Electron Redistribution within the Nitrogenase Active Site FeMo-Cofactor during Reductive Elimination of H2to Achieve NN Triple-Bond Activation

Dmitriy A. Lukoyanov, Zhi Yong Yang, Dennis R. Dean, Lance C. Seefeldt, Simone Raugei, Brian M. Hoffman*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

1 Scopus citations


Nitrogen fixation by nitrogenase begins with the accumulation of four reducing equivalents at the active-site FeMo-cofactor (FeMo-co), generating a state (denoted E4(4H)) with two [Fe-H-Fe] bridging hydrides. Recently, photolytic reductive elimination (re) of the E4(4H) hydrides showed that enzymatic re of E4(4H) hydride yields an H2-bound complex (E4(H2,2H)), in a process corresponding to a formal 2-electron reduction of the metal-ion core of FeMo-co. The resulting electron-density redistribution from Fe-H bonds to the metal ions themselves enables N2 to bind with concomitant H2 release, a process illuminated here by QM/MM molecular dynamics simulations. What is the nature of this redistribution? Although E4(H2,2H) has not been trapped, cryogenic photolysis of E4(4H) provides a means to address this question. Photolysis of E4(4H) causes hydride-re with release of H2, generating doubly reduced FeMo-co (denoted E4(2H)*), the extreme limit of the electron-density redistribution upon formation of E4(H2,2H). Here we examine the doubly reduced FeMo-co core of the E4(2H)∗ limiting-state by 1H, 57Fe, and 95Mo ENDOR to illuminate the partial electron-density redistribution upon E4(H2,2H) formation during catalysis, complementing these results with corresponding DFT computations. Inferences from the E4(2H)∗ ENDOR results as extended by DFT computations include (i) the Mo-site participates negligibly, and overall it is unlikely that Mo changes valency throughout the catalytic cycle; and (ii) two distinctive E4(4H) 57Fe signals are suggested as associated with structurally identified "anchors"of one bridging hydride, two others with identified anchors of the second, with NBO-analysis further identifying one anchor of each hydride as a major recipient of electrons released upon breaking Fe-H bonds.

Original languageEnglish (US)
Pages (from-to)21679-21690
Number of pages12
JournalJournal of the American Chemical Society
Issue number52
StatePublished - Dec 30 2020

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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