Elicitation of thiomolybdates from the iron-molybdenum cofactor of nitrogenase. Comparison with synthetic Fe-Mo-S complexes.

Aerial oxidation of the iron-molybdenum cofactor (FeMoco) of Azotobacter vinelandii nitrogenase has been shown to yield either the tetrathiomolybdate ion ([MoS4]2-) or the oxotrithiomolybdate ion ([MoOS3]2-), depending on the reaction conditions. Thus, when N-methylformamide (NMF) solutions of FeMoco either were titrated with measured aliquots of air or were diluted with air-saturated NMF, [MoOS3]2- was found to be the predominant product while dilution of NMF solutions of FeMoco with air-saturated methanol produced [MoS4]2- almost exclusively. Similar aerial oxidation of solutions of chemically synthesized Fe-Mo-S clusters showed that significant information about the molybdenum environment in these species could be deduced from the nature of the elicited thiomolybdates. The differences in decomposition products as a function of solvent are postulated to be due to the loss through precipitation of the reducing agent sodium dithionite on addition of methanol but not NMF. These overall decomposition results are discussed in the context of recent X-ray absorption spectroscopic data which suggest the presence of an 'MoS3' core in FeMoco. A possible mechanism whereby [MoS4]2- might be rapidly formed from this core is presented.