Iron oxide

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Electrochemically oxidized iron (rust) Almindeligt rust - jernoxid.jpg
Electrochemically oxidized iron (rust)

Iron oxides are chemical compounds composed of iron and oxygen. There are sixteen known iron oxides and oxyhydroxides, the best known of which is rust, a form of iron(III) oxide. [1]

Contents

Iron oxides and oxyhydroxides are widespread in nature and play an important role in many geological and biological processes. They are used as iron ores, pigments, catalysts, and in thermite, and occur in hemoglobin. Iron oxides are inexpensive and durable pigments in paints, coatings and colored concretes. Colors commonly available are in the "earthy" end of the yellow/orange/red/brown/black range. When used as a food coloring, it has E number E172.

Oxides

Iron oxide pigment. The brown color indicates that iron is at the oxidation state +3. IronOxidePigmentUSGOV.jpg
Iron oxide pigment. The brown color indicates that iron is at the oxidation state +3.
Green and reddish brown stains on a limestone core sample, respectively corresponding to oxides/hydroxides of Fe and Fe . Red and green iron oxides.jpg
Green and reddish brown stains on a limestone core sample, respectively corresponding to oxides/hydroxides of Fe and Fe .

Hydroxides

Thermal expansion

Iron oxideCTE (× 10−6 °C−1)
Fe2O314.9 [7]
Fe3O4>9.2 [7]
FeO12.1 [7]

Oxide-hydroxides

Microbial degradation

Several species of bacteria, including Shewanella oneidensis, Geobacter sulfurreducens and Geobacter metallireducens, metabolically utilize solid iron oxides as a terminal electron acceptor, reducing Fe(III) oxides to Fe(II) containing oxides. [11]

Environmental effects

Methanogenesis replacement by iron oxide reduction

Under conditions favoring iron reduction, the process of iron oxide reduction can replace at least 80% of methane production occurring by methanogenesis. [12] This phenomenon occurs in a nitrogen-containing (N2) environment with low sulfate concentrations. Methanogenesis, an Archaean driven process, is typically the predominant form of carbon mineralization in sediments at the bottom of the ocean. Methanogenesis completes the decomposition of organic matter to methane (CH4). [12] The specific electron donor for iron oxide reduction in this situation is still under debate, but the two potential candidates include either titanium (III) or compounds present in yeast. The predicted reactions with titanium (III) serving as the electron donor and phenazine-1-carboxylate (PCA) serving as an electron shuttle is as follows:

Ti(III)-cit + CO2 + 8H+ → CH4 + 2H2O + Ti(IV) + cit                           ΔE = 240 + 300 mV
Ti(III)-cit + PCA (oxidized) → PCA (reduced) + Ti(IV) + cit                ΔE = 116 + 300 mV
PCA (reduced) + Fe(OH)3 → Fe2+ + PCA (oxidized)                         ΔE = 50 + 116 mV [12]

Titanium (III) is oxidized to titanium (IV) while PCA is reduced. The reduced form of PCA can then reduce the iron hydroxide (Fe(OH)3).

Hydroxyl radical formation

On the other hand when airborne, iron oxides have been shown to harm the lung tissues of living organisms by the formation of hydroxyl radicals, leading to the creation of alkyl radicals. The following reactions occur when Fe2O3 and FeO, hereafter represented as Fe3+ and Fe2+ respectively, iron oxide particulates accumulate in the lungs. [13]

O2 + e → O2 [13]

The formation of the superoxide anion (O2) is catalyzed by a transmembrane enzyme called NADPH oxidase. The enzyme facilitates the transport of an electron across the plasma membrane from cytosolic NADPH to extracellular oxygen (O2) to produce O2. NADPH and FAD are bound to cytoplasmic binding sites on the enzyme. Two electrons from NADPH are transported to FAD which reduces it to FADH2. Then, one electron moves to one of two heme groups in the enzyme within the plane of the membrane. The second electron pushes the first electron to the second heme group so that it can associate with the first heme group. For the transfer to occur, the second heme must be bound to extracellular oxygen which is the acceptor of the electron. This enzyme can also be located within the membranes of intracellular organelles allowing the formation of O2 to occur within organelles. [14]

2O2 + 2 H+ → H2O2 + O2 [13] [15]

