Sulfur molybdenum cluster, iron

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

Iron-molybdenum-sulfur clusters will be considered in Section 62.1.14, which deals with aspects of the nitrogen cycle. 

Despite the lack of success in FeMoco synthesis, the quest for the FeMoco has motivated the study of numerous iron-molybdenum-sulfur clusters. " MoFe3S4 cubanes represent half of the FeMoco, and therefore have been extensively studied. Catecholates are often used to model the homocitrate ligand tetrachlorocatecholate (CU-cat) is common, but other adds are also used. ... 

It is recalled that in Chapter 9, Section 2, the electrochemical behaviour of the FeMo cofactor from FeMo-nitrogenase, was reported. It possesses a heteronuclear iron-molybdenum-sulfur (MoFe7S9) cluster, which has similarities with the above discussed iron-sulfur proteins. 

Methods of preparing molybdenum(V) complexes with M02O4 cores have been published. However, those of sulfur-bridged molybdenum(V) complexes with M02O3S and M02O2S2 cores remain difficult. Molybdenum-sulfur cluster compounds attract much attention since almost all molybdenum enzymes contain both molybdenum and sulfur, being always accompanied by iron. 

Fig. 1. Schematic illustration of the enzyme nitrogenase being composed of the molybdenum-iron (MoFe) protein, an oc2p2 tetramer with two unique iron-sulfur clusters (P-cluster) and two iron-molybdenum cofactors (FeMoco), and the iron protein with one [4Fe-4S]-cluster and two ATP binding sites.

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. 

More complex iron-sulfur clusters are also known to exist. These include the iron-molybdenum cofactor of nitrogenase (Thornely and Lowe, 1984) and probably larger clusters in which the only metal is iron (Hagen, 1987). They are characterized by highly anisotropic EPR spectra from S > ground states the nitrogenase cluster, for example is S = j and has EPR features near g = 4 and g = 2. 

Figure 10-9. The structure of the unique iron-molybdenum cluster found at the active site of nitro-genase. The molybdenum is co-ordinated to an extremely unusual homocitrate ligand. There is another iron-sulfur cluster close to this site.

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