Effects of molybdate,sulfide, and tetrathiomolybdate on copper metabolism in rats

Species differences in the response to dietary MoO₄^2? as a metabolic antagonist of Cu are considered briefly. Suggestions that (i) the potency of MoO₄^2? as a Cu antagonist is enhanced by normally innocuous dietary concentrations of S^2? and (ii) that MoS₄^2? may be a more effective antagonist than either MoO₄^2? or S^2? were investigated in a series of studies with rats. Diets including MoS₄^2? but not of MoO₄^2? or S^2? alone promoted a decline in hepatic Cu and ceruloplasmin activity and induced clinical signs of Cu deficiency. Evidence of concurrent anomalies in the partition of Cu between tissues and in the distribution of Cu between proteins of plasma and kidney cytosol suggested that such effects were partly attributable to the development of systemic defects in Cu metabolism. The relationship of such findings to the suggested involvement of MoS₄^2? or its derivatives in the etiology of Mo-induced Cu deficiency in ruminant animals is considered.     

Synthesis, structure, and redox behaviour of a novel cis-dioxomolybdenum(VI) dinuclear complex with a quadradentate dithiocarbamate

A novel dinuclear cis-dioxomolybdenum(VI) complex [{MoO₂(Bz₂endtc)}₂] coordinated with a quadradentate dithiocarbamate (Bz₂endtc²⁻: ((2-(dithiocarboxybenzylamino)ethyl)benzylamino)-methanedithioate(2−)) has been synthesised. The structural features of [{MoO₂(Bz₂endtc)}₂] have been elucidated by X-ray crystal analysis, elemental analysis and ¹³C NMR, IR and FAB⁺ mass spectroscopy: two almost identical cis-dioxomolybdenum(VI) centres are bridged by the two Bz₂endtc²⁻ ligands and each molybdenum(VI) centre has a distorted octahedral geometry with four sulphur atoms and two terminal oxo ligands lying in a cis position to each other. There is unlikely to be electronic interaction between the two cis-dioxomolybdenum(VI) centres in [{MoO₂(Bz₂endtc)}₂] because the MoMo distance is long (=7.337 Å). In the [{MoO₂(Bz₂endtc)}₂]/PPh₃ system, the oxygen atom transfer reaction (Eq. (A)) occurs to give a tetranuclear oxomolybdenum(VI,V) complex formulated as [MoO₂(Bz₂endtc)₂Mo₂O₃(Bz₂endtc)₂MoO₂] which has one μ-oxomolybdenum(V) moiety.

(A)     

Copper and molybdenum absorption by rats given ammonium tetrathiomolybdate

Previous studies have shown that the tetrathiomolybdate ion [MoS_4^2?] is a potent antagonist of Cu metabolism. Effects of orally administered MoS_4^2? on the absorption and tissue distribution of ⁶⁴Cu in rats have now been investigated. Four or 12 mg Mo/kg diet, when given as MoS_4^2?, strongly inhibited ⁶⁴Cu absorption and modified the fate of absorbed Cu, decreasing hepatic and renal uptake but increasing plasma retention of ⁶⁴Cu. These effects were not induced by equivalent dietary concentrations of Mo as MoS_4^2? or when S^2? was given as CaS. Clinical and biochemical effects induced by orally administered MoS_4^2? were abolished by increasing dietary concentrations of Cu. Such treatment also inhibited the absorption and tissue retention of ⁹⁹Mo derived from ⁹⁹MoS_4^2?. Intraperitoneal administration of Cu ameliorated clinical effects attributable to MoS_4^2? but neither inhibited ⁹⁹Mo absorption nor the appearance of systemic defects in Cu metabolism. Since the absorption of MoS_4^2? (or its derivatives) from the gastrointestinal tract is inhibited by Cu, it is evident that the site of its action as an antagonist influencing either the absorption or the subsequent metabolic fate of Cu depends upon the ratio Cu/MoS_4^2? in the diet.     

Analysis of molybdenite bioleaching by Thiobacillus ferrooxidans in the absence of iron (II)

Summary Thiobacillus ferrooxidans attachment on MoS² and Mo dissolution are increased by the addition of the tensioactive agent Tween 80 in absence of iron(II), which suggests that the poor bioleaching of MoS² is caused by its hydrophobic character. Additionally, inhibition ofThiobacillus ferrooxidans growth by the presence of MoO₄ ^2– and the effect of variable amounts of Tween 80 on bacteria growth and on MoS² bioleaching are considered in this paper. Data confirm the need of bacterial attachment to insoluble substrate for bioleaching by the direct mechanism.     

