Iron redox species

Figure 4. Distribution of dissolved iron redox species for varying concentrations of reduced and oxidized iron, (a) Hornet effluent (b) Boulder Creek (c) Spring Creek (d) Spring Creek reservoir. Analyses for these four sarriples are...

The second part of the database contains reactions for the various secondary species, minerals, and gases. These reactions are balanced in terms of the basis and redox species, avoiding (to the extent practical) electron transfer. Species and minerals containing ferric iron, for example, are balanced in terms of the redox species Fe+++,... 

The intense blue color of the reaction mixture was assigned to the paramagnetic [Fe(HDMG)2(MeIM)(DTBSQ )]+ complex which is characterized by a broad spectral band at imax 680 nm and a distinct doublet with g = 2.00425 and cqn = 3.135 G in the visible and ESR spectra, respectively. This iron(II) species is not involved in a direct redox step and acts only as a reservoir for the semi-quinone radical. 

Faust, B. C., A Review of the Photochemical Redox Reactions of Iron(III) Species in Atmospheric, Oceanic, and Surface Waters Influences on Geochemical Cycles and Oxidant Formation, in Aquatic and Surface Photochemistry (G. Helz, R. Zepp, and D. Crosby, Eds.), Chap, f, pp. 3-37, Lewis, Boca Raton, FL, 1994b. 

Both stopped-flow and rapid freeze quench kinetic techniques show that the substrate reduces the flavin to its hydroquinone form at a rate faster than catalytic turnover Reoxidation of the flavin hydroquinone by the oxidized Fe4/S4 center leads to formation of a unique spin-coupled species at a rate which appears to be rate limiting in catalysis. Formation of this requires the substrate since dithionite reduction leads to flavin hydroquinone formation and a rhombic ESR spectrum typical of a reduced iron-sulfur protein . The appearance of such a spin-coupled flavin-iron sulfur species suggests the close proximity of the two redox centers and provides a valuable system for the study of flavin-iron sulfur interactions. The publication of further studies of this interesting system is looked forward to with great anticipation. 

This chapter discusses the chemical mechanisms influencing the fate of trace elements (arsenic, chromium, and zinc) in a small eutrophic lake with a seasonally anoxic hypolimnion (Lake Greifen). Arsenic and chromium are redox-sensitive trace elements that may be directly involved in redox cycles, whereas zinc is indirectly influenced by the redox conditions. We will illustrate how the seasonal cycles and the variations between oxic and anoxic conditions affect the concentrations and speciation of iron, manganese, arsenic, chromium, and zinc in the water column. The redox processes occurring in the anoxic hypolimnion are discussed in detail. Interactions between major redox species and trace elements are demonstrated. 

Spectrophotometric techniques combined with flow injection analysis (FIA) and on-line preconcentration can meet the required detection limits for natural Fe concentrations in aquatic systems (Table 7.2) by also using very specific and sensitive ligands, such as ferrozine [3-(2-bipyridyl)-5,6-bis(4-phenylsulfonic acid)-l,2,4-triazine], that selectively bind Fe(II). Determining Fe(II) as well as the total Fe after on-line reduction of Fe(III) to Fe(II) with ascorbic acid allows a kind of speciation.37 A drawback is that the selective complexing agents can shift the iron redox speciation in the sample. For example, several researchers have reported a tendency for ferrozine to reduce Fe(III) to Fe(II) under certain conditions.76 Most ferrozine methods involve sample acidification, which may also promote reduction of Fe(III) in the sample. Fe(II) is a transient species in most seawater environments and is rapidly oxidized to Fe(III) therefore, unacidified samples are required in order to maintain redox integrity.8 An alternative is to couple FIA with a chemiluminescence reaction.77-78... 

As alluded to above, input data for total iron, Fe(II) and/or Fe(III) are accepted by the model, with solute modeling calculations done using whatever data are input. If either Fe(II) or Fe(III) are present, Fe(total) is ignored if Fe(II) only is present, speciation is done among Fe(II) complexes only, and likewise for Fe(III). To accomplish this, the reactions of the iron section have been extensively rewritten (10) and a procedure, named SPLIT IRON, has been added, which performs the mass balance calculations separately for Fe(II) and Fe(III) when they are input separately. An E value is calculated from the computed activities of Fe " and Fe " and may, by user option, be used to distribute other redox species in lieu of an input E value. If only Fe(total) is input, the input E value is used to distribute all redox species including Fe " " and Fe " if there is only Fe(total) input, and no input E value, all Fe calculations are bypassed. 

Superoxide generated by xanthine oxidase or in the redox cycling of paraquat can cause the reductive release of F3 from ferritin, a process that is dependent on the activity of microsomal NADPH-cytochrome P-450 reductase [119]. Iron appears to be an essential component in the formation of reactive species such as superoxide and hydroxyl radical via redox cycling of cephaloridine. Addition of EDTA or of the specific iron chelator desferrioxamine to an incubation system containing renal cortex microsomes and cephaloridine depressed cephaloridine-induced peroxidation of microsomal lipids significantly EDTA showed a weaker effect than desferrioxamine at equimolar concentrations. By chelating F3 preferentially [120], desferrioxamine reduced the availability of F2 produced by the iron redox cycle and decreased cephaloridine-stimu-lated peroxidation of membrane lipids [36, 37]. 

Faust BC (1994) A review of the photochemical redox reactions of iron(III) species in atmospheric, oceanic, and surface waters Influences on geochemical cycles and oxidant formation. In Aquatic and Surface Photochemistry. RG Zepp, DG Crosby, GR Helz (eds) p 3-37. Boca Raton, Florida Lewis Publishers... 

The terminal oxidase in an energy-transducing, cytochrome-based electron-transport system maintains electron flow by coupling cytochrome oxidation to dioxygen (O2) reduction. Members of this protein class are referred to as cytochrome oxidases they carry out Oj-binding and redox chemistry at transition metal-containing active sites. Although iron is the most commonly used metal and may occur as a protoheme or iron-chlorin species in the protein, this section is concerned only with mitochondrial cytochrome oxidase, which contains 2 mol of Cn and 2 mol of heme a bound Fe per function unit. Biochemistry of the protein will not be considered here, instead the focus will be on the stmcture of the metal centers, on the reactions they catalyze and on models for these centers. 

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