Showing papers on "Ferric published in 2000"

An ecologically effective water treatment technique using electrochemically generated hydroxyl radicals for in situ destruction of organic pollutants: Application to herbicide 2,4-D

Abstract: The electrochemical production of Fenton’s reagent by simultaneous reduction of dioxygen and ferric ions on a carbon felt electrode, permits a controlled, in situ generation of hydroxyl (OH AE ) radicals. The possibility of using electrochemically produced OH AE radicals for solving environmental problems is investigated. Continuous and controlled production of hydroxyl radicals was achieved by electrochemical reduction of O2 in the presence of a catalytic amount of ferric or ferrous ion. These radicals are used for remediation of water containing toxic-persistent-bioaccumulative organic pollutants through their transformation into biodegradable compounds or through their mineralization into H2O and CO2. A widely used herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), was selected as a model for a toxic organic pollutant. High pressure liquid chromatography (HPLC) was used to quantify the distribution of the hydroxylated products obtained. Rate constants for the hydroxylation reactions of 2,4-D, 2,4-dichlorophenol (2,4-DCP), 2,4-dichlororesorcinol (2,4-DCR) and 4,6-dichlororesorcinol (4,6-DCR) were determined. The mineralization of 2,4-D and its derivatives was followed by total organic carbon (TOC) measurements. More than 95% of 2,4-D and the intermediates generated during the electrolysis can be mineralized.

Iron absorption and transport: An update

Abstract: Iron is vital for all living organisms. However, excess iron is hazardous because it produces free radical formation. Therefore, iron absorption is carefully regulated to maintain an equilibrium between absorption and body loss of iron. In countries where heme is a significant part of the diet, most body iron is derived from dietary heme iron because heme binds few of the luminal intestinal iron chelators that inhibit absorption of non-heme iron. Uptake of luminal heme into enterocytes occurs as a metalloporphyrin. Intracellularly, iron is released from heme by heme oxygenase so that iron leaves the enterocyte to enter the plasma as non-heme iron. Ferric iron is absorbed via a beta(3) integrin and mobilferrin (IMP) pathway that is not shared with other nutritional metals. Ferrous iron uptake is facilitated by DMT-1 (Nramp-2, DCT-1) in a pathway shared with manganese. Other proteins were recently described which are believed to play a role in iron absorption. SFT (Stimulator of Iron Transport) is postulated to facilitate both ferric and ferrous iron uptake, and Hephaestin is thought to be important in transfer of iron from enterocytes into the plasma. The iron concentration within enterocytes reflects the total body iron and either upregulates or satiates iron-binding sites on regulatory proteins. Enterocytes of hemochromatotics are iron-depleted similarly to the absorptive cells of iron-deficient subjects. Iron depletion, hemolysis, and hypoxia each can stimulate iron absorption. In non-intestinal cells most iron uptake occurs via either the classical clathrin-coated pathway utilizing transferrin receptors or the poorly defined transferrin receptor independent pathway. Non-intestinal cells possess the IMP and DMT-1 pathways though their role in the absence of iron overload is unclear. This suggests that these pathways have intracellular functions in addition to facilitating iron uptake.

Microbial iron transport via a siderophore shuttle: A membrane ion transport paradigm

Abstract: A mechanism of ion transport across membranes is reported. Microbial transport of Fe(3+) generally delivers iron, a growth-limiting nutrient, to cells via highly specific siderophore-mediated transport systems. In contrast, iron transport in the fresh water bacterium Aeromonas hydrophila is found to occur by means of an indiscriminant siderophore transport system composed of a single multifunctional receptor. It is shown that (i) the siderophore and Fe(3+) enter the bacterium together, (ii) a ligand exchange step occurs in the course of the transport, and (iii) a redox process is not involved in iron exchange. To the best of our knowledge, there have been no other reports of a ligand exchange mechanism in bacterial iron transport. The ligand exchange step occurs at the cell surface and involves the exchange of iron from a ferric siderophore to an iron-free siderophore already bound to the receptor. This ligand exchange mechanism is also found in Escherichia coli and seems likely to be widely distributed among microorganisms.

