly centered at the μ2-C yldiide ligands.
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X-ray Magnetic Circular Dichroism Spectroscopy Applied to Nitrogenase and Related Models: Experimental Evidence for a Spin-Coupled Molybdenum(III) Center. Angew Chem Int Ed Engl 2019; 58:9373-9377. [PMID: 31119827 PMCID: PMC6772009 DOI: 10.1002/anie.201901899] [Citation(s) in RCA: 28] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2019] [Revised: 04/17/2019] [Indexed: 11/23/2022]
Abstract
Nitrogenase enzymes catalyze the reduction of atmospheric dinitrogen to ammonia utilizing a Mo‐7Fe‐9S‐C active site, the so‐called FeMoco cluster. FeMoco and an analogous small‐molecule (Et4N)[(Tp)MoFe3S4Cl3] cubane have both been proposed to contain unusual spin‐coupled MoIII sites with an S(Mo)=1/2 non‐Hund configuration at the Mo atom. Herein, we present Fe and Mo L3‐edge X‐ray magnetic circular dichroism (XMCD) spectroscopy of the (Et4N)[(Tp)MoFe3S4Cl3] cubane and Fe L2,3‐edge XMCD spectroscopy of the MoFe protein (containing both FeMoco and the 8Fe‐7S P‐cluster active sites). As the P‐clusters of MoFe protein have an S=0 total spin, these are effectively XMCD‐silent at low temperature and high magnetic field, allowing for FeMoco to be selectively probed by Fe L2,3‐edge XMCD within the intact MoFe protein. Further, Mo L3‐edge XMCD spectroscopy of the cubane model has provided experimental support for a local S(Mo)=1/2 configuration, demonstrating the power and selectivity of XMCD.
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X‐ray Magnetic Circular Dichroism Spectroscopy Applied to Nitrogenase and Related Models: Experimental Evidence for a Spin‐Coupled Molybdenum(III) Center. Angew Chem Int Ed Engl 2019. [DOI: 10.1002/ange.201901899] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Spectroscopic and Quantum Chemical Investigation of Benzene-1,2-dithiolate-Coordinated Diiron Complexes with Relevance to Dinitrogen Activation. Inorg Chem 2019; 58:5111-5125. [DOI: 10.1021/acs.inorgchem.9b00177] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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38
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Site-Specific Oxidation State Assignments of the Iron Atoms in the [4Fe:4S] 2+/1+/0 States of the Nitrogenase Fe-Protein. Angew Chem Int Ed Engl 2019; 58:3894-3897. [PMID: 30698901 PMCID: PMC6519357 DOI: 10.1002/anie.201813966] [Citation(s) in RCA: 25] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2018] [Indexed: 12/05/2022]
Abstract
The nitrogenase iron protein (Fe-protein) contains an unusual [4Fe:4S] iron-sulphur cluster that is stable in three oxidation states: 2+, 1+, and 0. Here, we use spatially resolved anomalous dispersion (SpReAD) refinement to determine oxidation assignments for the individual irons for each state. Additionally, we report the 1.13-Å resolution structure for the ADP bound Fe-protein, the highest resolution Fe-protein structure presently determined. In the dithionite-reduced [4Fe:4S]1+ state, our analysis identifies a solvent exposed, delocalized Fe2.5+ pair and a buried Fe2+ pair. We propose that ATP binding by the Fe-protein promotes an internal redox rearrangement such that the solvent-exposed Fe pair becomes reduced, thereby facilitating electron transfer to the nitrogenase molybdenum iron-protein. In the [4Fe:4S]0 and [4Fe:4S]2+ states, the SpReAD analysis supports oxidation states assignments for all irons in these clusters of Fe2+ and valence delocalized Fe2.5+ , respectively.
