An atomic level model for the interactions of molybdenum nitrogenase with carbon monoxide, acetylene, and ethylene

A combination of density functional theory and molecular mechanics calculations has been used to study the possible interactions of CO, C(2)H(2), and C(2)H(4) with the central Fe and terminal Mo sites of the iron-molybdenum cofactor of nitrogenase. The most favorable binding mode for CO on the central section of the FeMoco appears to be end-on to a single Fe and results in a change from high to low spin for the ligating Fe atom. If a coordination site for CO is available on the Mo, this becomes the preferred CO binding site. Calculated nu(CO) infrared frequencies are compared with the experimental values given in the literature. C(2)H(2) binds weakly in a side-on orientation to a single Fe site; addition of a single H(+)/e(-) couple to the substrate results in spontaneous migration of the resulting -CH=CH(2) group from Fe to a central S atom of the cofactor. Further reduction liberates C(2)H(4) or alternatively can give an S=CHCH(3) intermediate, which then goes on to produce C(2)H(6). A model for C(2)H(2) reduction by nitrogenase is proposed, based on the results of the calculations and the extensive literature on this process.