An official website of the United States government
ABSTRACT Nitrogenase iron (Fe) proteins reduce CO 2 to CO and/or hydrocarbons under ambient conditions. Here, we report a 2.4-Å crystal structure of the Fe protein from Methanosarcina acetivorans ( Ma NifH), which is generated in the presence of a reductant, dithionite, and an alternative CO 2 source, bicarbonate. Structural analysis of this methanogen Fe protein species suggests that CO 2 is possibly captured in an unactivated, linear conformation near the [Fe 4 S 4 ] cluster of Ma NifH by a conserved arginine (Arg) pair in a concerted and, possibly, asymmetric manner. Density functional theory calculations and mutational analyses provide further support for the capture of CO 2 on Ma NifH while suggesting a possible role of Arg in the initial coordination of CO 2 via hydrogen bonding and electrostatic interactions. These results provide a useful framework for further mechanistic investigations of CO 2 activation by a surface-exposed [Fe 4 S 4 ] cluster, which may facilitate future development of FeS catalysts for ambient conversion of CO 2 into valuable chemical commodities. IMPORTANCE This work reports the crystal structure of a previously uncharacterized Fe protein from a methanogenic organism, which provides important insights into the structural properties of the less-characterized, yet highly interesting archaeal nitrogenase enzymes. Moreover, the structure-derived implications for CO 2 capture by a surface-exposed [Fe 4 S 4 ] cluster point to the possibility of developing novel strategies for CO 2 sequestration while providing the initial insights into the unique mechanism of FeS-based CO 2 activation.
more »
« less
Characterization of a Nitrogenase Iron Protein Substituted with a Synthetic [Fe 4 Se 4 ] Cluster
Abstract The Fe protein of nitrogenase plays multiple roles in substrate reduction and cluster maturation via its redox‐active [Fe4S4] cluster. Here we report the synthesis and characterization of a water‐soluble [Fe4Se4] cluster that is used to substitute the [Fe4S4] cluster of theAzotobacter vinelandiiFe protein (AvNifH). Biochemical, EPR and XAS/EXAFS analyses demonstrate the ability of the [Fe4Se4] cluster to adopt the super‐reduced, all‐ferrous state upon its incorporation intoAvNifH. Moreover, these studies reveal that the [Fe4Se4] cluster inAvNifH already assumes a partial all‐ferrous state ([Fe4Se4]0) in the presence of dithionite, where its [Fe4S4] counterpart inAvNifH exists solely in the reduced state ([Fe4S4]1+). Such a discrepancy in the redox properties of theAvNifH‐associated [Fe4Se4] and [Fe4S4] clusters can be used to distinguish the differential redox requirements for the substrate reduction and cluster maturation of nitrogenase, pointing to the utility of chalcogen‐substituted FeS clusters in future mechanistic studies of nitrogenase catalysis and assembly.