The formation of hydrogen peroxide (H
2
O
2
) can occur spontaneously when the environment has a lower pH especially at pH 7.4. [15] The enzyme superoxide dismutase can also catalyze this reaction. Once H
2
O
2
has been synthesized, it can diffuse through membranes to travel within and outside the cell due to its nonpolar nature. [14]

Fe2+ + H
2
O
2
→ Fe3+ + HO + OH
Fe3+ + H2O2 → Fe2+ + O2 + 2H+
H2O2 + O2 → HO + OH + O2 [13]

Fe2+ is oxidized to Fe3+ when it donates an electron to H2O2, thus, reducing H2O2 and forming a hydroxyl radical (HO) in the process. H2O2 can then reduce Fe3+ to Fe2+ by donating an electron to it to create O2. O2 can then be used to make more H2O2 by the process previously shown perpetuating the cycle, or it can react with H2O2 to form more hydroxyl radicals. Hydroxyl radicals have been shown to increase cellular oxidative stress and attack cell membranes as well as the cell genomes. [13]

HO + RH → R + H2O [13]

The HO radical produced from the above reactions with iron can abstract a hydrogen atom (H) from molecules containing an R-H bond where the R is a group attached to the rest of the molecule, in this case H, at a carbon (C). [13]

See also

Related Research Articles

Cytochrome Redox-active proteins containing a heme with a Fe atom as a cofactor

Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of binding. Four varieties are recognized by the International Union of Biochemistry and Molecular Biology (IUBMB), cytochromes a, cytochromes b, cytochromes c and cytochrome d. Cytochrome function is linked to the reversible redox change from ferrous to the ferric oxidation state of the iron found in the heme core. In addition to the classification by the IUBMB into four cytochrome classes, several additional classifications such as cytochrome o and cytochrome P450 can be found in biochemical literature.

Electron transport chain Process in which a series of electron carriers operate together to transfer electrons from donors to any of several different terminal electron acceptors to generate a transmembrane electrochemical gradient.

An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. Many of the enzymes in the electron transport chain are membrane-bound.

A superoxide is a compound that contains the superoxide ion, which has the chemical formula O
2
. The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature. Molecular oxygen (dioxygen) is a diradical containing two unpaired electrons, and superoxide results from the addition of an electron which fills one of the two degenerate molecular orbitals, leaving a charged ionic species with a single unpaired electron and a net negative charge of −1. Both dioxygen and the superoxide anion are free radicals that exhibit paramagnetism. Superoxide was historically also known as "hyperoxide".

Coenzyme Q – cytochrome c reductase Class of enzymes

The coenzyme Q : cytochrome c – oxidoreductase, sometimes called the cytochrome bc1 complex, and at other times complex III, is the third complex in the electron transport chain, playing a critical role in biochemical generation of ATP. Complex III is a multisubunit transmembrane protein encoded by both the mitochondrial and the nuclear genomes. Complex III is present in the mitochondria of all animals and all aerobic eukaryotes and the inner membranes of most eubacteria. Mutations in Complex III cause exercise intolerance as well as multisystem disorders. The bc1 complex contains 11 subunits, 3 respiratory subunits, 2 core proteins and 6 low-molecular weight proteins.

Heme Chemical coordination complex of an iron ion chelated to a porphyrin

Heme, or haem, is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver.

Hemerythrin InterPro Family

Hemerythrin (also spelled haemerythrin; Ancient Greek: αἷμα, romanized: haîma, lit. 'blood', Ancient Greek: ἐρυθρός, romanized: erythrós, lit. 'red') is an oligomeric protein responsible for oxygen (O2) transport in the marine invertebrate phyla of sipunculids, priapulids, brachiopods, and in a single annelid worm genus, Magelona. Myohemerythrin is a monomeric O2-binding protein found in the muscles of marine invertebrates. Hemerythrin and myohemerythrin are essentially colorless when deoxygenated, but turn a violet-pink in the oxygenated state.

