Structural and Mechanistic Insights into CO 2 Activation by Nitrogenase Iron Protein

The Fe protein of nitrogenase catalyzes the ambient reduction of CO 2 when its cluster is present in the all‐ferrous, [Fe 4 S 4 ] 0 oxidation state. Here, we report a combined structural and theoretical study that probes the unique reactivity of the all‐ferrous Fe protein toward CO 2 . Structural comparisons of the Azotobacter vinelandii Fe protein in the [Fe 4 S 4 ] 0 and [Fe 4 S 4 ] + states point to a possible asymmetric functionality of a highly conserved Arg pair in CO 2 binding and reduction. Density functional theory (DFT) calculations provide further support for the asymmetric coordination of O by the “proximal” Arg and binding of C to a unique Fe atom of the all‐ferrous cluster, followed by donation of protons by the proximate guanidinium group of Arg that eventually results in the scission of a C−O bond. These results provide important mechanistic and structural insights into CO 2 activation by a surface‐exposed, scaffold‐held [Fe 4 S 4 ] cluster.