ic9b01951_si_001.pdf (1.37 MB)
Spectroscopic Description of the E1 State of Mo Nitrogenase Based on Mo and Fe X‑ray Absorption and Mössbauer Studies
journal contribution
posted on 2019-08-23, 11:06 authored by Casey Van Stappen, Roman Davydov, Zhi-Yong Yang, Ruixi Fan, Yisong Guo, Eckhard Bill, Lance C. Seefeldt, Brian M. Hoffman, Serena DeBeerMo
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.