Showing papers on "Redox published in 2001"

The Biological Chemistry of the Elements: The Inorganic Chemistry of Life

Abstract: Part 1 The chemical and physical factors controlling the elements of life: the chemical elements in biology the principles of the chemical uptake and speciation of the elements in biology physical separation of the elements - compartments and zones in biology kinetic considerations of element reactions energy in biological systems and hydrogen biochemistry the functional value of the chemical elements in biological systems. Part 2 The roles of individual elements in biology: the role of biological macromolecules and polymers sodium and potassium - osmotic control, electrolyte equilibria and currents magnesium - primary metabolism and polynucleotide structures calcium - controls and triggers zinc - Lewis acid catalysis and regulation non-haem iron redox reactions and controls haem iron - coupled redox reactions manganese copper - extracellular redox reactions and matrix formation nickel and cobalt - primitive hydrogen and organmetallic biochemistry molybdenum and vanadium - nitrogen fixation and extracellular oxygen transfer phosphorus and silicon - condensation reactions of non-metals redox non-metals - sulphur, selenium and halogens. Part 3 The co-operative interaction of elements in living systems: biomineralization and shape homeostasis and morphogenesis man's interference in element distribution - medicines and pollutants.

Energetics of overall metabolic reactions of thermophilic and hyperthermophilic Archaea and Bacteria

Abstract: Thermophilic and hyperthermophilic Archaea and Bacteria have been isolated from marine hydrothermal systems, heated sediments, continental solfataras, hot springs, water heaters, and industrial waste They catalyze a tremendous array of widely varying metabolic processes As determined in the laboratory, electron donors in thermophilic and hyperthermophilic microbial redox reactions include H2, Fe(2+), H2S, S, S2O3(2-), S4O6(2-), sulfide minerals, CH4, various mono-, di-, and hydroxy-carboxylic acids, alcohols, amino acids, and complex organic substrates; electron acceptors include O2, Fe(3+), CO2, CO, NO3(-), NO2(-), NO, N2O, SO4(2-), SO3(2-), S2O3(2-), and S Although many assimilatory and dissimilatory metabolic reactions have been identified for these groups of microorganisms, little attention has been paid to the energetics of these reactions In this review, standard molal Gibbs free energies (DeltaGr(0)) as a function of temperature to 200 degrees C are tabulated for 370 organic and inorganic redox, disproportionation, dissociation, hydrolysis, and solubility reactions directly or indirectly involved in microbial metabolism To calculate values of DeltaGr(0) for these and countless other reactions, the apparent standard molal Gibbs free energies of formation (DeltaG(0)) at temperatures to 200 degrees C are given for 307 solids, liquids, gases, and aqueous solutes It is shown that values of DeltaGr(0) for many microbially mediated reactions are highly temperature dependent, and that adopting values determined at 25 degrees C for systems at elevated temperatures introduces significant and unnecessary errors The metabolic processes considered here involve compounds that belong to the following chemical systems: H-O, H-O-N, H-O-S, H-O-N-S, H-O-C(inorganic), H-O-C, H-O-N-C, H-O-S-C, H-O-N-S-C(amino acids), H-O-S-C-metals/minerals, and H-O-P For four metabolic reactions of particular interest in thermophily and hyperthermophily (knallgas reaction, anaerobic sulfur and nitrate reduction, and autotrophic methanogenesis), values of the overall Gibbs free energy (DeltaGr) as a function of temperature are calculated for a wide range of chemical compositions likely to be present in near-surface and deep hydrothermal and geothermal systems

Chemical properties of catechols and their molecular modes of toxic action in cells, from microorganisms to mammals.