more »
« less
- Award ID(s):
- 1651398
- PAR ID:
- 10368319
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Angewandte Chemie International Edition
- Volume:
- 61
- Issue:
- 19
- ISSN:
- 1433-7851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The one‐step syntheses, X‐ray structures, and spectroscopic characterization of synthetic iron clusters, bearing either inorganic sulfides or thiolate with interstitial carbide motifs, are reported. Treatment of iron carbide carbonyl clusters [Fen(μn‐C)(CO)m]x(n=5,6;m=15,16;x=0,−2) with electrophilic sulfur sources (S2Cl2, S8) results in the formation of several μ4‐S dimers of clusters, and moreover, iron‐sulfide‐(sulfocarbide) clusters. The core sulfocarbide unit {C−S}4−serves as a structural model for a proposed intermediate in the radicalS‐adenosyl‐L‐methionine biogenesis of the M‐cluster. Furthermore, the electrophilic sulfur strategy has been extended to provide the first ever thiolato‐iron‐carbide complex: an analogous reaction with toluylsulfenyl chloride affords the cluster [Fe5(μ5‐C)(SC7H7)(CO)13]−. The strategy described herein provides a breakthrough towards developing syntheses of biomimetic iron‐sulfur‐carbide clusters like FeMoco.more » « less
-
Abstract Mackinawite has unique structural properties and reactivities when compared to other iron sulfides. Herein we provide evidence for the mackinawite‐supported reduction of KCN into various reduced compounds under primordial conditions. We proposed a reaction mechanism based on the nucleophilic attack by the deprotonated mackinawite ‐SH surface groups at the carbon atom of HCN. The initial binding of the substrate and the subsequent reduction events are supported by DFT calculations and further experiments using other substrates, such as KSCN, KOCN and CS2. Until now, conversion of CN−into CH4and NH3has been limited to nitrogenase cofactors or molecular Fe‐CN complexes. Our study provides evidence for mackinawite‐supported cleavage of the C−N bond under ambient conditions, which opens new avenues for investigation of other substrates for mackinawite‐supported reactions while shedding light on the relevance of this type of reaction to the origin of life on Earth.more » « less
-
Abstract Theoretical and experimental investigations of various exfoliated samples taken from layered In4Se3crystals are performed. In spite of the ionic character of interlayer interactions in In4Se3and hence much higher calculated cleavage energies compared to graphite, it is possible to produce few‐nanometer‐thick flakes of In4Se3by mechanical exfoliation of its bulk crystals. The In4Se3flakes exfoliated on Si/SiO2have anisotropic electronic properties and exhibit field‐effect electron mobilities of about 50 cm2 V−1 s−1at room temperature, which are comparable with other popular transition metal chalcogenide (TMC) electronic materials, such as MoS2and TiS3. In4Se3devices exhibit a visible range photoresponse on a timescale of less than 30 ms. The photoresponse depends on the polarization of the excitation light consistent with symmetry‐dependent band structure calculations for the most expectedaccleavage plane. These results demonstrate that mechanical exfoliation of layered ionic In4Se3crystals is possible, while the fast anisotropic photoresponse makes In4Se3a competitive electronic material, in the TMC family, for emerging optoelectronic device applications.more » « less
-
Under anaerobic conditions, ferrous iron reacts with sulfide producing FeS, which can then undergo a temperature, redox potential, and pH dependent maturation process resulting in the formation of oxidized mineral phases, such as greigite or pyrite. A greater understanding of this maturation process holds promise for the development of iron-sulfide catalysts, which are known to promote diverse chemical reactions, such as H + , CO 2 and NO 3 − reduction processes. Hampering the full realization of the catalytic potential of FeS, however, is an incomplete knowledge of the molecular and redox processess ocurring between mineral and nanoparticulate phases. Here, we investigated the chemical properties of iron-sulfide by cyclic voltammetry, Raman and X-ray absorption spectroscopic techniques. Tracing oxidative maturation pathways by varying electrode potential, nanoparticulate n (Fe 2+ S 2− ) (s) was found to oxidize to a Fe 3+ containing FeS phase at −0.5 V vs. Ag/AgCl (pH = 7). In a subsequent oxidation, polysulfides are proposed to give a material that is composed of Fe 2+ , Fe 3+ , S 2− and polysulfide (S n 2− ) species, with its composition described as Fe 2+ 1−3 x Fe 3+ 2 x S 2− 1− y (S n 2− ) y . Thermodynamic properties of model compounds calculated by density functional theory indicate that ligand oxidation occurs in conjunction with structural rearrangements, whereas metal oxidation may occur prior to structural rearrangement. These findings together point to the existence of a metastable FeS phase located at the junction of a metal-based oxidation path between FeS and greigite (Fe 2+ Fe 3+ 2 S 2− 4 ) and a ligand-based oxidation path between FeS and pyrite (Fe 2+ (S 2 ) 2− ).more » « less