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Comparison of multireference ab initio wavefunction methodologies for X-ray absorption edges: A case study on [Fe(II/III)Cl4]2–/1– molecules. J Chem Phys 2019; 150:104106. [DOI: 10.1063/1.5051613] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023] Open
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Site‐Specific Oxidation State Assignments of the Iron Atoms in the [4Fe:4S]
2+/1+/0
States of the Nitrogenase Fe‐Protein. Angew Chem Int Ed Engl 2019. [DOI: 10.1002/ange.201813966] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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41
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Cubane-Type [Mo 3 S 4 M] Clusters with First-Row Groups 4-10 Transition-Metal Halides Supported by C 5 Me 5 Ligands on Molybdenum. Chemistry 2018; 24:17138-17147. [PMID: 30204282 DOI: 10.1002/chem.201804083] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/09/2018] [Indexed: 02/02/2023]
Abstract
A synthetic protocol was developed for a series of cubane-type [Mo3 S4 M] clusters that incorporate halides of first-row transition metals (M) from Groups 4-10. This protocol is based on the anionic cluster platform [Cp*3 Mo3 S4 ]- ([1]- ; Cp*=η5 -C5 Me5 ), which crystallizes when K(18-crown-6) is used as the counter cation. Treatment of in situ-generated [1]- with such transition-metal halides led to the formation of [Mo3 S4 M] clusters, in which the M/halide ratio gradually changes from 1:2 to 1:1.5 and to 1:1, when moving from early to late transition metals. This trend suggests a tendency for early transition metals to tolerate higher oxidation states and adopt larger ionic radii relative to late transition metals. The properties of the [Mo3 S4 Fe] cluster 6 a were investigated in detail by using 57 Fe Mössbauer spectroscopy and computational methods.
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How Do Ring Size and π-Donating Thiolate Ligands Affect Redox-Active, α-Imino-N-heterocycle Ligand Activation? Inorg Chem 2018; 57:1935-1949. [PMID: 29411979 DOI: 10.1021/acs.inorgchem.7b02748] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Considerable effort has been devoted to the development of first-row transition-metal catalysts containing redox-active imino-pyridine ligands that are capable of storing multiple reducing equivalents. This property allows abundant and inexpensive first-row transition metals, which favor sequential one-electron redox processes, to function as competent catalysts in the concerted two-electron reduction of substrates. Herein we report the syntheses and characterization of a series of iron complexes that contain both π-donating thiolate and π-accepting (α-imino)-N-heterocycle redox-active ligands, with progressively larger N-heterocycle rings (imidazole, pyridine, and quinoline). A cooperative interaction between these complementary redox-active ligands is shown to dictate the properties of these complexes. Unusually intense charge-transfer (CT) bands, and intraligand metrical parameters, reminiscent of a reduced (α-imino)-N-heterocycle ligand (L•-), initially suggested that the electron-donating thiolate had reduced the N-heterocycle. Sulfur K-edge X-ray absorption spectroscopic (XAS) data, however, provides evidence for direct communication, via backbonding, between the thiolate sulfur and the formally orthogonal (α-imino)-N-heterocycle ligand π*-orbitals. DFT calculations provide evidence for extensive delocalization of bonds over the sulfur, iron, and (α-imino)-N-heterocycle, and TD-DFT shows that the intense optical CT bands involve transitions between a mixed Fe/S donor, and (α-imino)-N-heterocycle π*-acceptor orbital. The energies and intensities of the optical and S K-edge pre-edge XAS transitions are shown to correlate with N-heterocycle ring size, as do the redox potentials. When the thiolate is replaced with a thioether, or when the low-spin S = 0 Fe(II) is replaced with a high-spin S = 3/2 Co(II), the N-heterocycle ligand metrical parameters and electronic structure do not change relative to the neutral L0 ligand. With respect to the development of future catalysts containing redox-active ligands, the energy cost of storing reducing equivalents is shown to be lowest when a quinoline, as opposed to imidazole or pyridine, is incorporated into the ligand backbone of the corresponding Fe complex.
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Pair Natural Orbital Restricted Open-Shell Configuration Interaction (PNO-ROCIS) Approach for Calculating X-ray Absorption Spectra of Large Chemical Systems. J Phys Chem A 2018; 122:1215-1227. [DOI: 10.1021/acs.jpca.7b10880] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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46
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X-ray Absorption Spectroscopy Combined with Time-Dependent Density Functional Theory Elucidates Differential Substitution Pathways of Au(I) and Au(III) with Zinc Fingers. Inorg Chem 2017; 57:218-230. [DOI: 10.1021/acs.inorgchem.7b02406] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
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Advanced X-ray Spectroscopic Methods for Studying Iron-Sulfur-Containing Proteins and Model Complexes. Methods Enzymol 2017; 599:427-450. [PMID: 29746249 DOI: 10.1016/bs.mie.2017.09.008] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
In this chapter, a brief overview of X-ray spectroscopic methods that may be utilized to obtain insight into the geometric and electronic structure of iron-sulfur proteins is provided. These methods include conventional methods, such as metal and ligand K-edge X-ray absorption, as well as more advanced methods including nonresonant and resonant X-ray emission. In each section, the basic information content of the spectra is highlighted and important experimental considerations are discussed. Throughout the chapter, recent applications to iron-sulfur-containing models and proteins are highlighted.
