Electronic dimensions of FeMo-co, the active site of nitrogenase, and its catalytic intermediates

The iron-molybdenum cofactor (FeMo-co), which is the catalytic center for the enzymatic conversion of N(2) to NH(3), has the composition [NFe(7)MoS(9)(homocitrate)], and, with a cluster of eight transition metal atoms and nine sulfur atoms, has a complex delocalized electronic structure. The electronic dimensions of FeMo-co and of each of its derivatives appear as sets of electronic states lying close in energy. These electronic dimensions naturally partner the geometrical changes and the reactivity patterns during the catalytic cycle, and also connect with spectroscopic investigations of the mechanism. This paper describes straightforward computational procedures for the determination and management of the low-lying electronic states of FeMo-co and of its coordinated intermediates and transition states during density functional simulations of steps in the catalytic mechanism. General principles for the distribution of electron spin density over all atoms are presented, using several proposed intermediates as examples. A tough general irony arises in the distribution of spin density over FeMo-co and its derivatives: the less interesting atoms get the spin, and the most interesting atoms do not.