Electronic Structure of Activated Bleomycin: Oxygen Intermediates in Heme versus Non-Heme Iron

Abstract: Bleomycin (BLM) is a glycopeptide antibiotic produced by the fungus Streptomyces verticillus which is used clinically for anticancer therapy. It is most active as an iron complex. A detailed spectroscopic and theoretical study of the ferric form of the drug, Fe(III)BLM, and its activated form, ABLM, is reported. ABLM, which has been shown to be a ferric hydroperoxide complex, is the last detectable intermediate in the reaction cycle of BLM that leads to a DNA radical and subsequent cleavage. Both forms of the drug are low-spin Fe(III) complexes and were studied with electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), and absorption (ABS) spectroscopy. In addition, resonance Raman (rR) and circular dichroism (CD) spectra are reported for Fe(III)BLM. The g matrix was analyzed in detail, and Griffith's model for low-spin d5 complexes was experimentally evaluated. A ligand field analysis of the d−d region of the optical spectra resulted in a complete determination of the ligand field par...

Separate pathways for cellular uptake of ferric and ferrous iron

Abstract: Separate pathways for transport of nontransferrin ferric and ferrous iron into tissue cultured cells were demonstrated Neither the ferric nor ferrous pathway was shared with either zinc or copper

Regulation of Ferritin-Mediated Cytoplasmic Iron Storage by the Ferric Uptake Regulator Homolog (Fur) of Helicobacter pylori

Abstract: Homologs of the ferric uptake regulator Fur and the iron storage protein ferritin play a central role in maintaining iron homeostasis in bacteria. The gastric pathogen Helicobacter pylori contains an iron-induced prokaryotic ferritin (Pfr) which has been shown to be involved in protection against metal toxicity and a Fur homolog which has not been functionally characterized in H. pylori. Analysis of an isogenic fur-negative mutant revealed that H. pylori Fur is required for metal-dependent regulation of ferritin. Iron starvation, as well as medium supplementation with nickel, zinc, copper, and manganese at nontoxic concentrations, repressed synthesis of ferritin in the wild-type strain but not in the H. pylori fur mutant. Fur-mediated regulation of ferritin synthesis occurs at the mRNA level. With respect to the regulation of ferritin expression, Fur behaves like a global metal-dependent repressor which is activated under iron-restricted conditions but also responds to different metals. Downregulation of ferritin expression by Fur might secure the availability of free iron in the cytoplasm, especially if iron is scarce or titrated out by other metals.

Identification of active sites and adsorption complexes in Fe/MFI catalysts for NOx reduction

Abstract: FTIR, extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES) and ESR were used to identify the active sites and the adsorption complexes present in a variety of Fe/MFI samples prepared in different ways and displaying vastly different activities and selectivities in the reduction of NOx to N2 with hydrocarbons. Iron oxide particles, charged ferric oxide nano-clusters, isolated iron ions and oxygen-bridged binuclear iron ions have been identified with various degrees of reliability. In contact with appropriate gases, NO+ ions, mono- and dinitrosyl groups, nitro groups, nitrate ions and superoxide ions have been identified. Peroxide ions, though not detectable with the methods used here, were postulated by other authors on the basis of density functional calculations. The binuclear iron oxo-ions are more abundant in the best catalysts with high Fe/Al ratio that were prepared by sublimation. They are the most probable sites for NO oxidation to NO2 and its further conversion to adsorbed nitro groups and nitrate ions, steps that are crucial for NOx reduction. Superoxide and/or peroxide ions are likely involved in the NO oxidation to NO2. This process is fast when Fe/MFI is exposed to a mixture of NO+O2, but much slower if NO is chemisorbed first, before exposure to O2.

