The Electronic Structure of the Iron-Molybdenum Cofactor of Nitrogenase:

Our atmosphere is composed of 80% dinitrogen, yet none of it is bioavailable. Before the invention of the Haber-Bosch process in 1913, only bacteria were known to be capable of fixing dinitrogen. Ammonia produced through the costly, fossil-fuel reliant Haber-Bosch process is an essential raw material of artificial fertilizers that are responsible for supporting 40% of the world population. Beyond the environmental and economic benefits of an alternative means of fixing dinitrogen, it is also exciting from a fundamental science point of view. Biological nitrogen fixation is catalyzed by the heterodimeric metalloenzyme nitrogenase, composed of the Fe-protein and the MoFe-protein (Figure 1A). The Feprotein is involved in electron transfer to the P-cluster ([Fe8S7]) of the MoFe-protein, from which electrons flow to the active site metallocofactor of formula [MFe7S9X], where M is Mo, V or Fe, and X is an unknown interstitial atom (thought to be C, N or O). This paper will focus on the molybdenum containing FeMo-co (Figure 1B), which is the most frequently studied and the most active of the three cofactor varieties. FeMo-co binds, activates and reduces dinitrogen, but the substrate binding mode and mechanistic details are yet to be understood. Electronic structural elements including the charge and spin coupling of the cluster are crucial to our understanding of nitrogenase, yet they, too, are not well defined. I will discuss the evolution of our knowledge of the electronic structure of FeMo-co.