Chemical State of Molybdenum in Nitrogenase

The measurement of an atom’s K or L x-ray absorption edge energy and line shape can provide useful information about its chemical environment. ’ The “chemical shift ” of the absorption edge reflects the net charge on an atom. Fur-thermore, for transition metal complexes, the fine structure of the metal edge reflects the distribution of empty molecu-lar orbitals and hence the nature of the surrounding ligands. We report here a good correlation between the chemical shift of the Mo K edge and the calculated “coordination charge ” for a wide range of Mo compounds. The Mo K ab-sorption edge for the Mo-Fe component of nitrogenase has also been observed, and its position has been used to deter-mine the possible chemical states of Mo in the resting en-zyme. These results suggest that absorption edge measure-mefits will be useful in the characterization of the chemica