Electron Redistribution within the Nitrogenase Active Site FeMo-Cofactor During Reductive Elimination of H_2 to Achieve N≡N Triple-Bond Activation

摘要

Nitrogen fixation by nitrogenase begins with the accumulation of four reducing equivalents at the active-site FeMo-cofactor (FeMo-co), generating a state (denoted E_4(4H)) with two [Fe-H-Fe] bridging hydrides. Recently, photolytic reductive elimination (re) of the E_4(4H) hydrides showed that enzymatic re of E_4(4H) hydride yields an H_2-bound complex (E_4(H_2,2H)), 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 N_2 to bind with concomitant H_2 release, a process illuminated here by QM/MM molecular dynamics simulations. What is the nature of this redistribution? Although E_4(H_2,2H) has not been trapped, cryogenic photolysis of E_4(4H) provides a means to address this question. Photolysis of E_4(4H) causes hydride-re with release of H_2, generating doubly reduced FeMo-co (denoted E_4(2H)*), the extreme limit of the electron-density redistribution upon formation of E_4(H_2,2H). Here we examine the doubly reduced FeMo-co core of the E_4(2H)* limiting-state by ~1H, ~(57)Fe, and ~(95)Mo ENDOR to illuminate the partial electron-density redistribution upon E_4(H_2,2H) formation during catalysis, complementing these results with corresponding DFT computations. Inferences from the E_4(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 E_4(4H) ~(57)Fe 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.