Ferrous iron reductant

Uranium is subsequendy stripped reductively from the loaded solvent using a bleed stream of the raffinate acid to which ferrous iron has been added to reduce uranium to its nonextractable, quadravalent state. Raffinate is acid from which uranium has already been extracted. By controlling the organic-to-aqueous volume phase ratios in the extraction and stripping circuits, uranium is concentrated by a factor of approximately 70. 

Iron Absorption. A very important effect of ascorbic acid is the enhancement of absorption of nonheme iron from foods. Ascorbic acid also enhances the reduction of ferric iron to ferrous iron. This is important both in increasing iron absorption and in its function in many hydroxylation reactions (140,141). In addition, ascorbic acid is involved in iron metaboHsm. It serves to transfer iron to the Hver and to incorporate it into ferritin. 

Reduction. Many dyes, particularly azo dyestuffs, are susceptible to destmctive reduction. The reducing agents that can be used are sodium hydrosulfite, thiourea dioxide, sodium borohydtide, zinc sulfoxylate, and ferrous iron. 

Enterocytes in the proximal duodenum are responsible for absorption of iron. Incoming iron in the Fe " state is reduced to Fe " by a ferrireductase present on the surface of enterocytes. Vitamin C in food also favors reduction of ferric iron to ferrous iron. The transfer of iron from the apical surfaces of enterocytes into their interiors is performed by a proton-coupled divalent metal transporter (DMTl). This protein is not specific for iron, as it can transport a wide variety of divalent cations. 

The quadrupole doublet has an isomer shift corresponding to iron in the ferric or Fe " state. After reduction in H2 at 675 K the catalyst consists mainly of metallic iron, as evidenced by the sextet, along with some unreduced iron, which gives rise to two doublet contributions of Fe " and Fe " in the centre. The overall degree of iron reduction, as reflected by the relative area under the bcc ion sextet, is high. Fischer-Tropsch synthesis at 575 K in CO and FI2 converts the metallic iron into the Flagg carbide, Fe5C2. The unreduced iron is mainly present as Fe ". Exposure of the carburized catalyst to the air at room temperature leaves most of the carbide phase unaltered but oxidizes the ferrous to ferric iron. 

These different systems come into operation under different conditions both environmental and in terms of growth requirements. As we will see later in this chapter, yeasts do not appear to have a mechanism for iron excretion, so that their cellular iron homeostasis, as in E. coli, relies on tight control of uptake and eventually storage. As we will see when we examine these iron uptake systems in detail, most of them require ferrous iron, rather than ferric. This implies that the first step required for iron transport is the reduction of Fe3+ to Fe2+ by membrane-bound reductases. 

Figure 8.3 A model of iron transport across the intestine. Reduction of ferric complexes to the ferrous form is achieved by the action of the brush border ferric reductase. The ferrous form is transported across the brush border membrane by the proton-coupled divalent cation transporter (DCT1) where it enters an unknown compartment in the cytosol. Ferrous iron is then transported across the basolateral membrane by IREG1, where the membrane-bound copper oxidase hephaestin (Hp) promotes release and binding of Fe3+ to circulating apotransferrin. Except for hephaestin the number of transmembrane domains for each protein is not shown in full. Reprinted from McKie et al., 2000. Copyright (2000), with permission from Elsevier Science.

It therefore appears that for at least domains 4 and 6, hCP possesses metal binding sites in addition to the six integral copper atoms. Binding of ferrous iron at these sites could result in its oxidation and its removal from the plasma or in the reduction of oxygen-based free radicals. Binding of copper could result in its transport through the plasma (vide infra). 

Liger et al. (1999), for example, studied the reduction of uranyl (UVI) by dissolved ferrous iron,... 

Reaction in the simulation begins slowly, but proceeds more rapidly as biomass accumulates, reflecting the reaction s autocatalytic nature. The reaction rate continues to increase until most of the acetate is consumed, at which point it slows abruptly to a near stop. In the simulation, dissolved ferrous iron is present in excess amount. The bisulfide produced as a result of bacterial sulfate reduction reacts with the iron,... 

The P-cluster, located at the interface of MoFe-protein s a- and (3-subunits, is believed to function as the electron transfer mediator between Fe-protein and the N2 reduction site at the M center. The P-cluster is contained within a hydrophobic environment and located approximately 10 A below the MoFe-protein surface. Three cysteine side chains from each subunit bind to iron ions in the P-cluster. The cluster is now known to exist in Pox and PN forms in active enzyme, both with stoichiometry FegS7. The PN form, with its octahedrally coordinated central sulfur, has the structure shown in Figure 6.6. As can be seen in Table 6.3, the PN form contains all ferrous irons, corresponding to the P (5 = 0) state, whereas the Pox form corresponds to the P2+ (5=3 or 4) form. 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

Dissolved iron(III) is (i) an intermediate of the oxidative hydrolysis of Fe(II), and (ii) results from the thermal non-reductive dissolution of iron(III)(hydr)oxides, a reaction that is catalyzed by iron(II) as discussed in Chapter 9. Hence, iron(II) formation in the photic zone may occur as an autocatalytic process (see Chapter 10.4). This is also true for the oxidation of iron(II). As has been discussed in Chapter 9.4, the oxidation of iron(II) by oxygen is greatly enhanced if the ferrous iron is adsorbed at a mineral (or biological) surface. Since mineral surfaces are formed via the oxidative hydrolysis of Fe(II), this reaction proceeds as an autocatalytic process (Sung and Morgan, 1980). Both the rate of photochemical iron(II) formation and the rate of oxidation of iron(II) are strongly pH-dependent the latter increases with... 

The anaerobic reduction of metal ions, including ferric iron (Fe(III)) to ferrous iron (Fe(II)) by microorganisms, has been observed for >80 years (Harders 1919) and was reviewed several times from various viewpoints (Jones 1986 Lovley 1991, 1995 Nealson and Saffarini 1995). In general terms the process can be described by ... 

When a spadeful of wet, anaerobic soil is brought to the surface and allowed to dry, air enters through drying cracks and the soil tends to become uniformly oxidized and turn a uniform brown. Whereas when oxidation occurs without drying—as, for example, near a root releasing O2 into wet soil—it is far less uniform and reddish-brown ferric oxide deposits form on and near the oxidizing source. The difference depends on the relative rates of movement of O2 into the soil and of ferrous iron and other reductants in the opposite direction, and the rates of reaction. 

This generalization has some qualifications. Recently, nitrile hydratase was reported to be a low-spin Fe " enzyme. Demonstration of the spin state is incomplete at this time and the low-spin claim for the enzyme may be revised. Also, the binding of NO to high-spin ferrous iron (S = 2) results in a one-electron reduction and intermediate spin (5 = i) ferric iron. Finally, low-spin ferric iron has been observed for cyano 3,4-protochatechuate dioxygenase (Whittaker and Lipscomb, 1984). 

Proposed model for oxygen activation in cytochrome P450. Reduction by a single electron enables the ferrous iron to bind oxygen. Addition of a second electron generates an iron peroxide heme, which then cleaves to form water and an electrophilic perferryl species. 

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