Microbial Oxidation of Fe(II) and Mn(II) at Circumneutral pH

Abstract: Over the past decade it has become clear that iron and manganese can be important electron acceptors for anaerobic respiration carried out by a diverse array of prokaryotes. This chapter is biased toward iron, in part because a number of recent findings concerning the role of microbes in iron (Fe) oxidation make this area of particular interest at the organismal level and in part because an excellent review on manganese (Mn) oxidation was recently published. It is also biased toward organisms rather than molecules, since virtually nothing is know about the molecules, involved in Fe oxidation at neutral pH, although there is an emerging story in this regard concerning Mn. In terms of biological reactivity, the two most relevant oxidation states of iron are Fe(II), the reduced ferrous form, and Fe(III), the oxidized ferric form. To illustrate the commonalities and differences that are manifested by the sites, four quite different examples are discussed. The examples are Marselisborg, Loihi, Plant Rhizosphere, and Anaerobic Environments. Representatives of some of the Fe oxidizers that are known to occur in the habitats are discussed in detail in this chapter. The oxidation of the manganous ion, Mn(II), to the manganic form, Mn(IV), is a two-electron transfer, which can proceed via one-electron steps through an unstable intermediate, Mn(III). Examples of three quite different Mn-oxidizing organisms are discussed in detail. The three Mn-oxidizing organisms are L. discophora, Marine Bacillus sp. strain SG-1, and Metallogenium.

Intrinsic Barriers for Electron and Hydrogen Atom Transfer Reactions of Biomimetic Iron Complexes

Abstract: Self-exchange reactions between high-spin iron complexes of 2,2'-bi-imidazoline (H2bim) have been investigated by the dynamic NMR line-broadening technique. Addition of the ferric complex (Fe III (H2bim)3) 3+ causes broadening of the 1 H NMR resonances of the ferrous analogue, (Fe II (H2bim)3) 2+ . This indicates electron self-exchange with ke - ) (1.7 ( 0.2) 10 4 M -1 s -1 at 298 K in MeCN-d3 (I ) 0.1 M). Similar broadening is observed when the deprotonated ferric complex (Fe III (Hbim)(H2bim)2) 2+ is added to (Fe II (H2bim)3) 2+ . Because these reactants differ by a proton and an electron, this is a net hydrogen atom exchange reaction. Kinetic and thermodynamic results preclude stepwise mechanisms of sequential proton and then electron transfer, or electron and then proton transfer. Concomitant electron and proton (H ¥ ) transfer occurs with bimolecular rate constant kH ¥ ) (5.8 ( 0.6) 10 3 M -1 s -1 . This is a factor of 3 smaller than ke - under the same conditions. The H-atom exchange reaction exhibits a primary kinetic isotope effect kNH/kND ) 2.3 ( 0.3 at 324 K, whereas no such effect is detected in the electron exchange reaction. Proton self-exchange between the two ferric complexes, (Fe III (Hbim)(H2bim)2) 2+ and (Fe III (H2bim)3) 3+ , has also been investigated and is found to be faster than both the electron and H-atom transfer reactions. From kinetic analyses and the application of simple Marcus theory, an order of intrinsic reaction barriers IH ¥ > Ie - > IH + is derived. The reorganization energies are discussed in terms of their inner-sphere and outer-sphere components.

Reductive Dissolution of Ferric Iron Minerals by Acidiphilium SJH

Abstract: The acidophilic heterotrophic eubacterium Acidiphilium SJH was shown to catalyze the reductive dissolution of a wide range of ferric iron?containing minerals (akageneite, goethite, jarosite, natrojarosite, and amorphous ferric hydroxide) and of the mixed ferrous/ferric mineral magnetite The specific rates of dissolution varied with the structural stabilities of the minerals, such that amorphous ferric hydroxide was the most rapid and jarosite and akageneite were the slowest of the minerals tested The reductive dissolution of both amorphous ferric hydroxide and magnetite was faster in pH 20 than in higher pH (28?30) cultures, even though Acidiphilium SJH has a pH optimum close to pH 3 Contact between bacteria and ferric mineral was not necessary for reductive dissolution to occur Adding EDTA or diethylenetriamine pentaacetic acid to bacterial cultures accelerated the solubilization of goethite and amorphous ferric hydroxide Although cell-free spent media and heat-killed Acidiphilium SJH also appear

Interleukin-8 levels in human lung epithelial cells are increased in response to coal fly ash and vary with the bioavailability of iron, as a function of particle size and source of coal.

