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The One-Electron Reduced Active-Site FeFe-Cofactor of Fe-Nitrogenase Contains a Hydride Bound to a Formally Oxidized Metal-Ion Core.
- Source :
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Inorganic chemistry [Inorg Chem] 2022 Apr 11; Vol. 61 (14), pp. 5459-5464. Date of Electronic Publication: 2022 Mar 31. - Publication Year :
- 2022
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Abstract
- The nitrogenase active-site cofactor must accumulate 4e <superscript>-</superscript> /4H <superscript>+</superscript> (E <subscript>4</subscript> (4H) state) before N <subscript>2</subscript> can bind and be reduced. Earlier studies demonstrated that this E <subscript>4</subscript> (4H) state stores the reducing-equivalents as two hydrides, with the cofactor metal-ion core formally at its resting-state redox level. This led to the understanding that N <subscript>2</subscript> binding is mechanistically coupled to reductive-elimination of the two hydrides that produce H <subscript>2</subscript> . The state having acquired 2e <superscript>-</superscript> /2H <superscript>+</superscript> (E <subscript>2</subscript> (2H)) correspondingly contains one hydride with a resting-state core redox level. How the cofactor accommodates addition of the first e <superscript>-</superscript> /H <superscript>+</superscript> (E <subscript>1</subscript> (H) state) is unknown. The Fe-nitrogenase FeFe-cofactor was used to address this question because it is EPR-active in the E <subscript>1</subscript> (H) state, unlike the FeMo-cofactor of Mo-nitrogenase, thus allowing characterization by EPR spectroscopy. The freeze-trapped E <subscript>1</subscript> (H) state of Fe-nitrogenase shows an S = 1/2 EPR spectrum with g = [1.965, 1.928, 1.779]. This state is photoactive, and under 12 K cryogenic intracavity , 450 nm photolysis converts to a new and likewise photoactive S = 1/2 state (denoted E <subscript>1</subscript> (H)*) with g = [2.009, 1.950, 1.860], which results in a photostationary state, with E <subscript>1</subscript> (H)* relaxing to E <subscript>1</subscript> (H) at temperatures above 145 K. An H/D kinetic isotope effect of 2.4 accompanies the 12 K E <subscript>1</subscript> (H)/E <subscript>1</subscript> (H)* photointerconversion. These observations indicate that the addition of the first e <superscript>-</superscript> /H <superscript>+</superscript> to the FeFe-cofactor of Fe-nitrogenase produces an Fe-bound hydride, not a sulfur-bound proton. As a result, the cluster metal-ion core is formally one-electron oxidized relative to the resting state. It is proposed that this behavior applies to all three nitrogenase isozymes.
Details
- Language :
- English
- ISSN :
- 1520-510X
- Volume :
- 61
- Issue :
- 14
- Database :
- MEDLINE
- Journal :
- Inorganic chemistry
- Publication Type :
- Academic Journal
- Accession number :
- 35357830
- Full Text :
- https://doi.org/10.1021/acs.inorgchem.2c00180