Connecting the Geometric and Electronic Structures of the Nitrogenase Iron–Molybdenum Cofactor through Site-selective Labeling

Understanding the chemical bonding in the catalytic cofactor of the Mo nitrogenase (FeMo-co) is foundational for building a mechanistic picture of biological nitrogen fixation. A persistent obstacle in these efforts has been that the 57Fe-based spectroscopic data—although rich with information—reflects all seven Fe sites, and it has therefore not been possible to map individual spectroscopic responses to specific sites in the 3-D structure. We herein overcome this challenge by incorporating 57Fe into a single site of FeMo-co. Spectroscopic analysis of the reduced and oxidized forms of the resting state provides unprecedented insights into the local electronic structure of the terminal (Fe1) site, including its oxidation state and spin orientation. This leads to the discovery that Fe1 is a site of redox reactivity during oxidation of the resting state, and on this basis, we suggest a possible role for Fe1 as an electron reservoir during N2 reduction catalysis.