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X-ray spectroscopic observation of an interstitial carbide in NifEN-bound FeMoco precursor.

ABSTRACT: The iron-molybdenum cofactor (FeMoco) of nitrogenase contains a biologically unprecedented ?(6)-coordinated C(4-) ion. Although the role of this interstitial atom in nitrogenase catalysis is unknown, progress in understanding its biosynthetic origins has been made. Here we report valence-to-core Fe K? X-ray emission spectroscopy data to show that this C(4-) ion is present in the Fe(8)S(9) "L-cluster," which is the immediate precursor to FeMoco prior to the insertion of molybdenum and coordination by homocitrate. These results accord with recent evidence supporting a role for the S-adenosylmethionine-dependent enzyme NifB in the incorporation of carbon into the FeMoco center of nitrogenase.

SUBMITTER: Lancaster KM

PROVIDER: S-EPMC3555700 | biostudies-literature | 2013 Jan

REPOSITORIES: biostudies-literature

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X-ray spectroscopic observation of an interstitial carbide in NifEN-bound FeMoco precursor.

Lancaster Kyle M KM Hu Yilin Y Bergmann Uwe U Ribbe Markus W MW DeBeer Serena S

Journal of the American Chemical Society 20130108 2

The iron-molybdenum cofactor (FeMoco) of nitrogenase contains a biologically unprecedented μ(6)-coordinated C(4-) ion. Although the role of this interstitial atom in nitrogenase catalysis is unknown, progress in understanding its biosynthetic origins has been made. Here we report valence-to-core Fe Kβ X-ray emission spectroscopy data to show that this C(4-) ion is present in the Fe(8)S(9) "L-cluster," which is the immediate precursor to FeMoco prior to the insertion of molybdenum and coordinatio ...[more]

PMID: 23276198

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Project description: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.

Dataset Information

X-ray spectroscopic observation of an interstitial carbide in NifEN-bound FeMoco precursor.

Publications

X-ray spectroscopic observation of an interstitial carbide in NifEN-bound FeMoco precursor.

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