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QM/MM Study of the Nitrogenase MoFe Protein Resting State: Broken-Symmetry States, Protonation States, and QM Region Convergence in the FeMoco Active Site. Inorg Chem 2017; 56:13417-13429. [DOI: 10.1021/acs.inorgchem.7b02158] [Citation(s) in RCA: 57] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
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Electronic and molecular structure relations in diiron compounds mimicking the [FeFe]-hydrogenase active site studied by X-ray spectroscopy and quantum chemistry. Dalton Trans 2017; 46:12544-12557. [PMID: 28905949 DOI: 10.1039/c7dt02720f] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
Synthetic diiron compounds of the general formula Fe2(μ-S2R)(CO)n(L)6-n (R = alkyl or aromatic groups; L = CN- or phosphines) are versatile models for the active-site cofactor of hydrogen turnover in [FeFe]-hydrogenases. A series of 18 diiron compounds, containing mostly a dithiolate bridge and terminal ligands of increasing complexity, was characterized by X-ray absorption and emission spectroscopy in combination with density functional theory. Fe K-edge absorption and Kβ main-line emission spectra revealed the varying geometry and the low-spin state of the Fe(i) centers. Good agreement between experimental and calculated core-to-valence-excitation absorption and radiative valence-to-core-decay emission spectra revealed correlations between spectroscopic and structural features and provided access to the electronic configuration. Four main effects on the diiron core were identified, which were preferentially related to variation either of the dithiolate or of the terminal ligands. Alteration of the dithiolate bridge affected mainly the Fe-Fe bond strength, while more potent donor substitution and ligand field asymmetrization changed the metal charge and valence level localization. In contrast, cyanide ligation altered all relevant properties and, in particular, the frontier molecular orbital energies of the diiron core. Mutual benchmarking of experimental and theoretical parameters provides guidelines to verify the electronic properties of related diiron compounds.
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Iron L 2,3-Edge X-ray Absorption and X-ray Magnetic Circular Dichroism Studies of Molecular Iron Complexes with Relevance to the FeMoco and FeVco Active Sites of Nitrogenase. Inorg Chem 2017; 56:8147-8158. [PMID: 28653855 PMCID: PMC5516708 DOI: 10.1021/acs.inorgchem.7b00852] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
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Herein, a systematic study of a series
of molecular iron model complexes has been carried out using Fe L2,3-edge X-ray absorption (XAS) and X-ray magnetic circular
dichroism (XMCD) spectroscopies. This series spans iron complexes
of increasing complexity, starting from ferric and ferrous tetrachlorides
([FeCl4]−/2–), to ferric and ferrous
tetrathiolates ([Fe(SR)4]−/2–),
to diferric and mixed-valent iron–sulfur complexes [Fe2S2R4]2–/3–.
This test set of compounds is used to evaluate the sensitivity of
both Fe L2,3-edge XAS and XMCD spectroscopy to oxidation
state and ligation changes. It is demonstrated that the energy shift
and intensity of the L2,3-edge XAS spectra depends on both
the oxidation state and covalency of the system; however, the quantitative
information that can be extracted from these data is limited. On the
other hand, analysis of the Fe XMCD shows distinct changes in the
intensity at both L3 and L2 edges, depending
on the oxidation state of the system. It is also demonstrated that
the XMCD intensity is modulated by the covalency of the system. For
mononuclear systems, the experimental data are correlated with atomic
multiplet calculations in order to provide insights into the experimental
observations. Finally, XMCD is applied to the tetranuclear heterometal–iron–sulfur
clusters [MFe3S4]3+/2+ (M = Mo, V),
which serve as structural analogues of the FeMoco and FeVco active
sites of nitrogenase. It is demonstrated that the XMCD data can be
utilized to obtain information on the oxidation state distribution
in complex clusters that is not readily accessible for the Fe L2,3-edge XAS data alone. The advantages of XMCD relative to
standard K-edge and L2,3-edge XAS are highlighted. This
study provides an important foundation for future XMCD studies on
complex (bio)inorganic systems. A systematic Fe L2,3-edge X-ray absorption (XAS) and X-ray magnetic circular dichroism
(XMCD) study of iron tetrachlorides ([FeCl4]−/2−), iron tetrathiolates ([Fe(SR)4]−/2−), diferric and mixed-valent iron−sulfur dimers [Fe2S2R4]2−/3− and heterometal−iron−sulfur
tetramers [MFe3S4]3+/2+ (M = Mo,
V) is reported. The changes in XAS and XMCD energies and intensities
across this set of complexes are presented together with atomic multiplet
calculations. The advantages of XMCD as an electronic structure probe
of complex clusters are highlighted.
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