The energetics of N2 reduction by vanadium containing nitrogenase

Abstract

The main class of nitrogenases has a molybdenum in its cofactor. A mechanism for Mo-nitrogenase has recently been described. In the present study, another class of nitrogenases has been studied, the one with a vanadium instead of a molybdenum in its cofactor. It is generally believed that these classes use the same general mechanism to activate nitrogen. The same methodology has been used here as the one used for Mo-nitrogenase. N₂ activation is known to occur after four reductions in the catalytic cycle, in the E₄ state. The main features of the mechanism for Mo-nitrogenase found in the previous study are an activation process in four steps prior to catalysis, the release of a sulfide during the activation steps and the formation of H₂ from two hydrides in E₄, just before N₂ is activated. The same features have been found here for V-nitrogenase. A difference is that five steps are needed in the activation process, which explains why the ground state of V-nitrogenase is a triplet (even number) and the one for Mo-nitrogenase is a quartet (odd number). The reason an additional step is needed for V-nitrogenase is that V³⁺ can be reduced to V²⁺, in contrast to the case for Mo³⁺ in Mo-nitrogenase. The fact that V³⁺ is Jahn–Teller active has important consequences. N₂H₂ is formed in E₄ with reasonably small barriers.