Molybdate active site models

One striking feature of the active site stmctures of the enzymes in Figure 3.14 is the prevalence of five-coordinate molybdenum and the dithiolene ligation. Prior to structural information on the enzymes, a significant body of modeling work was focused on accomplishing OAT for monooxo molybdenum(IV) and dioxo molybde-num(VI) (Equation 3.12).47 The earlier model compounds featured a variety of donor ancillary ligands based on O, N, and S, but not dithiolene. Furthermore, these complexes were six-coordinate and none of them mimicked the desoxo observed in the reduced state of DMSO reductase. Nevertheless, important kinetics and... 

A variety of selenomolybdates are also accessible via reactions of molybdate and thiomolybdates with H2Se in aqueous solutions. Salts of [MoOSeS2] are attractive precursors for models of the E. barkeri NDH active site (2). 

Numerous studies have attempted to elucidate the role of Mo in the passivity of stainless steel. It has been proposed from XPS studies that Mo forms a solid solution with CrOOH with the result tiiat Mo is inhibited from dissolving trans-passively [9]. Others have proposed that active sites are rapidly covered with molybdenum oxyhydroxide or molybdate salts, thereby inhibiting localized corrosion [10]. Yet another study proposed that molybdate is formed by oxidation of an Mo dissolution product [11]. The oxyanion is then precipitated preferentially at active sites, where repassivation follows. It has also been proposed that in an oxide lattice dominated by three-valent species of Cr and Fe, ferrous ions will be accompanied by point defects. These defects are conjectured to be canceled by the presence of four- and six-valent Mo species [1]. Hence, the more defect-free film will be less able to be penetrated by aggressive anions. A theoretical study proposed a solute vacancy interaction model in which Mo " is assumed to interact electrostatically with oppositely charged cation vacancies [ 12]. As a consequence, the cation vacancy flux is gradually reduced in the passive film from the solution side to the metal-film interface, thus hindering vacancy condensation at the metal-oxide interface, which the authors postulate acts as a precursor for localized film breakdown [12]. 

Evidence in support of several of these models has been reported. XPS studies of the passive and transpassive films formed on Mo in deaerated 0.1 M HCl [3] established that molybdate was absent from both surface films. In a later study the same authors used a twin potentiostat arrangement, with a second working electrode of either Fe, Cr, or Ni that was polarized near an Mo electrode at the same potential (-180mV vs. SCE) [18]. At this potential Mo and Cr are passive, while Ni and Fe are active. In this work it was shown that for the Fe-Mo and Ni-Mo electrode couples, iron or nickel molybdate was observed on the passive Mo surface. In the case of the Cr-Mo couple, molybdate was observed only on the passive film of Cr. This work was also able to show that transpassivity of Mo at 250 mV (SCE) was suppressed in the presence of Fe, which formed a molybdate salt on the surface of Mo. This indicated evidence of a possible mechanism by which Mo can remain passive in stainless steels at higher potentials than the transpassive potential of Mo. In addition, this work supported the idea that soluble molybdate anions can redeposit at active sites. 

Molybdate, a structural analog of phosphate, is a potent inhibitor of pink uteroferrin s acid phosphatase activity, but has negligible effect on its optical spectrum and leaves the protein in its EPR-active form It converts the EPR spectrum of uteroferrin from a rhombic (g = 1.93, 1.75, 1.59) to an axial (g = 1.97, gi = 1.52) type which remains invariant to subsequent additions of phosphate, suggesting that both anions compete for the same binding site on the protein . With both ESEEM (Fig. 5A) and ENDOR (Fig. 5B) spectroscopy, which offer the advantage that interpretation does not depend on model compounds, a superhyperfine interaction of Mo-molybdate with the S = 1/2... 

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