Iron- and aluminium-induced depletion of molybdenum in acidic environments impedes the nitrogen cycle

Anthropogenic nitrate contamination is a serious problem in many natural environments. Nitrate removal by microbial action is dependent on the metal molybdenum (Mo), which is required by nitrate reductase for denitrification and dissimilatory nitrate reduction to ammonium. The soluble form of Mo, molybdate (MoO₄²⁻), is incorporated into and adsorbed by iron (Fe) and aluminium (Al) (oxy) hydroxide minerals. Herein we used Oak Ridge Reservation (ORR) as a model nitrate-contaminated acidic environment to investigate whether the formation of Fe- and Al-precipitates could impede microbial nitrate removal by depleting Mo. We demonstrate that Fe and Al mineral formation that occurs as the pH of acidic synthetic groundwater is increased, decreases soluble Mo to low picomolar concentrations, a process proposed to mimic environmental diffusion of acidic contaminated groundwater. Analysis of ORR sediments revealed recalcitrant Mo in the contaminated core that co-occurred with Fe and Al, consistent with Mo scavenging by Fe/Al precipitates. Nitrate removal by ORR isolate Pseudomonas fluorescens N2A2 is virtually abolished by Fe/Al precipitate-induced Mo depletion. The depletion of naturally occurring Mo in nitrate- and Fe/Al-contaminated acidic environments like ORR or acid mine drainage sites has the potential to impede microbial-based nitrate reduction thereby extending the duration of nitrate in the environment.     

The relationship of Mo,molybdopterin, and the cyanolyzable sulfur in the Mo cofactor

Reconstitution of the apoprotein of the molybdoenzyme nitrate reductase in extracts of the Neurospora crassa mutant nit-1 with molybdenum cofactor released by denaturation of purified molybdoenzymes is efficient in the absence of exogenous MoO₄^2? under defined conditions. Evidence is presented that this molybdate-independent reconstitution is due to transfer of intact Mo cofactor, a complex of Mo and molybdopterin (MPT), the organic constituent of the cofactor. This complex can be separated from denatured protein by gel filtration, and from excess MoO₄^2? by reverse-phase HPLC. Sulfite oxidase, native xanthine dehydrogenase, and cyanolyzed xanthine dehydrogenase are equipotent Mo cofactor donors. Other well-studied inactive forms of xanthine dehydrogenase are also shown to be good cofactor sources. Using xanthine dehydrogenase specifically radiolabeled in the cyanolyzable sulfur, it is shown that this terminal ligand of Mo is rapidly removed from Mo cofactor under the conditions used for reconstitution.     

Over 100‐nm‐Thick MoOx Films with Superior Hole Collection and Transport Properties for Organic Solar Cells

Herein, a novel and effective method to prepare n‐doped MoO_x films with highly improved conductivity is reported. The MoO_x films are readily prepared by spin‐coating an aqueous solution containing ammonium molybdate tetrahydrate and vitamin C (VC). As confirmed by UV–vis absorption, X‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy measurements, Mo(VI) is partially reduced to Mo(V) by VC, resulting in the n‐doping of MoO_x. The conductivity of the n‐doped MoO_x (H:V‐Mo) film can be enhanced by four orders of magnitude compared to pristine MoO_x (H‐Mo), that is, from 1.2 × 10⁻⁷ to 1.1 × 10⁻³ S m⁻¹. The device using a 10 nm H:V‐Mo anode interlayer (AIL) exhibits comparable photovoltaic performance to a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)‐modified device. More importantly, the hole transport and collection properties of the H:V‐Mo AILs show outstanding tolerance to thickness variation, that is, with increasing thickness of the H:V‐Mo AIL from 10 to 150 nm, the V _oc and fill factor values of the devices remain unchanged. The device based on the blade‐coated H:V‐Mo AIL also has a high power conversion efficiency of 10.6%. To the best of the authors' knowledge, this work demonstrates the first example to prepare metal oxide AILs with outstanding tolerance to thickness, which is promising for the future large‐area manufacturing.     

95Mo NMR spectra of MoO2(C8H7N2S)2 and X-ray crystal structure of Mo2O4(C8H7N2S)2(C3H7NO)2

The facile preparation of MoO₂(C₈H₇N₂S)₂ is given and its complex behaviour in dimethylformamide, as revealed by ⁹⁵Mo NMR spectroscopy, discussed. The X-ray crystal structure of one of the products obtained from this dimethylformamide solution is briefly described. This structure indicates that the Mo(VI) has been reduced to Mo(V) and is based on the [Mo₂O₄]²⁺ core.     