Abstract: Particulate air pollution contains iron, and some of the pathological effects after inhalation may be due to radical species produced by iron-catalyzed reactions. We tested the hypothesis that iron present in coal fly ash (CFA) could induce the expression and synthesis of the inflammatory cytokine interleukin-8 (IL-8). CFA, containing as much as 14% iron, was used as a model combustion source particle. Three coal types were used to generate three size fractions enriched in particles [submicron ( 10 micrometer. Treatment of human lung epithelial (A549) cells for 4 h with CFA from Utah enriched in <1 micrometer particles (20 microgram/cm(2)) resulted in a 2.6-fold increase in mRNA levels for IL-8. IL-8 levels were increased in the medium by as much as 8-fold when cells were treated with the fraction enriched in the smallest size Utah CFA for 24 h. IL-8 production was completely inhibited when the CFA was pretreated with the metal chelator desferrioxamine B, suggesting that a transition metal was responsible for the induction, probably iron. Treatment with a soluble form of iron, ferric ammonium citrate (FAC), mimicked the IL-8 level increase observed with CFA. There was a direct relationship, above a threshold level of bioavailable iron, between the levels of IL-8 and bioavailable iron in A549 cells treated with CFA or FAC. Further, the relationship between IL-8 and bioavailable iron for CFA was indistinguishable from that for FAC. These results strongly suggest that iron can induce IL-8 in A549 cells and that iron was the likely component of CFA that induced IL-8. CFA-induced IL-8 production was inhibited by tetramethylthiourea or dimethyl sulfoxide, suggesting that radical species were involved in the induction. These results demonstrate that iron present in CFA may be responsible for production and release of inflammatory mediators by the lung epithelium through generation of radical species and suggest that iron may contribute to the exacerbation of respiratory problems by particulate air pollution.

Inhibition of bacterially promoted uranium reduction: ferric (Hydr)oxides as competitive electron acceptors.

Abstract: The reduction of uranyl (U(VI)) to the relatively insoluble tetravalent form (U(IV)) by Shewanella alga (BrY), a dissimilatory metal-reducing bacteria, was studied in the presence of environmentally relevant iron hydrous oxides. Because this process is dependent on U(VI) being used as the terminal electron acceptor (TEA) during anaerobic respiration, it is important to understand how other potential TEAs might affect this process. When cell suspensions of BrY were added to uranyl acetate (400 μM), uranyl was removed from solution within 10 h. Similarly, uranyl in the presence on goethite (11.1 μmol of U/m2 of solid) underwent dramatic reduction (>90%) with active BrY cells. In contrast, when ferrihydrite was available (0.67 μmol of U/m2 of solid) only 48% of the initial U(VI) was removed after 10 h. When varying ratios of goethite and ferrihydrite were incorporated into cell suspensions, the extent of uranyl reduction was inversely related to the fraction of ferrihydrite present. Increasing uranyl concent...

The iron oxidation and hydrolysis chemistry of Escherichia coli bacterioferritin.

