Spectroscopic Description of the E1 State of Mo Nitrogenase Based on Mo and Fe X‑ray Absorption and Mössbauer Studies

Authors

Casey Van Stappen, Max Planck Institute for Chemical Energy Conversion
Roman Davydov, Northwestern University
Zhi-Yong Yang, Utah State UniversityFollow
Ruixi Fan, Carnegie Mellon University
Yisong Guo, Carnegie Mellon University
Eckhard Bill, Max Planck Institute for Chemical Energy Conversion
Lance C. Seefeldt, Utah State UniversityFollow
Brian M. Hoffman, Northwestern University
Serena DeBeer, Max Planck Institute for Chemical Energy Conversion

Document Type

Article

Journal/Book Title

Inorganic Chemistry

Publication Date

8-23-2019

Publisher

American Chemical Society

Volume

58

First Page

12365

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 License.

Last Page

12376

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 highenergy-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 redoxinnocent. 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.

Recommended Citation

Stappen, Casey, et al. "Spectroscopic Description of the E1 State of Mo Nitrogenase Based on Mo and Fe X-ray Absorption and Mössbauer Studies." Inorganic Chemistry, 2019, pp. 12365-12376. https://doi.org/10.1021/acs.inorgchem.9b01951