Cr isotopic insights into ca. 1.9 Ga oxidative weathering of the continents using the Beaverlodge Lake paleosol,Northwest Territories,Canada

The ca. 1.9 Ga Beaverlodge Lake paleosol was studied using redox‐sensitive Cr isotopes in order to determine the isotopic response to paleoweathering of a rhyodacite parent rock 500 million years after the Great Oxidation Event. Redox reactions occurring in modern weathering environments produce Cr(VI) that is enriched in heavy Cr isotopes compared to the igneous inventory. Cr(VI) species are soluble and easily leached from soils into streams and rivers, thus, leaving particle‐reactive and isotopically light Cr(III) species to build up in soils. The Beaverlodge Lake paleosol and two other published weathering profiles of similar age, the Flin Flon and Schreiber Beach paleosols, are not as isotopically light as modern soils, indicating that rivers were not as isotopically heavy at that time. Considering that the global average δ⁵³Cr value for the oxidative weathering flux of Cr to the oceans today is just 0.27 ± 0.30‰ (1σ) based on a steady‐state analysis of the modern ocean Cr cycle, the oxidative weathering flux of Cr to the oceans at ca. 1.9 Ga would have likely been shifted to lower δ⁵³Cr values, and possibly lower than the igneous inventory (–0.12 ± 0.10‰, 2σ). Mn oxides are the main oxidant of Cr(III) in modern soils, but there is no evidence that they formed in the studied paleosols. Cr(VI) may have formed by direct oxidation of Cr(III) using molecular oxygen or H₂O₂, but neither pathway is as efficient as Mn oxides for producing Cr(VI). The picture that emerges from this and other studies of Cr isotope variation in ca. 1.9 Ga paleosols is of atmospheric oxygen concentrations that are high enough to oxidize iron, but too low to oxidize Mn, resulting in low Cr(VI) inventories in Earth surface environments.     

Solubility and dissolution of iron oxides

In most soils, FeIII oxides (group name) are the common source of Fe for plant nutrition. Since this Fe has to be supplied via solution, the solubility and the dissolution rate of the Fe oxides are essential for the Fe supply. Hydrolysis constants and solubility products (K_sp) describing the effect of pH on FeIII ion concentration in solution are available for the well-known Fe oxides occurring in soils such as goethite, hematite, ferrihydrite. K_sp values are usually extremely low ((Fe³⁺)·(OH)³=10^–37–10^–44). However, for each mineral type, K_sp may increase by several orders of magnitude with decreasing crystal size and it decreases with increasing Al substitution assuming ideal solid solution between the pure end-members. Based on such calculations a poorly crystalline goethite with a crystal size of 5 nm may well reach the solubility of ferrihydrite. The variations in K_sp are of relevance for soils because crystal size and Al substitution of soil Fe oxides vary considerably and can now be determined relatively easily.The concentration of Fe²⁺ in soil solutions is often much higher than that of Fe(III) ions. Therefore, redox potential strongly influences the activity of FeII. At a given pH and E_h, the activity of FeII is higher the higher K_sp of the FeIII oxide and thus also varies with the type of Fe oxide present.Besides the solubility, it is the dissolution rate which governs the supply of soluble Fe to the plant roots. Dissolution of Fe oxides takes place either by protonation, complexation or, most important, by reduction. Numerous dissolution rate studies with various FeIII oxides were conducted in strong mineral acids (protonation) and they have shown that besides the Fe oxide species, crystal size and/or crystal order and substitution are important determinative factors. For example, in soils, small amounts of a more highly soluble meta- or instable Fe oxide such as ferrihydrite with a large specific surface (several hundred m²g^–1) may be essential for the Fe supply to the plant root. Its higher dissolution rate can also be used to quantify its amount in soils. Ferrihydrite can be an important component in soils with high amounts of organic matter and/or active redox dynamics, whereas highly aerated and strongly weathered soils are usually very low in ferrihydrite. On the other hand, dissolution rates of goethites decrease as their Al substitution increases.Much less information exists on the rate of reductive and chelative dissolution of Fe oxides which generally simulate soil conditions better than dissolution by protonation. Here again, type of oxide, crystal size and substitution are important factors. Organic anions such as oxalate, which are adsorbed at the surface, may weaken the Fe³⁺-O bonds and thereby increase reductive dissolution. As often observed in weathering, the dissolution features of the crystals appear to follow zones of weakness in the crystal.     

