Fe-Mo-S clusters

Figure 4 Mo-Fe-S clusters containing two cubane-type MoFe3S4 cores, which are the extensively studied, synthetic models of FeMo-co...

Figure 7 Mo-Fe-S cluster consisting of the metal-sulfur connecting scheme akin to that in PN-cluster...

The dependence of rate constants for approach to equilibrium for reaction of the mixed oxide-sulfide complex [Mo3((i3-S)((i-0)3(H20)9] 1+ with thiocyanate has been analyzed into formation and aquation contributions. These reactions involve positions trans to p-oxo groups, mechanisms are dissociative (391). Kinetic and thermodynamic studies on reaction of [Mo3MS4(H20)io]4+ (M = Ni, Pd) with CO have yielded rate constants for reaction with CO. These were put into context with substitution by halide and thiocyanate for the nickel-containing cluster (392). A review of the chemistry of [Mo3S4(H20)9]4+ and related clusters contains some information on substitution in mixed metal derivatives [Mo3MS4(H20)re]4+ (M = Cr, Fe, Ni, Cu, Pd) (393). There are a few asides of mechanistic relevance in a review of synthetic Mo-Fe-S clusters and their relevance to nitrogenase (394). 

P-cluster pairs in, 17 307-308 structure of, 17 307-310 Mo-Fe-S clusters, 17 315 Mohavite, 4 133t Mohs hardness scale, 1 3-4 selected materials, l 3t Moire interferometry equations in, 17 426-428 experimental setup for, 17 428-429 in nondestructive evaluation,... 

Four distinct types of Fe-S center have now been found in proteins, ranging from mono- to tetranuclear in addition, a novel Mo-Fe-S cluster is present in the enzyme nitrogenase. Synthetic analogs of most of these have been prepared and used to provide insight into the intrinsic properties of the metal-sulfur centers in the absence of protein-imposed constraints. The strategies used to prepare both Fe-S and Mo-Fe-S clusters are described they range from spontaneous self-assembly to the designed synthesis of clusters with specific structural features. 

Synthetic efforts at preparing model complexes for the FeMo-cofactor have largely focussed on two types of Mo-Fe-S cluster, both of which are prepared via self-assembly reactions using tetrathiomolybdate as starting material. The first of these is the "linear" type of cluster, containing the MoS2Fe unit formed by coordination of discrete MoS - units to Fe. The second is the... 

The latter are generally obtained as dimeric "double cubanes", which must be cleaved to produce the single cubane clusters. The preparation and properties of both linear (51,71,72) and cubane (51,71) Mo-Fe-S clusters have been reviewed in detail, and will not be discussed further here. 

O.lA shorter. Third, the linear clusters exhibit no reaction chemistry with small molecules at Mo, but such chemistry is well-established for the single cubane MoFe3S clusters (71). It is clear that new strategies to produce synthetic Mo-Fe-S clusters with proper stoichiometry. Mo oxidation state, and reactivity are necessary two such approaches are described below. 

A) clusters. To date, it has not proven possible to isolate a cluster in which only one of the trigonal faces is capped by an Mo(C0)3 fragment, although such species are in equilibrium with the dicapped cluster in solution (7A). This system represents one of the very few Mo-Fe-S clusters in which the detailed structure of the product was anticipated prior to its synthesis. If the problems associated with forming a monocapped species and subsequent oxidative decarbonylation at Mo can be overcome, these complexes will Indeed be the closest synthetic representations to FeMo-co yet achieved. 

D = -0.7 cm", E/D = 0.26 for the S = 3/2 center in dithionite-reduced AvV (B. J. Hales, unpublished results). MCD magnetization data for dithionite-reduced Avl and Avl (8) indicate that the EPR-detectable S = 3/2 paramagnets are also responsible for the temperature-dependent MCD transitions. Therefore, by analogy with the MoFe protein it seems probable that the S = 3/2 paramagnetic chromophore in dithionite-reduced Avl is a V-Fe-S cluster. However, the EPR and MCD studies indicate that this cluster has magnetic and electronic properties distinct from that of the Mo-Fe-S clusters in conventional Mo nitrogenases. 

Nitrogenase Trimethylamine oxidase Range of microbes E. coli 2 Mo, Fe, S cluster, iron-sulfur cluster... 

Figure 73 Some Mo-Fe-S clusters. The coordination sphere of the iron atoms is completed by thiolate ligands...

Several reports currently are available for the preparation of Mo-Fe-S clusters most take advantage of the techniques developed for the successful synthesis of the chemical models for the ferredoxins (125). A similar spontaneous assembly occurs when MoS42 and Fe(SR)3 are mixed in MeOH in the presence of excess thiol. In this instance, however, a more complex product results consisting of two MoFe3S4(SR)3 cube-like structures bridged via the two Mo atoms by a sulfide and two thiolate groups, [ (EtSFe)3S4Mo 2S(SEt)2]3 (124). A similar product has been obtained independently by a similar method... 

Contents E. I. Solomon, K. W.Penfield, D.E. Wilcox Active Sites in Copper Proteins. An Electronic Structure Overview. -B.A.Averill Fe-S and Mo-Fe-S Clusters as Models for the Active Site of Nitrogenase. - N.D. Chasteen The Biochemistry of Vanadium. -KKustin, G.C. McLeod, T.R. Gilbert,... 

Combined EPR and 57Fe Mossbauer spectroscopic studies of the MoFe protein in various overall oxidation states39 40,42,44 48,49) have provided strong evidence for the presence of six metal clusters two M centers that are the protein-bound form of the FeMo-cofactor (a novel Mo-Fe-S cluster) and four unusual tetranuclear iron clusters referred to as the P clusters. These will be discussed separately below. In addition, Mossbauer spectra of MoFe proteins from Azotobacter vinelandii, Clostridium pas-teurianum, and Klebsiella pneumoniae all show an additional component termed S39 40 42, which accounts for 6% of the total Fe present (2 Fe per molecule) and which has Mossbauer parameters (AEq = 1.35 mm/s d = 0.60 mm/s) different from those expected for likely impurities such as high-spin Fe2+ or Fe3+ ( adventitious iron). At present, it is difficult to decide whether species S is an unusual and persistent impurity or an integral part of the MoFe protein. 

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