Site‐Specific Oxidation State Assignments of the Iron Atoms in the [4Fe:4S](2+/1+/0) States of the Nitrogenase Fe‐Protein

The nitrogenase iron protein (Fe‐protein) contains an unusual [4Fe:4S] iron‐sulphur cluster that is stable in three oxidation states: 2+, 1+, and 0. Here, we use spatially resolved anomalous dispersion (SpReAD) refinement to determine oxidation assignments for the individual irons for each state. Additionally, we report the 1.13‐Å resolution structure for the ADP bound Fe‐protein, the highest resolution Fe‐protein structure presently determined. In the dithionite‐reduced [4Fe:4S](1+) state, our analysis identifies a solvent exposed, delocalized Fe(2.5+) pair and a buried Fe(2+) pair. We propose that ATP binding by the Fe‐protein promotes an internal redox rearrangement such that the solvent‐exposed Fe pair becomes reduced, thereby facilitating electron transfer to the nitrogenase molybdenum iron‐protein. In the [4Fe:4S](0) and [4Fe:4S](2+) states, the SpReAD analysis supports oxidation states assignments for all irons in these clusters of Fe(2+) and valence delocalized Fe(2.5+), respectively.