Spectroscopic Characterization of an Eight‐Iron Nitrogenase Cofactor Precursor that Lacks the “9th Sulfur”

Nitrogenases catalyze the reduction of N 2 to NH 4 + at its cofactor site. Designated the M‐cluster, this [MoFe 7 S 9 C( R ‐homocitrate)] cofactor is synthesized via the transformation of a [Fe 4 S 4 ] cluster pair into an [Fe 8 S 9 C] precursor (designated the L‐cluster) prior to insertion of Mo and homocitrate. We report the characterization of an eight‐iron cofactor precursor (designated the L*‐cluster), which is proposed to have the composition [Fe 8 S 8 C] and lack the “9 th sulfur” in the belt region of the L‐cluster. Our X‐ray absorption and electron spin echo envelope modulation (ESEEM) analyses strongly suggest that the L*‐cluster represents a structural homologue to the l ‐cluster except for the missing belt sulfur. The absence of a belt sulfur from the L*‐cluster may prove beneficial for labeling the catalytically important belt region, which could in turn facilitate investigations into the reaction mechanism of nitrogenases.