Polymeric ions and polynuclear complexes of thio- and oxythiomolybdates

Evidence that thio- or oxythiomolybdates may be involved in the inhibitory action of Mo upon Cu metabolism is considered briefly. There are indications that structurally related polymeric derivatives rather than free monomeric (MoO_nS_4-n)^2- species may be involved. Claims that pH adjustment of an aqueous solution of (MoS₄)^2- yields the polymeric derivatives (Mo₂S₇)^2-, (Mo₄S₁₃)^2-, or (Mo₄S₁₅)^6- could not be substantiated. The composition and spectral characteristics of (NMe₄) derivatives of products formed within the pH range 4.8–7.3 indicated the persistence of (MoS₄)^2-. More acid solutions yielded MoS₃; traces of (MoOS₃)^2- formed in 2 M NaOH. Studies of the reaction between (MoS₄)^2- and Fe(III) salen confirmed the formation of a binuclear complex {(salen)Fe}Mo₂S₇{Fe(salen)} and indicated that the product retains a strong affinity for Cu. Analogous reactions with (MoOS₃)^2- or (MoO₂S₂)^2- also yielded binuclear Fe-complexes but which involved only the monomeric Mo-containing species as bridging ligand. Differences in infrared spectra and in the affinity of the products for Cu(II) indicated the preferential formation of thio- rather than oxo-bridged complexes. The physiological implications of these findings are considered.     

Investigation on the removal of Mo(VI) from Mo-Re containing wastewater by chemically modified persimmon residua

Persimmon waste was chemically modified by crosslinking with concentrated sulfuric acid to obtain a novel kind of adsorption gel, which was termed as crosslinked persimmon tannin (CPT), hereinafter. The adsorption behaviors of Mo(VI) with other coexisting metal ions onto the CPT gel were investigated. The gel exhibited selectivity only for Mo(VI) ions evidenced by the high value of separation factor of molybdenum and rhenium (β_Mo/Re = 164.37), and the adsorption mechanism of Mo(VI) as a multispecies was studied. The molybdenum adsorption behavior conforms to the Langmuir model with a remarkably high adsorption capacity of 0.56 mol/kg. A kinetic study for the adsorption of molybdenum at various temperatures confirmed that the endothermic adsorption process followed pseudo-second order kinetics. Moreover, its excellent adsorption properties and applicability for Mo(VI) were demonstrated by the removal and separation of Mo(VI) from different Mo-Re containing industrial wastewaters.     

Synthesis, structures, electrochemistry and properties of dioxo-molybdenum(VI) and -tungsten(VI) complexes with novel asymmetric N2OS, and partially symmetric N2S2, NOS2N-capped tripodal ligands

A new class of asymmetric N-capped (dianionic/trianionic) tripodal proligands [H_x(Lⁿ)] (x = 2, n = 1-6; x = 3, n = 7, 8) which possess pendant arms with N₂OS, N₂S₂ or NOS₂ donor groups and with different chelate ring sizes {5,5,5} or {5,6,5} has been prepared. Treatment of these ligands with [WO₂Cl₂(dme)] (dme = 1,2-dimethoxyethane) in the presence of base (triethylamine or KOH) leads to the formation of cis-dioxotungsten(VI) complexes of the types [WO₂(Lⁿ)] (n = 1-6) and K[WO₂(Lⁿ)] (n = 7, 8). Reaction of these tetradentate ligands with [MoO₂(acac)₂] (acac = acetylacetonate) gives the corresponding Mo(VI) analogues [MoO₂(Lⁿ)] (n = 1-6) and K[MoO₂(Lⁿ)] (n = 7, 8). Moreover, a new five coordinate dioxomolybdenum(VI) complex with an NS₂ tridentate ligand [MoO₂(L⁹)] has been synthesised using similar procedure. All these compounds have been spectroscopically characterised and the molecular structures of [MoO₂(Lⁿ)] (n = 2, 6) and [WO₂(L⁶)] have been established by X-ray diffraction analysis. The electrochemistry and the catalytic activity for oxidation of allylic and benzylic alcohols of these dioxo complexes have also been investigated.     