Abstract: Catechols can undergo a variety of chemical reactions. In this review, we particularly focus on complex formations and the redox chemistry of catechols, which play an inportant role in the toxicity of catechols. In the presence of heavy metals, such as iron or copper, stable complexes can be formed. In the presence of oxidizing agents, catechols can be oxidized to semiquinone radicals and in a next step to o-benzoquinones. Heavy metals may catalyse redox reactions in which catechols are involved. Further chemical properties like the acidity constant and the lipophilicity of different catechols are shortly described as well. As a consequence of the chemical properties and the chemical reactions of catechols, many different reactions can occur with biomolecules such as DNA, proteins and membranes, ultimately leading to non-repairable damage. Reactions with nucleic acids such as adduct formation and strand breaks are discussed among others. Interactions with proteins causing protein and enzyme inactivation are described. The membrane-catechol interactions discussed here are lipid peroxidation and uncoupling. The deleterious effect of the interactions between catechols and the different biomolecules is discussed in the context of the observed toxicities, caused by catechols.

Oxidation reactivities of dibenzothiophenes in polyoxometalate/H2O2 and formic acid/H2O2 systems

Abstract: Dibenzothiophene, 4-methyldibenzothiophene, and 4,6-dimethyldibenzothiophene are typical thiophenic sulfur compounds that exist in diesel fuels Using toluene solutions of the model compounds, experiments were carried out to compare the reactivity of the different dibenzothiophenes in oxidation reactions, a key step for oxidative desulfurizations A series of polyoxometalate/H2O2 systems were evaluated for dibenzothiophene oxidation The H2O2 solutions of phosphotungstic acid and its salt were very active catalyst systems for the model compound oxidation, while their molybdenum counterpart systems were much less active The H2O2 solutions of silicotungstic and silicomolybdic compounds were the least active catalyst systems for the reaction Oxidation reactivities decreased in the order of dibenzothiophene>4-methyldibenzothiophene>4,6-dimethyldibenzothiophene, the same reactivity trend that exists in HDS However, the oxidation of the dibenzothiophenes was achieved under mild reaction conditions and it was easy to increase reaction temperature or reaction time to achieve high oxidation conversions, even for the least reactive 4,6-dimethyldibenzothiophene Apparent activation energies of dibenzothiophene, 4-methyldibenzothiophene, and 4,6-dimethyldibenzothiophene oxidation were 538, 560, and 587 kJ/mol, respectively These activation energies indicated a decrease in reactivity of dibenzothiophenes as methyl substitutes increased at the 4 and 6 positions on dibenzothiophene rings Interestingly, in a formic acid/H2O2 system, the oxidation reactivity of the dibenzothiophenes showed the reverse trend, suggesting that steric hindrance might play a role when bulky polyoxoperoxo species, which likely form in a hydrogen peroxide solution, act as catalysts

Synthesis of “Solid Solution” and “Core-Shell” Type Cobalt−Platinum Magnetic Nanoparticles via Transmetalation Reactions

Abstract: In this article, we report the synthesis of “solid solution” and “core-shell” types of well-defined Co−Pt nanoalloys smaller than 10 nm. The formation of these alloys is driven by redox transmetalation reactions between the reagents without the need for any additional reductants. Also the reaction proceeds selectively as long as the redox potential between the two metals is favorable. The reaction between Co2(CO)8 and Pt(hfac)2 (hfac = hexafluoroacetylacetonate) results in the formation of “solid solution” type alloys such as CoPt3 nanoparticles. On the other hand, the reaction of Co nanoparticles with Pt(hfac)2 in solution results in “CocorePtshell” type nanoalloys. Nanoparticles synthesized by both reactions are moderately monodispersed (σ < 10%) without any further size selection processes. The composition of the alloys can also be tuned by adjusting the ratio of reactants. The magnetic and structural properties of the obtained nanoparticles and reaction byproducts are characterized by TEM, SQUID, UV/v...

Electron Transfer Dynamics in Nanocrystalline Titanium Dioxide Solar Cells Sensitized with Ruthenium or Osmium Polypyridyl Complexes

Abstract: The electron transfer dynamics in solar cells that utilize sensitized nanocrystalline titanium dioxide photoelectrodes and the iodide/triiodide redox couple have been studied on a nanosecond time scale. The ruthenium and osmium bipyridyl complexes Ru(H2L‘)2(CN)2, Os(H2L‘)2(CN)2, Ru(H2L‘)2(NCS)2, and Os(H2L‘)2(NCS)2, where H2L‘ is 4,4‘-dicarboxylic acid 2,2‘-bipyridine, inject electrons into the semiconductor with a rate constant >108 s-1. The effects of excitation intensity, temperature, and applied potential on the recombination reaction were analyzed using a second-order kinetics model. The rates of charge recombination decrease with increasing driving force to the oxidized sensitizer, indicating that charge recombination occurs in the Marcus inverted region. The electronic coupling factors between the oxidized sensitizer and the injected electrons in TiO2 and the reorganization energies for the recombination reaction vary significantly for the different metal complexes. The charge recombination rates a...

