CHEM
In the literature: open
Where and how nitrogen binds to the nitrogenase cofactor
In plain words
Nitrogenase must add four electrons and protons to its metal cluster before nitrogen gas can bind and be broken. Which metal atom grabs the nitrogen and what the cluster looks like at that moment are uncertain.
Precise statement
For the E4 (Janus) state of FeMoco, determine the positions of the four added hydrogens (bridging hydrides, protonated sulfides), the site of N2 binding (Fe2/Fe6 face, Mo, or after sulfide displacement), and the energetics of reductive elimination of H2 coupled to N2 binding. An answer is a structure and mechanism consistent with ENDOR/EPR data and with converged electronic-structure energetics.
What would settle it
Spectroscopic and freeze-quench structural data on trapped intermediates matched by converged calculations of the competing binding modes.