Delicate dance becomes a ballet: The enzyme nitrogenase uses adenosine triphosphate and several unusual iron-sulfur cofactors to pump electrons into typically inert dinitrogen (N ₂ ), providing protons along the way. Previous work has shown that sulfur atoms in the iron-molybdenum cofactor (FeMoCo) are labile and suggests that replacement of one of the sulfurs by N ₂ is integral to the mechanism of N ₂ binding and reduction. Through the elimination of excess reducing agent during preparation, Kang et al. determined structures of Mo-nitrogenase in a resting conformation. Unexpectedly, they found that all three sulfurs at the outer edge of FeMoCo appear to be labile, with one subunit even having two of three sulfurs replaced by light, diatomic ligands. Biochemical and spectroscopic data indicate that the protein is active, holds tightly bound N ₂ , and is in the expected oxidation state. These results may prompt a reassessment of the possible mechanisms of N ₂ reduction and the role of dynamic belt ligands in FeMoCo. Science , this issue p. 1381

The preparation of [Fe^IV(O)(MePy₂tacn)]²⁺ (2, MePy₂tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) by reaction of [Fe^II(MePy₂tacn)(solvent)]²⁺ (1) and PhIO in CH₃CN and its full characterization are described. This compound can also be prepared photochemically from its iron(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of [Ru^II(bpy)₃]²⁺ as photosensitizer and a sacrificial electron acceptor (Na₂S₂O₈). Remarkably, the rate of the reaction of the photochemically prepared compound 2 toward sulfides increases 150-fold under irradiation, and 2 is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound 2 with sulfides under different conditions, which demonstrated that both light and [Ru^II(bpy)₃]²⁺ are necessary for the observed increase in the reaction rate. A combination of nanosecond time-resolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated [Ru^III(bpy)₃]³⁺ oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by 2 to the corresponding sulfoxide.