Structural, spectroscopic, and redox consequences of a central ligand in the femoco of nitrogenase: A density functional theoretical study

Broken symmetry density functional and electrostatics calculations have been used to shed light on which of three proposed atoms, C, N, or O, is most likely to be present in the center of the FeMoco, the active site of nitrogenase. At the Mo⁴⁺4Fe²⁺3Fe³⁺ oxidation level, a central N^3- anion results in (1) calculated Fe-N bond distances that are in very good agreement with the recent high-resolution X-ray data of Einsle et al.; (2) a calculated redox potential of 0.19 eV versus the standard hydrogen electrode (SHE) for FeMoco(oxidized) + e^- → FeMoco(resting), in good agreement with the measured value of -0.042 V in Azotobacter vinelandii; and (3) average Mössbauer isomer shift values (IS_av = 0.48 mm s^-1) compatible with experiment (IS_av = 0.40 mm s^-1). At the more reduced Mo⁴⁺6Fe²⁺1 Fe³⁺ level, the calculated geometry around a central N^3- anion still correlates well with the X-ray data, but the average Mössbauer isomer shift value (IS_av = 0.54 mm s^-1) and the redox potential of -2.21 eV show a much poorer agreement with experiment. These calculated structural, spectroscopic, and redox data indicate the most likely iron oxidation state for the resting FeMoco of nitrogenase to be 4Fe²⁺3Fe³⁺. At this favored oxidation state, oxygen or carbon coordination leads to (1) Fe-O distances in poor agreement and Fe-C distances in good agreement with experiment and (2) calculated redox potentials of +0.97 eV for O^2- and -1.31 eV for C^4-. The calculated structural parameters and/or redox data suggest either O^2- or C^4- is unlikely as a central anion.