Fe-S and Mo-Fe-S clusters as models for the active site of nitrogenase

Abstract

Studies on the enzyme nitrogenase have shown that it contains two novel metal-sulfur clusters: the iron-molybdenum cofactor (FeMo-co), which contains one Mo, 6–7 Fe, and ∼ 4 S atoms in a spincoupled paramagnetic (S=3/2) cluster; and the P clusters, variants of normal but highly reduced 4 Fe-4 S clusters in which three Fe atoms are somehow distinguished from the fourth and which, when oxidized, exhibit an usually high spin (S=5/2 or 7/2) ground state. No structures consistent with all available spectroscopic data have been proposed for either unit. Synthetic efforts aimed at developing models for FeMo-co have utilized tetrathiomolybdate as starting material, and have resulted in two well-characterized classes of Mo-Fe-S cluster: those containing the MoFe3S4 “cubane” core and those containing a “linear” MoS2Fe unit. The former apparently contain Mo in a relatively reduced valence state (ca. + 3) and exhibit Mo-S and Mo-Fe distances in good agreement with those found for the Mo site of nitrogenase; the latter contain Mo in a higher oxidation state (ca. + 6) and have significantly shorter Mo-S distances. Synthetic work related to the P clusters has centered on the properties of the [Fe4S4(SR)4]3− ions, for which two different structures with different magnetic properties are found, and on the properties of 4 Fe-4 S clusters with non-thiolate ligands.