Molybdenum atom

As yet, no X-ray crystal structures are available for any of the molybdenum enzymes in Table I. Therefore, present descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The two most powerful techniques for directly probing the molybdenum centers of enzymes are electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS), especially the extended X-ray absorption fine structure (EXAFS) from experiments at the Mo K-absorption edge. Brief summaries of techniques are presented in this section, followed by specific results for sulfite oxidase (Section III.B), xanthine oxidase (Section III.C), and model compounds (Section IV). 

These techniques are applicable only to paramagnetic Mo(V) centers, but the EPR parameters are extremely sensitive to coordination changes at the molybdenum center 17, 64). The molybdenum and ligand hyperfine splittings can provide additional information about the coordination environment of the molybdenum(V) species and the chemical reactions at the molybdenum center. EPR spectra from xanthine oxidase were first reported in 1959 by Bray et al. (65), and Bray and co-workers have continued to develop the application of EPR spectroscopy to molybdenum enzymes 17, 64). In 1966 it was shown (66) that mixing [Mo04] with dithiols produced EPR signals with (g) and (A( Mo)) values similar to those of xanthine oxidase. Only recently, however, have the structures of such thiolate complexes been determined (see Section IV.B.2.b). 39) and P (67) ENDOR spec- 

EXAFS at the Mo K-edge is usually applied to enzymes in their fully oxidized (Mo(VI)) or fiilly reduced (Mo(IV)) states (68), but studies of enzymes poised in the Mo(V) oxidation state have also been described 

To date only DMSO reductase from R. sphaeroides forma specialis denitrificans (71) (Section III.A.4) has been studied by resonance Raman spectroscopy (40). The oxidized and reduced forms of DMSO reductase show vibrations in the 335- to 385-cm region that shift upon enrichment of the enzyme with S and that have been assigned to Mo—S vibrations. The most prominent feature is the band at 350 cm in oxidized DMSO reductase, which shifts to 341 cm upon S enrichment. For the oxidized state of the enzyme (presumably Mo(VD) the 350-cm band has been assigned to a Mo—S(dithiolene) vibration 

Resonance Raman studies on model complexes containing Mo=0 and Mo=S groups have shown that the Mo=0 group is a very weak Raman scatterer and is unlikely to be detectable in an enzyme (.74). The Mo=S group showed maximum Raman enhancements of about 33 and may be detectable in oxidized xanthine oxidase and other enzymes possessing [MoOS] oxidized centers. 

---

Physical Properties. Molybdenum has many unique properties, leading to its importance as a refractory metal (see Refractories). Molybdenum, atomic no. 42, is in Group 6 (VIB) of the Periodic Table between chromium and tungsten vertically and niobium and technetium horizontally. It has a silvery gray appearance. The most stable valence states are +6, +4, and 0 lower, less stable valence states are +5, +3, and +2. 

However, when either P(CgH )(CH2)2 or P(CgH )2(CH2) is used to form cis- or /n j -M(N2)2(PR3)4j M = Mo or W, respectively, followed by treatment with acid, ammonia yields of about 2 mol or 0.7 mol pet mole of complex for M = W and Mo, respectively, are produced (193,194). These and related data have been used to suggest a possible stepwise sequence for the reduction and protonation of N2 on a single molybdenum atom ia nitrogeaase (194). However, acidificatioa leads to complete destmctioa of the complex. Using both the stabilizing effect of the chelating phosphine triphos,... 

Fig. 18. Production of x-ray fluorescence K and E series from molybdenum atom.

Mechanism for RMC formation is proposed. Transfer of electrons from ascorbic acid proceeds through Me(III) atoms to molybdenum atoms in mixed POM. Me(III) atoms in heteropolyanion can be oxidized to Me(V) by Mo(VI) making possible easy oxidation of AA. 

The element molybdenum (atomic weight 95.95) constitutes 0.08% of the weight of nitrate reductase. If the molecular weight of nitrate reductase is 240,000, what is its likely quaternary structure ... 

Fe—Fe bond can be assigned structures 201 or 202 based on spectral data. The other product of this reaction is 193 (R = r-Bu), however, it is produced in minor amounts. Complexes 199 (R = R = r-Bu, R = Ph, R = r-Bu) were obtained. Reaction of 146 (M = Mo, R = Ph, R = R = Ft, R = r" = Me) with (benzyli-deneacetone)iron carbonyl gives rise to the bimetallic complex 200 (M = Mo), which reacts further with the free phosphole to form the bimetallic heteronuclear sandwich 203. The preferable coordination of the molybdenum atom to the dienic system of the second phosphole nucleus is rather unusual. The molybdenum atom is believed to have a greater tendency to coordinate via the trivalent phosphorus atom than via the dienic system. 

