Possible model reactions for the nitrate reductases

Garner, C. David; Hyde, Michael R.; Mabbs, Frank E.; Routledge, Vincent I.

doi:10.1038/252579a0

“…The Raman shift of the oxo species to a lower frequency upon reduction was likely due to weakening of the Mo=O bond strength that is associated with the lowered valence state of Mo . The axial symmetry of Mo V =O species has been suggested to be critical for facilitating the overlap of the 4d xy orbital of the metal with the π* orbital of the nitrato group, thus lowering the energy of the latter and promoting the intramolecular electron transfer after formation of the nitrato complex …”

Section: Resultsmentioning

confidence: 91%

Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media

Li

¹

,

Go

²

,

Ooka

³

et al. 2020

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Nitrate is a pervasive aquatic contaminant of global environmental concern. In nature, the most effective nitrate reduction reaction (NRR) is catalyzed by nitrate reductase enzymes at neutral pH, using a highly‐conserved Mo center ligated mainly by oxo and thiolate groups. Mo‐based NRR catalysts mostly function in organic solvents with a low water stability. Recently, an oxo‐containing molybdenum sulfide nanoparticle that serves as an NRR catalyst at neutral pH was first reported. Herein, in a nanoparticle‐catalyzed NRR system a pentavalent MoV(=O)S4 species, an enzyme mimetic, served as an active intermediate for the NRR. Potentiometric titration analysis revealed that a redox synergy among MoV−S, S radicals, and MoV(=O)S4 likely play a key role in stabilizing MoV(=O)S4, showing the importance of secondary interactions in facilitating NRR. The first identification and characterization of an oxo‐ and thiolate‐ligated Mo intermediates pave the way to the molecular design of efficient enzyme mimetic NRR catalysts in aqueous solution.

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“…The exclusive usage of this oxo‐ and thiolate‐coordinated, mononuclear Mo active‐site configuration in nature is a long‐standing enigma that has stimulated intensive research regarding the specific role of Mo in the NRR. For example, the capacity of oxo‐Mo compounds with non‐dithiolene ligands for reducing either NO 3 − to NO 2 gas in organic media or NO 2 − in acidic aqueous solution (pH=−0.8∼1) has been investigated. At low pH, however, protonated species (HNO 3 , p K a =−1.3) are the substrates of the NRR and are far more reactive than NO 3 − because of disruption of the D 3 h resonant structure .…”

Section: Introductionmentioning

confidence: 99%

Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media

Li

¹

,

Go

²

,

Ooka

³

et al. 2020

View full text Add to dashboard Cite

Nitrate is a pervasive aquatic contaminant of global environmental concern. In nature, the most effective nitrate reduction reaction (NRR) is catalyzed by nitrate reductase enzymes at neutral pH, using a highly‐conserved Mo center ligated mainly by oxo and thiolate groups. Mo‐based NRR catalysts mostly function in organic solvents with a low water stability. Recently, an oxo‐containing molybdenum sulfide nanoparticle that serves as an NRR catalyst at neutral pH was first reported. Herein, in a nanoparticle‐catalyzed NRR system a pentavalent MoV(=O)S4 species, an enzyme mimetic, served as an active intermediate for the NRR. Potentiometric titration analysis revealed that a redox synergy among MoV−S, S radicals, and MoV(=O)S4 likely play a key role in stabilizing MoV(=O)S4, showing the importance of secondary interactions in facilitating NRR. The first identification and characterization of an oxo‐ and thiolate‐ligated Mo intermediates pave the way to the molecular design of efficient enzyme mimetic NRR catalysts in aqueous solution.

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“…Similar π-type overlap was observed in the case of several guest molecules like C 2 H 2 , CO, N 2 , or nitrate, coordinated in cis to terminal oxide in a variety of oxomolybdenum complexes including, e.g., those with dithiolene or dithiocarbamato ligands. 40 It was pointed out by Garner et al 41 that if such molecules bind in cis to ModO, then the symmetry is correct for the metal d xy orbitals to overlap with ligand π* orbitals. The interaction between these redox orbitals provides a pathway for the electron transfer process, which in the present case consists in the flow of the unpaired electron density from the d xy donor orbital of molybdenum toward the antibonding 3π* acceptor orbital of N 2 O ligand.…”

Section: Discussionmentioning

confidence: 99%

EPR Investigation of the Activation of N₂O on Mo/SiO₂Catalysts via Electron Transfer: From N₂O as a Ligand to Adsorbed O^-Ion

Sojka

¹

,

Che

²

1996

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EPR spectroscopy has been used to monitor changes occurring at the molecular level upon adsorption of N 2 O on grafted Mo/SiO 2 catalysts using naturally abundant and 95 Mo-enriched molybdenum. The tetracoordinated Mo 4c 5+ surface species produced upon reduction with d x 2 -y 2 SOMO coordinates N 2 O at 298 K and rearranges into a pentacoordinated complex of apparent C 4V symmetry with d xy ground state indicating attachment of the N 2 O ligand in equatorial position. Upon temperature-induced metal to ligand electron transfer (MLET) between 3π* and d xy redox orbitals, O -radicals are produced with an activation energy of 20 ( 4 kJ/mol measured in the 323-393 K range. The mechanism of this process and the hindrance to electron transfer are discussed in terms of intrinsic barriers and within the framework of potential energy surfaces. From the spin Hamiltonian parameters of O -, the unpaired electron density on molybdenum and the splitting of energy levels of O -species were calculated to be 4.3% and ∆E x,y-z ) 1.5 eV, respectively. The crystal field approach was used to discuss the O --Mo 6+ bonding.

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Possible model reactions for the nitrate reductases

Cited by 30 publications

References 14 publications

Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media

Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media

Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media

EPR Investigation of the Activation of N₂O on Mo/SiO₂Catalysts via Electron Transfer: From N₂O as a Ligand to Adsorbed O^-Ion

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Possible model reactions for the nitrate reductases

Cited by 30 publications

References 14 publications

Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media

Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media

Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media

EPR Investigation of the Activation of N2O on Mo/SiO2Catalysts via Electron Transfer: From N2O as a Ligand to Adsorbed O-Ion

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EPR Investigation of the Activation of N₂O on Mo/SiO₂Catalysts via Electron Transfer: From N₂O as a Ligand to Adsorbed O^-Ion