Abstract
Mo nitrogenase (N2ase) utilizes a two-component protein system, the catalytic MoFe and its electron-transfer partner FeP, to reduce atmospheric dinitrogen (N2) to ammonia (NH3). The FeMo cofactor contained in the MoFe protein serves as the catalytic center for this reaction and has long inspired model chemistry oriented toward activating N2. This field of chemistry has relied heavily on the detailed characterization of how Mo N2ase accomplishes this feat. Understanding the reaction mechanism of Mo N2ase itself has presented one of the most challenging problems in bioinorganic chemistry because of the ephemeral nature of its catalytic intermediates, which are difficult, if not impossible, to singly isolate. This is further exacerbated by the near necessity of FeP to reduce native MoFe, rendering most traditional means of selective reduction inept. We have now investigated the first fundamental intermediate of the MoFe catalytic cycle, E1, as prepared both by low-flux turnover and radiolytic cryoreduction, using a combination of Mo Kα high-energy-resolution fluorescence detection and Fe K-edge partial-fluorescence-yield X-ray absorption spectroscopy techniques. The results demonstrate that the formation of this state is the result of an Fe-centered reduction and that Mo remains redox-innocent. Furthermore, using Fe X-ray absorption and 57Fe Mössbauer spectroscopies, we correlate a previously reported unique species formed under cryoreducing conditions to the natively formed E1 state through annealing, demonstrating the viability of cryoreduction in studying the catalytic intermediates of MoFe.
Highlights
The conversion of dinitrogen (N2) to bioavailable ammonia (NH3) is a fundamental step in the biogeochemical nitrogen cycle.[1]
Few previous investigations have aimed at exploring the electronic and geometric structures of the E1 state of MoFe, and no conclusive evidence has been provided regarding the site of reduction on the FeMoco cluster in E1.66,67 To this end, perhaps the most significant effort undertaken to date involved the measurement of selectively 57FeMoco-enriched MoFe using 57Fe Mössbauer spectroscopy to ascertain the electronic properties of the catalytic cluster across a series of oxidation states.[46]
The results of the Mo Kα HERFD X-ray absorption spectroscopy (XAS) demonstrate clearly that one-electron reduction of MoFe does not result in a Mo-centered reduction under either of these conditions
Summary
The conversion of dinitrogen (N2) to bioavailable ammonia (NH3) is a fundamental step in the biogeochemical nitrogen cycle.[1]. MoFe functions along with a [4Fe-4S] clustercontaining iron protein (FeP), which serves as the native reductant of MoFe. Mo N2ase and the FeMoco cluster have long inspired model chemistry for the activation of N2 and other small molecules. Following the discovery of Mo as an essential component of Mo N2ase,[5] a field of chemistry focused around the tuning of single and polynuclear Mo complexes to bind and reduce N2 ensued.[6−14] This route has been somewhat successful activating manoddehl acsopmrpolveixdeesd.6,9s,o11m,1e2,1o5−f 2t2heHofiwrsetvecra,tailnytircecNen2t-. Current Proposed Relationship between the Previously Observed Natively Reduced (MR) and Cryoreduced (MI) Species Formed at the FeMoco Clustera aElements are colored as follows: Mo, cyan; Fe, orange; S, yellow; N, blue; O, red; C, gray
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