Consider individual atoms of an element deposited on a thin substrate highly transparent to x-rays—say atoms of molybdenum upon paper. Let a characteristic line (say molybdenum K ) be excited by a polychromatic beam, x-ray source and detector both being located above the sample. So long as the number of molybdenum atoms is small, they will not noticeably attenuate the incident beam, nor will an x-ray quantum radiated by any molybdenum atom be absorbed by any other. Under these conditions, the intensity of the characteristic line will be proportional to the number of molybdenum atoms and hence to the thickness of the molybdenum film. 

M0S2CI3, monoclinic (P2i/c). Figure 23 shows the translational unit, with atom designations. The sulfur atoms form pairs having a distance of 1.98 A. The molybdenum atoms also occur in pairs enclosed by two bridging S2 groups. With two chlorine atoms in terminal and... 

M03S7X4 (X = Cl, Br), monoclinic (P2i/c). Figure 24 shows the asymmetrical unit of the crystal structure. The three, independent molybdenum atoms form an almost equilateral triangle. Six of the seven sulfur atoms occur in three S2 groups, each one bridging one Mo-Mo... 

FeMoco can be extracted from the MoFe protein into A(-methylfor-mamide (NMF) solution 32) and has been analyzed extensively using a wide range of spectroscopic techniques both bound to the protein and in solution after extraction from it (33). The extracted FeMoco can be combined with the MoFe protein polypeptides, isolated from strains unable to synthesize the cofactor, to generate active protein. The structure of the FeMoco is now agreed 4, 5, 7) as MoFeTSg homocitrate as in Fig. 4. FeMoco is bound to the a subunit through residues Cys 275, to the terminal tetrahedral iron atom, and His 442 to the molybdenum atom (residue numbers refer to A. vinelandii). A number of other residues in its environment are hydrogen bonded to FeMoco and are essential to its activity (see Section V,E,2). The metal... 

Homocitrate is bound to the molybdenum atom by its 2-carboxy and 2-hydroxy groups and projects down from the molybdenum atom of the cofactor toward the P clusters. This end of FeMoco is surrounded by several water molecules (5, 7), which has led to the suggestion that homocitrate might be involved in proton donation to the active site for substrate reduction. In contrast, the cysteine-ligated end of FeMoco is virtually anhydrous. 

The molyhdopterin cofactor, as found in different enzymes, may be present either as the nucleoside monophosphate or in the dinucleotide form. In some cases the molybdenum atom binds one single cofactor molecule, while in others, two pterin cofactors coordinate the metal. Molyhdopterin cytosine dinucleotide (MCD) is found in AORs from sulfate reducers, and molyhdopterin adenine dinucleotide and molyb-dopterin hypoxanthine dinucleotide were reported for other enzymes (205). The first structural evidence for binding of the dithiolene group of the pterin tricyclic system to molybdenum was shown for the AOR from Pyrococcus furiosus and D. gigas (199). In the latter, one molyb-dopterin cytosine dinucleotide (MCD) is used for molybdenum ligation. Two molecules of MGD are present in the formate dehydrogenase and nitrate reductase. 

In order to explain why the bridging vinylidene group of MoCo2(/r 3-)F-CCH2) (COlsI / -CyHOr" (Fig. 83) leans toward the molybdenum atom rather than a cobalt atom, EHMO calculations have been employed. Results showed that the... 

An earlier study by Baumgartner and Reichold (11) shows further that even single metal atoms can react with available CO molecules to produce carbonyls. By irradiating a mixture of powdered Cr(CO)j and UjOg, they were able to catch the fission product Mo and isolate it as Mo(CO)g. This clearly indicates that molecule formation is not dependent on previously formed bonds. The yields of Mo(CO)6 (60%) were too high to represent only the primary fission product molybdenum atoms, and indicate that some short-lived precursors of Mo ( Zr, Nb) may also have formed at least tentative metal-CO bonds, and produced Mo(CO)g after )3-decay. 

Understanding the mechanism of reactions on the catalyst surface requires an adequate description of the surface it must modelled either by infinite slab or by clusters having similar properties. The interesting feature of the M0O3 surface is the existence of three structurally different oxygen atoms, a terminal one O] coordinated to one molybdenum atom, and two bridge-like oxygen atoms On and Om, coordinated to two and three Mo atoms, respectively. 

---