Abstract: Bacterioferritins are members of a class of spherical shell-like iron storage proteins that catalyze the oxidation and hydrolysis of iron at specific sites inside the protein shell, resulting in formation of a mineral core of hydrated ferric oxide within the protein cavity. Electrode oximetry/pH stat was used to study iron oxidation and hydrolysis chemistry in E. coli bacterioferritin. Consistent with previous UV-visible absorbance measurements, three distinct kinetic phases were detected, and the stoichiometric equations corresponding to each have been determined. The rapid phase 1 reaction corresponds to pairwise binding of 2 Fe(2+) ions at a dinuclear site, called the ferroxidase site, located within each of the 24 subunits, viz., 2Fe(2+) + P(Z) --> [Fe(2)-P](Z) + 4H(+), where P(Z) is the apoprotein of net charge Z and [Fe(2)-P](Z) represents a diferrous ferroxidase complex. The slower phase 2 reaction corresponds to the oxidation of this complex by molecular oxygen according to the net equation: [Fe(2)-P](Z) + (1)/(2)O(2) --> [Fe(2)O-P](Z) where [Fe(2)O-P](Z) represents an oxidized diferric ferroxidase complex, probably a mu-oxo-bridged species as suggested by UV-visible and EPR spectrometric titration data. The third phase corresponds to mineral core formation according to the net reaction: 4Fe(2+) + O(2) + 6H(2)O --> 4FeO(OH)((core)) + 8H(+). Iron oxidation is inhibited by the presence of Zn(2+) ions. The patterns of phase 2 and phase 3 inhibition are different, though inhibition of both phases is complete at 48 Zn(2+)per 24mer, i.e., 2 Zn(2+) per ferroxidase center.

Catalase-peroxidase (Mycobacterium tuberculosis KatG) catalysis and isoniazid activation.

Abstract: Resonance Raman spectra of native, overexpressed M. tuberculosis catalase-peroxidase (KatG), the enzyme responsible for activation of the antituberculosis antibiotic isoniazid (isonicotinic acid hydrazide), have confirmed that the heme iron in the resting (ferric) enzyme is high-spin five-coordinate. Difference Raman spectra did not reveal a change in coordination number upon binding of isoniazid to KatG. Stopped-flow spectrophotometric studies of the reaction of KatG with stoichiometric equivalents or small excesses of hydrogen peroxide revealed only the optical spectrum of the ferric enzyme with no hypervalent iron intermediates detected. Large excesses of hydrogen peroxide generated oxyferrous KatG, which was unstable and rapidly decayed to the ferric enzyme. Formation of a pseudo-stable intermediate sharing optical characteristics with the porphyrin pi-cation radical-ferryl iron species (Compound I) of horseradish peroxidase was observed upon reaction of KatG with excess 3-chloroperoxybenzoic acid, peroxyacetic acid, or tert-butylhydroperoxide (apparent second-order rate constants of 3.1 x 10(4), 1.2 x 10(4), and 25 M(-1) s(-1), respectively). Identification of the intermediate as KatG Compound I was confirmed using low-temperature electron paramagnetic resonance spectroscopy. Isoniazid, as well as ascorbate and potassium ferrocyanide, reduced KatG Compound I to the ferric enzyme without detectable formation of Compound II in stopped-flow measurements. This result differed from the reaction of horseradish peroxidase Compound I with isoniazid, during which Compound II was stably generated. These results demonstrate important mechanistic differences between a bacterial catalase-peroxidase and the homologous plant peroxidases and yeast cytochrome c peroxidase, in its reactions with peroxides as well as substrates.

Iron Compounds as Coatings on Polystyrene Latex and as Hollow Spheres.

Abstract: Sub-micrometer-sized anionic polystyrene latices have been coated with uniform layers of iron compounds by aging, at elevated temperature, dispersions of the polymer colloid in the presence of aqueous solutions of ferric chloride, urea, hydrochloric acid, and polyvinylpyrrolidone. The thickness of the deposited layers could be altered by suitable adjustment of the reactant concentrations, and they could also be increased by further aging of the coated particles in the presence of aqueous solutions of ferric chloride. Hollow colloidal spheres of iron compounds were obtained by calcination of the so-coated polystyrene latices at elevated temperature in air. Different chemical compositions of hollow colloidal spheres were obtained by heating them in hydrogen.