Is there computational support for an unprotonated carbon in the E4 state of nitrogenase?

In the key enzyme for nitrogen fixation in nature, nitrogenase, the active site has a metal cluster with seven irons and one molybdenum bound by bridging sulfurs. Surprisingly, there is also a carbon in the center of the cluster, with a role that is not known. A mechanism has been suggested experimentally, where two hydrides leave as a hydrogen molecule in the critical E4 state. A structure with two hydrides, two protonated sulfurs and an unprotonated carbon has been suggested for this state. Rather recently, DFT calculations supported the experimental mechanism but found an active state where the central carbon is protonated all the way to CH3 . Even more recently, another DFT study was made that instead supported the experimentally suggested structure. To sort out the origin of these quite different computational results, additional calculations have here been performed using different DFT functionals. The conclusion from these calculations is very clear and shows no computational support for an unprotonated carbon in E4 . © 2017 Wiley Periodicals, Inc.