Computational Model Study of the Experimentally Suggested Mechanism for Nitrogenase

The mechanism for N-2 activation in the E-4 state of nitrogenase was investigated by model calculations. In the experimentally suggested mechanism, the E-4 state is obtained after four reductions to the ground state. In a recent theoretical study, results for a different mechanism have been found in excellent agreement with available Electron Paramagnetic Resonance (EPR) experiments for E-4. The two hydrides in E-4 leave as H-2 concertedly with the binding of N-2. The mechanism suggested differs from the experimentally suggested one by a requirement for four activation steps prior to catalysis. In the present study, the experimentally suggested mechanism is studied using the same methods as those used in the previous study on the theoretical mechanism. The computed results make it very unlikely that a structure obtained after four reductions from the ground state has two hydrides, and the experimentally suggested mechanism does therefore not agree with the EPR experiments for E-4. Another structure with only one hydride is here suggested to be the one that has been observed to bind N-2 after only four reductions of the ground state.