A Proposed Complete-Cycle Mechanism for Conversion of N₂ to NH₃ by Mo-Nitrogenase

How does the enzyme nitrogenase convert N₂ to NH₃ under ambient conditions? This perspective expounds the Dance chemical mechanism comprising 40 explicit intermediates and 31 reaction steps, from start to finish, to effect the enzyme stoichiometry N₂ + 8H⁺ + 8e^- → 2NH₃ + H₂. Energy profiles and kinetic barriers are derived from density functional simulations with a 483+ atom quantum model. The key to the mechanism is discovery of the unique N₂ capture step, in which N₂ tumbles into a preformed gallery of H atoms, and is activated there by concerted formation of two HN and three FeN bonds, forming bound HNNH. This sets up subsequent hydrogenations that break N-N and form NH₃ in concert. Retention of bridging S2B adjacent to bound intermediates is a crucial stereochemical component. An inconsistency with kinetic data is still to be resolved. The well-known H₂/N₂ exchange occurs because H₂ is formed where its dissociation creates the space required for N₂ capture. Possible quantum tunneling by H atoms is considered. I discuss architecture and function in the reaction space and surrounding protein. An enabling attribute of this mechanism is the favourable geometry and propitious stereochemistry involved in each of the chemical steps.