The influence of pH and chloride on the retention of cadmium,lead, mercury,and zinc by soils

The extent of contamination of soils by toxic heavy metals not only depends on the rate of loading of the metal but also on the nature of the adsorbing surfaces, the degree of alkalinity or acidity of the soil and the presence of aqueous complexant ligands. This work reports on the role of pH on the retention of Cd, Hg, Pb and Zn by two soils and on the influence of the chloride, Cl‐, ion on the chemical speciation and retention of the four metals. Batch adsorption experiments were conducted from pH 3 to 7 in the presence of either 0.1 M LiCl or LiClO₄. The results of the study showed that high concentrations of Cl^‐ ions can greatly decrease the retention of Hg and have an increasingly lesser effect on Cd, Pb and Zn retention. The effect of the Cl^‐ons was directly related to the metal‐Cl formation constants. The results of computer modeling of Cd and Hg retention by goethite and humic acid fractions indicated the relative importance of aqueous vs. surface complexation on metal retention. For organic surfaces, which do not form ternary surface complexes, the presence of aqueous complexant ligands should always decrease the adsorption of the metal. For mineral surfaces, which do form ternary surface complexes, there may be increased or decreased metal retention depending on the formation constant of the aqueous metal‐ligand species, the intrinsic complexation constants for the various binary and ternary complexes of the metal and the concentration of the complexant ligand. Thus for Hg, which forms very strong aqueous species with Cl^‐ ions, reduced adsorption on goethite was predicted in the presence of 0.1 M LiCl, while enhanced adsorption was predicted for Cd and Pb. The results suggest caution in the disposal of Cl‐containing wastes onto metal‐contaminated soils. The deleterious effects of Cl^‐ ion addition would be greatest for soils with relatively high organic matter contents and low contents of hydrous ferric oxides.     

Tetrathiomolybdate(VI) as an antidote in acute intoxication by copper(II) and other toxic metal ions

Tetrathiomolybdate(VI), MoS₄^2?, has been found to act as an effective antidote for acute copper(II) intoxication in mice. Both (NH₄)₂MoS₄ and Na₂MoS₄ were used for this purpose with approximately the same results. The sodium salt is less toxic (LD50 = 537 mg/kg, ip) than the ammonium salt (LD50 = 176mg/kg, ip). The sodium salt was found to be an effective antidote for acute intoxication for some divalent metal ions (Zn²⁺ and Ni²⁺), but not for others (Hg²⁺, Cd²⁺. The sodium salt was also ineffective as an antidote in acute intoxication by arsenic(III), antimony(III) and bismuth(III), and methylmercuric chloride.     

Solution structure of molybdic acid from Raman spectroscopy and DFT analysis

Protonation of produces the well-characterized polymolybdates, but at concentrations below 10⁻³ M the dominant species is monomeric molybdic acid, H₂MoO₄. It is likely to be the species adsorbed on manganese oxide, a process thought to control levels in the ocean, because of the strong proton dependence of adsorption. The molecular structure of H₂MoO₄ is elusive, since it occurs only in dilute solutions. Using 244 nm laser excitation, near resonance with O → Mo charge-transfer electronic transitions of H₂MoO₄, we have detected a 919 cm⁻¹ Raman band assignable to ν_sMoO. Using DFT, we have computed geometries and vibrational modes for the various structures consistent with the H₂MoO₄ formula. We tested the computations on a series of Mo(VI) oxo complexes with known vibrational frequencies, at several levels of theory. Best agreement with experimental values, at reasonable computational cost, was obtained with the B3LYP functional, employing a LANL2DZ ECP basis set for Mo and the 6-311+G(2df,p) basis set for O and H. Among the possible H₂MoO₄ structures only those based on the MoO₃ unit, with one, two or three coordinated water molecules, gave a scaled frequency for ν_sMoO that was within two standard deviations of 919 cm⁻¹. Best agreement was obtained for MoO₃(H₂O)₃. The MoO₂ and MoO structures gave frequencies that were too high. The Mo(OH)₆ structure could be excluded, because its vibrational frequencies shift down strongly upon H/D exchange, whereas the 919 cm⁻¹H₂MoO₄ band shifts up 1 cm⁻¹ in D₂O.     

