The correlation of redox potential, HOMO energy, and oxidation state in metal sulfide clusters and its application to determine the redox level of the FeMo-co active-site cluster of nitrogenase.

This paper describes a procedure that permits the total charge state (i.e., oxidation state) of a complex molecule to be obtained from its redox potential data by comparison with good data (both charge state and redox potential) for reference compounds that are chemically similar. The link between the reference data and the unknown compound is made by the calculated energies of the Fermi level or highest occupied molecular orbital (HOMO). The HOMO energies are calculated by unrestricted density functional methods (DMol) for the reference compounds in their known charge states, and a graphical correlation of HOMO energy and redox potential for oxidation (corresponding to loss of an electron from the HOMO) is constructed. The measured redox potential of the unknown is then applied to the correlation to yield the HOMO energy of the unknown, against which the calculated HOMO energies for various charge states of the unknown are assessed. This method is generally applicable. Using 26 reference data, the method is used here to determine the resting redox state, [NFe6MoS9]0, of the core of the FeMo cofactor (FeMo-co, bound to the MoFe protein) which is the active site of nitrogen-fixing enzymes. The analysis also shows that if the atom at the center of FeMo-co is C rather than N, then FeMo-co must be protonated in its resting state, but if FeMo-co is N-centered, it would not be protonated in the resting state.