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

The Fe protein of nitrogenase catalyzes the ambient reduction of CO when its cluster is present in the all-ferrous, [Fe S ] oxidation state. Here, we report a combined structural and theoretical study that probes the unique reactivity of the all-ferrous Fe protein toward CO . Structural comparisons of the Azotobacter vinelandii Fe protein in the [Fe S ] and [Fe S ] states point to a possible asymmetric functionality of a highly conserved Arg pair in CO 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 activation by a surface-exposed, scaffold-held [Fe S ] cluster.