A breakthrough column study for removal of malachite green using coco-peat

Abstract

A continuous adsorption study in a fixed bed column using coco-peat (CP) as an adsorbent was carried out for the removal of toxic malachite green (MG) from contaminated water. Fixed bed column studies were carried out to check field application viability. Various parameters like particle size, pH, concentration, dose and interference were exercised to optimize dye removal. Data obtained from breakthrough column studies were evaluated using Thomas and BDST model. Thomas rate constants K_t (0.22?ml min^?1 mg^?1) and adsorption capacity q_o (181.04?mg g^?1) were estimated and found to favor efficiency of CP. Thomas model was tested with several parameters like flow rate, concentration, and bed depth. Upon increase in input dye concentration, flow rate and bed height, adsorption coefficients increased. According to BDST model, maximum dye uptake of 468.26?mg/l was obtained with an input dye concentration of 5?mg/l. HYBRID and MPSD error functions were tested and found that Thomas model fits best. Dilute hydrochloric acid was found best for desorption. Real wastewater from textile industry was analyzed and confirmed the prospect of large-scale industrial application. In conclusion, coco-peat can be used as a promising bio-sorbent in column bed for scavenging of MG from contaminated water.     

Isolation of [{HB(Me2pz)3}MoO2(OC6H4S)]2, a binuclear Mo(VI) complex containing a disulfide bond

Aerial recrystallization of the mononuclear molybdenum(V) complex {HB(Me₂pz)₃}MoO(OC₆H₄-o-S) produced the novel binuclear Mo(VI) complex [{HB(Me₂pz)₃}MoO₂(OC₆H₄- o-S)]₂ which contains a disulfide bond. The dimer crystallizes as the dioxane solvate in the space group C2/c with cell parameters a=23.46(2), b=11.100(4), c=21.571(8) Å, β=104.23(4)°, Z=4. Final R_w=0.062 (2236 reflections with F_o>3σ(F_o), 301 variable parameters). The dimer contains two crystallographically identical distorted octahedral MoO₂²⁺ centers. One face of each octahedron is occupied by two oxo ligands and a phenolate oxygen atom; the opposite face is occupied by three nitrogens of the HB(Me₂pz)₃⁻ ligand. The two Mo(VI) centers of the dimers are linked by a disulfide bond formed upon oxidation of the 2-mercaptophenolate ligand of the original molybdenum(V) compound.     

Specific Surface Area Effect on Adsorption of Chlorpyrifos and TCP by Soils and Modelling

The adsorption of chlorpyrifos and TCP (3,5,6, trichloro-2-pyridinol) was determined in four soils (Mollisol, Inceptisol, Entisol, Alfisol) having different specific surface areas (19–84 m² g^?1) but rather similar organic matter content (2.4–3.5%). Adsorption isotherms were derived from batch equilibration experiments at 25°C. After liquid-liquid extraction, the chlorpyrifos and TCP concentrations in the solution phase were determined by gas chromatography with an electron capture detector. Adsorption coefficients were calculated using the Freundlich adsorption equation. High K_F coefficients for chlorpyrifos (15.78) and TCP (6.54) were determined for the Entisol soil, while low K_F coefficients for chlorpyrifos (5.32) and TCP (3.93) were observed in the Alfisol soil. In all four soils, adsorption of chlorpyrifos was higher than that of TCP. A surface complexation model, the constant capacitance model, was well able to fit the adsorption isotherms of both chlorpyrifos and TCP on all four soils. The results showed that specific surface area affected adsorption of both chlorpyrifos and TCP. Among the soil properties, specific surface area could be a better indicator than organic matter content alone for adsorption of chlorpyrifos and TCP by soils that contained low organic matter.     

Electron paramagnetic resonance properties and oxidation-reduction potentials of the molybdenum,flavin, and iron-sulfur centers of chicken liver xanthine dehydrogenase

The oxidation-reduction potentials of the various prosthetic groups in the native and desulfo forms of chicken liver xanthine dehydrogenase, determined by potentiometric titration in 0.05 m potassium phosphate buffer, pH 7.8, are: Mo(VI)/Mo(V) (native), ?357 mV; Mo(VI)/Mo(V) (desulfo), ?397 mV; Mo(V)/Mo(IV) (native), ?337 mV; Mo(V)/Mo(IV) (desulfo), ?433 mV; FAD/FADH · ?345 mV; FADH · FADH₂, ? 377 mV; (Fe/S)I_ox/(Fe/S)I_red, ?280 mV; (Fe/S)II_ox/(Fe/S)II_red, ? 275 mV. Titration at pH 6.8 revealed that the Mo and FAD centers but not the Fe/S centers are in prototropic equilibrium. Spectroscopic studies on the native and deflavinated enzymes show that environment of the flavin in xanthine dehydrogenase differs from that in bovine milk xanthine oxidase.