Electron Inventory, Kinetic Assignment (En), Structure, and Bonding of Nitrogenase Turnover Intermediates with C2H2 and CO

Improved 1H ENDOR data from the SEPR1 intermediate formed during turnover of the nitrogenase α-195Gln MoFe protein with C21,2H2 in 1,2H2O buffers, taken in context with the recent study of the intermediate formed from propargyl alcohol, indicate that SEPR1 is a product complex, likely with C2H4 bound as a ferracycle to a single Fe of the FeMo-cofactor active site. 35 GHz CW and Mims pulsed 57Fe ENDOR of 57Fe-enriched SEPR1 cofactor indicates that it exhibits the same valencies as those of the CO-bound cofactor of the lo-CO intermediate formed during turnover with CO, [Mo4+, Fe3+, Fe62+, S92-(d43)]+1, reduced by m = 2 electrons relative to the resting-state cofactor. Consideration of 57Fe hyperfine coupling in SEPR1 and lo-CO leads to a picture in which CO bridges two Fe of lo-CO, while the C2H4 of SEPR1 binds to one of these. To correlate these and other intermediates with Lowe−Thorneley (LT) kinetic schemes for substrate reduction, we introduce the concept of an “electron inventory”. It partitions the nu...