Hydride Conformers of the Nitrogenase FeMo-cofactor Two-Electron Reduced State E(2)(2H), Assigned Using Cryogenic Intra Electron Paramagnetic Resonance Cavity Photolysis

[Image: see text] Early studies in which nitrogenase was freeze-trapped during enzymatic turnover revealed the presence of high-spin (S = (3)/(2)) electron paramagnetic resonance (EPR) signals from the active-site FeMo-cofactor (FeMo-co) in electron-reduced intermediates of the MoFe protein. Historically denoted as 1b and 1c, each of the signals is describable as a fictitious spin system, S′ = (1)/(2), with anisotropic g′ tensor, 1b with g′ = [4.21, 3.76, ?] and 1c with g′ = [4.69, ∼3.20, ?]. A clear discrepancy between the magnetic properties of 1b and 1c and the kinetic analysis of their appearance during pre-steady-state turnover left their identities in doubt, however. We subsequently associated 1b with the state having accumulated 2[e(–)/H(+)], denoted as E(2)(2H), and suggested that the reducing equivalents are stored on the catalytic FeMo-co cluster as an iron hydride, likely an [Fe–H–Fe] hydride bridge. Intra-EPR cavity photolysis (450 nm; temperature-independent from 4 to 12 K) of the E(2)(2H)/1b state now corroborates the identification of this state as storing two reducing equivalents as a hydride. Photolysis converts E(2)(2H)/1b to a state with the same EPR spectrum, and thus the same cofactor structure as pre-steady-state turnover 1c, but with a different active-site environment. Upon annealing of the photogenerated state at temperature T = 145 K, it relaxes back to E(2)(2H)/1b. This implies that the 1c signal comes from an E(2)(2H) hydride isomer of E(2)(2H)/1b that stores its two reducing equivalents either as a hydride bridge between a different pair of iron atoms or an Fe–H terminal hydride.