Electron‐Transfer Chemistry of the Iron–Molybdenum Cofactor of Nitrogenase: Delocalized and Localized Reduced States of FeMoco which Allow Binding of Carbon Monoxide to Iron and Molybdenum

Abstract

The electron-transfer chemistry of the isolated iron-molybdenum cofactor of nitrogenase (FeMoco) has been studied by electrochemical and spectroelectrochemical methods. Two interconverting forms of the cofactor arise from a redox-linked ligand isomerism at the terminal iron atom; this is attributed to rotamerism of an anionic N-methyl formamide ligand bound at this site. FeMoco in its EPR-silent oxidised state is shown to undergo three successive one-electron transfer steps. We argue that the first and second redox processes are associated with electron-transfer delocalised over the iron-sulfur core of the cofactor, whilst the third irreversible process is localised on molybdenum. This is strongly reinforced by spectroelectrochemical studies under (12)CO and (13)CO which reveal two independent carbon monoxide binding sites that are specifically associated with the second (iron core) and third (molybdenum) electron-transfer processes and which give rise to terminal nu((12)CO) bands at 1885 and 1920 cm(-1) respectively. Moreover, in parallel with earlier studies on the enzyme system, it is shown that at low CO concentration, carbon monoxide binds to the cofactor in bridging modes, with nu(CO) bands at 1835 and 1808 cm(-1) that are interconverted by single-electron transfer. Importantly we show that the contentious overall 2e difference in the assignment of the metal oxidation levels in the resting state of the enzyme-bound cofactor, arising from analysis of (57)Fe ENDOR and Mössbauer data, can be resolved in the light of the electron-transfer chemistry of the isolated cofactor described herein.

Catalysis by Enzymes: The Biological Ammonia Synthesis

“…It has been shown that the isolated FeMoco can reduce acetylene ( [63] and references therein) Furthermore, Pickett et al have shown that the isolated FeMo cofactor can be put in an electrochemical system and can be reduced by an applied potential and catalyze the formation of H 2 [64]. It has also been shown that the isolated FeMoco binds CO and shows vibrational frequency shifts similar to those observed in the enzyme [65,66]. However, as in the enzyme, CO does not bind to the FeMoco in the resting state, but only at more reduced states.…”

IR‐Monitored Photolysis of CO‐Inhibited Nitrogenase: A Major EPR‐Silent Species with Coupled Terminal CO Ligands

“…Recently, the rates and relative intensities of these ν(CO) bands were shown to be sensitive to variation of the side-chain at the α -70 position. [13c] Other CO-related IR bands have been observed during spectro-electrochemical studies of FeMoco (the solvent-extracted version of the FeMo-cofactor), [14] where bands at 1808 and 1835 cm −1 were proposed to arise from bridging CO, whereas features at 1885 and 1920 cm −1 were assigned to terminally bound CO species.…”

Analysis of active sites for N2 and H+ reduction on FeMo-cofactor of nitrogenase

“…In addition, it is well known that CO inhibits all substrate reductions in the catalytic process except for H + and spectroscopy has demonstrated that CO can bind to central Fe sites on FeMo-co [31,32] . It has been suggested that several substrates such as N 2 and C 2 H 2 , can interact with the 4Fe4S face of FeMo-co, composed of Fe atoms 2, 3, 6, and 7 (numbered from the X-ray structure) from the work by Igarashi et al [12] and Barney et al 19] .…”