Can the E 1 state in nitrogenase tell if there is an activation process prior to catalysis?

Model calculations have been performed for the singly reduced ground state of Mo-nitrogenase, usually termed E 1 . Contradictory conclusions have been reached in two recent experimental studies. In a study based on EPR, it was concluded that there is a bridging hydride in E 1 , while in an X-ray study it was concluded that there is no hydride in E 1 . Therefore, the EPR study implies that there is an oxidation of the cofactor going from E 0 to E 1 , the X-ray study implies a reduction. DFT methods have here been used, which have previously been benchmarked on a set of redox enzymes that led to the conclusion that the accuracy is about 3 kcal mol -1 in all cases, even for redox transitions. The methodology should therefore be adequate for resolving the question of the hydride presence in E 1 . As a comparison, calculations are performed on both Mo- and V-nitrogenase with the same conclusion. The conclusion from the calculations has far reaching consequences for the mechanism of nitrogenase.