Reduction of C₁ Substrates to Hydrocarbons by the Homometallic Precursor and Synthetic Mimic of the Nitrogenase Cofactor

Abstract

Solvent-extracted nitrogenase cofactors can reduce C substrates (CN, CO and CO) to hydrocarbons in reactions driven by a strong reductant, SmI (E = -1.55 V vs SCE). Here we show that a synthetic [EtN][FeS(SEt)] cluster (designated the Fe-cluster), which mimics the homometallic [FeSC] core of the nitrogenase cofactor (designated the L-cluster), is capable of conversion of C substrates into hydrocarbons in the same reactions. Comparison of the yields and product profiles between these homometallic clusters and their heterometallic counterparts points to possible roles of the heterometal, interstitial carbide and belt sulfur-bridged iron atoms in catalysis. More importantly, the observation that a "simplified", homometallic cofactor mimic can perform Fischer-Tropsch-like hydrocarbon synthesis suggests future biotechnological adaptability of nitrogenase-based biomimetic compounds for recycling C substrates into useful chemical and fuel products.

A Comparative Analysis of the CO‐Reducing Activities of MoFe Proteins Containing Mo‐ and V‐Nitrogenase Cofactors

“…[26,27] The hydrocarbon products were analyzed as described elsewhere. [4,5,23] Ammonium was determined by a HPLC fluorescence method, [28] and hydrogen was analyzed as described previously. [29]…”

“…As the crystal structure of VnfDGK is now available, it is conceivable that key segments of VnfDGK could be systematically introduced into a NifDK scaffold in order to pinpoint the structural components that are essential for the enzymatic reduction of CO. Furthermore, given the recent success of incorporating synthetic FeS compounds (hetero‐ or homometallic) into the cofactor‐deficient, apo‐NifDK, the efforts of varying the protein scaffold could be combined with the screening of biogenic cofactor homologues or synthetic cofactor mimics containing different heterometals/components; this could serve as a potential platform for engineering artificial CO‐reducing enzymes in the future.…”

“…Activity assays : All nitrogenase activity assays were carried out as described previously . The hydrocarbon products were analyzed as described elsewhere . Ammonium was determined by a HPLC fluorescence method, and hydrogen was analyzed as described previously…”

“…This observation strongly suggestst hat the protein environmenti samajor contributor to the observed deuterium effects and points to the structural variations between homologous nitrogenase proteins, such as those that have an impact on the folding and quaternarystructures of the protein scaffold, or the chemical alterations, such as those that change the pK a values of key protein residues, as worthy directions for furthere xploration of the mechanism of nitrogenase-catalyzed CO reduction.A st he crystal structure of VnfDGK is now available, [18] it is conceivable that keys egments of VnfDGK could be systematically introduced into aN ifDK scaffold in order to pinpoint the structural components that are essential for the enzymatic reduction of CO. Furthermore, given the recent successo fi ncorporating synthetic FeS compounds (hetero-or homometallic) into the cofactor-deficient, apo-NifDK, [21] the efforts of varying the protein scaffold could be combined with the screening of biogenic cofactor homologues or synthetic cofactor mimics [22,23] containing different heterometals/components;t his could serve as ap otentialp latform for engineering artificial CO-reducing enzymesi nt he future.…”

Activation of CO2 by Vanadium Nitrogenase

“…These newer studies employ organic proton sourcess uch as 2,6-dimethylpyridinium and triethylammonium salts under buffering conditions and use the strong reductant Sm II I 2 to drive catalytic substrate reduction. [100,102,103] The use of Sm II I 2 as ar eductant in organic chemistry has been well-studied, althought he redox potential of this powerful electron source is highly sensitive to its coordinating ligands. [104,105] Based on the observed formation of ad istinct Sm II I 2 species in the nitrogenase cofactor experiments, aD MF-coordinated Sm II I 2 complex is believed to be formed in situ.…”