Nitrogenase vanadium complexes

More complicated aggregate structures are found in the vanadium-copper and vanadium-silver species [ViMiS SPh CR ]" (M = Cu or Ag) (133), in which the metal sulfido core is in the shape of a cube [Fig. 14(d)] bulk magnetic susceptibility measurements have shown that there is probably a direct V —V bond within these species. These and related complexes are of interest since a vanadium-containing nitrogenase has been discovered (136). 

Aryloxo and thiolato vanadium complexes as chemical models of the active site of vanadium nitrogenase 05CCR(249)2144. 

Coupling the results from the Mo model complex [57] with the explanation for the presence and possible action of homocitrate, these results provide the best model to date for N2 reduction by conventional molybdenum nitrogenases. Vanadium-dependent nitrogenases probably undergo similar reactions, but iron-only nitrogenases [66] and the unique molybdenum-dependent nitrogenase from Streptomyces thermoautrophicus [67] present further mysteries. 

More complex assemblies of iron and sulfur, sometimes extended to other metals like nickel, molybdenum, vanadium, or other iron centers are found in some enzymes, that catalyze the transformation of small molecules [1, 14]. Among these centers, we will focus next on the P cluster and the FeMo cofactor of nitrogenase and on the H cluster of the iron-only hydrogenase. 

The next two entries to Table 3 are cited for completeness. Nitrogenase is treated in Chapter 7 and CO dehydrogenase in Chapter 9. Nitrogenase contains a very complex iron-sulfur cluster that includes another metal, molybdenum or vanadium. The crystal structure of the Mo variant has been determined. There is a third variant, alternative nitrogenase [92], whose cluster apparently does not contain any heterometal. That cluster would thus be a perfect candidate for our definition of a redox-catalytic iron-sulfur cluster. Unfortunately, this third nitrogenase has thus far been characterized to a much lesser extent than the other two forms. For all nitrogenases holds that the binding of N2 to the cluster has not been established [53] therefore, formally these enzymes have not yet been positively identified as redox iron-sulfur catalysts. 

Acetylene is quantitatively reduced by V catecholate to ethylene with cw-di-deuteroethylene formed selectively from C2D2, also similarly to enzymatic reduction of C2D2 by nitrogenase. Distinct from N2 reduction, the pH range for the reduction of acetylene is much broader than for dinitrogen (from ca pH 5 to concentrated alkali solution). Kinetic studies of the oxidation of vanadium(II) catecholate complex by dinitrogen have led to the reaction equation ... 

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