Nitric oxide synthase Enzyme catalysing the formation of the gasotransmitter NO(nitric oxide)

Nitric oxide synthases (NOSs) are a family of enzymes catalyzing the production of nitric oxide (NO) from L-arginine. NO is an important cellular signaling molecule. It helps modulate vascular tone, insulin secretion, airway tone, and peristalsis, and is involved in angiogenesis and neural development. It may function as a retrograde neurotransmitter. Nitric oxide is mediated in mammals by the calcium-calmodulin controlled isoenzymes eNOS and nNOS. The inducible isoform, iNOS, involved in immune response, binds calmodulin at physiologically relevant concentrations, and produces NO as an immune defense mechanism, as NO is a free radical with an unpaired electron. It is the proximate cause of septic shock and may function in autoimmune disease.

Ferredoxins are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied to the "iron protein" first purified in 1962 by Mortenson, Valentine, and Carnahan from the anaerobic bacterium Clostridium pasteurianum.

Photocatalysis

In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalyzed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs, which generate free radicals (e.g. hydroxyl radicals: •OH) able to undergo secondary reactions. Its practical application was made possible by the discovery of water electrolysis by means of titanium dioxide (TiO2).

Heme oxygenase

Heme oxygenase, or haem oxygenase, is an enzyme that catalyzes the degradation of heme to produce biliverdin, ferrous ion, and carbon monoxide.

In enzymology, a manganese peroxidase (EC 1.11.1.13) is an enzyme that catalyzes the chemical reaction

Hydroxylamine oxidoreductase (HAO) is an enzyme found in the prokaryote Nitrosomonas europaea. It plays a critically important role in the biogeochemical nitrogen cycle as part of the metabolism of ammonia-oxidizing bacteria.

Nitric oxide reductase, an enzyme, catalyzes the reduction of nitric oxide (NO) to nitrous oxide (N2O). The enzyme participates in nitrogen metabolism and in the microbial defense against nitric oxide toxicity. The catalyzed reaction may be dependent on different participating small molecules: Cytochrome c (EC: 1.7.2.5, Nitric oxide reductase (cytochrome c)), NADPH (EC:1.7.1.14), or Menaquinone (EC:1.7.5.2).

Thiosulfate dehydrogenase

Thiosulfate dehydrogenase is an enzyme that catalyzes the chemical reaction:

Dioxygenase

Dioxygenases are oxidoreductase enzymes. Aerobic life, from simple single-celled bacteria species to complex eukaryotic organisms, has evolved to depend on the oxidizing power of dioxygen in various metabolic pathways. From energetic adenosine triphosphate (ATP) generation to xenobiotic degradation, the use of dioxygen as a biological oxidant is widespread and varied in the exact mechanism of its use. Enzymes employ many different schemes to use dioxygen, and this largely depends on the substrate and reaction at hand.

Haem peroxidases (or heme peroxidases) are haem-containing enzymes that use hydrogen peroxide as the electron acceptor to catalyse a number of oxidative reactions. Most haem peroxidases follow the reaction scheme:

Plastid terminal oxidase or plastoquinol terminal oxidase (PTOX) is an enzyme that resides on the thylakoid membranes of plant and algae chloroplasts and on the membranes of cyanobacteria. The enzyme was hypothesized to exist as a photosynthetic oxidase in 1982 and was verified by sequence similarity to the mitochondrial alternative oxidase (AOX). The two oxidases evolved from a common ancestral protein in prokaryotes, and they are so functionally and structurally similar that a thylakoid-localized AOX can restore the function of a PTOX knockout.

Hill reaction

The Hill reaction is the light-driven transfer of electrons from water to Hill reagents in a direction against the chemical potential gradient as part of photosynthesis. Robin Hill discovered the reaction in 1937. He demonstrated that the process by which plants produce oxygen is separate from the process that converts carbon dioxide to sugars.

Julia A. Kovacs is an American chemist specializing in bioinorganic chemistry. She is Professor of Chemistry at the University of Washington. Her research involves synthesizing small-molecule mimics of the active sites of metalloproteins, in order to investigate how cysteinates influence the function of non-heme iron enzymes, and the mechanism of the oxygen-evolving complex (OEC).

Transition metal nitrite complex

Transition metal complexes of nitrite describes families of coordination complexes containing one or more nitrite (NO2) ligands. Although the synthetic derivatives are only of scholarly interest, metal-nitrite complexes occur in several enzymes that participate in the nitrogen cycle.

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