Role of glutathione in adaptation and signalling during chilling and cold acclimation in plants.

Abstract: Glutathione is an important component of the ascorbate-glutathione cycle, which is involved in the regulation of hydrogen peroxide (H2O2) concentrations in plants. During chilling and cold acclimation, i.e. exposure to temperatures between 0 and 15 degrees C, H2O2 may accumulate. Excess electrons from the photosynthetic and respiratory electron transport chains can be used for the reduction of oxygen, thus producing superoxide radicals (O2.-); these are subsequently transformed to H2O2 via superoxide dismutase (SOD; EC 1.15.1.1). During the removal of excess H2O2, reduced glutathione (GSH) is converted to its oxidised form (GSSG), and GSH is regenerated by the activity of NADPH-dependent glutathione reductase (GR; EC 1.6.4.2). At low non-freezing temperatures, high GSH content and GR activity were detected in several plant species, indicating a possible contribution to chilling tolerance and cold acclimation. Changes in H2O2 concentration and GSH/GSSG ratio alter the redox state of the cells and may activate special defence mechanisms through a redox signalling chain. The finding that several defence genes have antioxidant responsive elements or GSSG binding sites in their regulatory regions supports the idea that redox signalling is involved in regulating gene expression in response to low temperature.

Methanol: A “Smart” Chemical Probe Molecule

Abstract: A novel chemisorption method was employed for the dissociative adsorption of methanol to surface methoxy intermediates in order to quantitatively determine the number of surface active sites on one-component metal oxide catalysts (MgO, CaO, SrO, BaO, Y2O3, La2O3, CeO2, TiO2, ZrO2, HfO2, V2O5, Nb2O5, Ta2O5, Cr2O3, MoO3, WO3, Mn2O3, Fe2O3, Co3O4, Rh2O3, NiO, PdO, PtO, CuO, Ag2O, Au2O3, ZnO, Al2O3, Ga2O3, In2O3, SiO2, GeO2, SnO2, P2O5, Sb2O3, Bi2O3, SeO2 and TeO2). The number of surface active sites for methanol dissociative adsorption corresponds to ∼3 μmol/m2 on average for many of the metal oxide catalysts. Furthermore, the methanol oxidation product distribution at low conversions reflects the nature of the surface active sites on metal oxides since redox sites yield H2CO, acidic sites yield CH3OCH3 and basic sites yield CO2. The distribution of the different types of surface active sites was found to vary widely for the different metal oxide catalysts. In addition, the commonality of the surface methoxy intermediate during dissociative chemisorption of methanol and methanol oxidation on oxide catalysts also allows for the quantitative determination of the turnover frequency (TOF) values. The TOF values for the various metal oxide catalysts were found to vary over seven orders of magnitude (10−3 to 104 s−1). An inverse relationship (for metal oxide catalysts displaying high (>85%) selectivity to either redox or acidic products) was found between the methanol oxidation TOF values and the decomposition temperatures of the surface M–OCH3 intermediates reflecting that the decomposition of the surface M–OCH3 species is the rate-determining step during methanol oxidation over the metal oxide catalysts.

Old iron, young copper: from Mars to Venus.

Abstract: Iron and copper are metals which play an important role in the living world. From a brief consideration of their chemistry and biochemistry we conclude that the early chemistry of life used water soluble ferrous iron while copper was in the water-insoluble Cu(I) state as highly insoluble sulphides. The advent of oxygen was a catastrophic event for most living organisms, and can be considered to be the first general irreversible pollution of the earth. In contrast to the oxidation of iron and its loss of bioavailability as insoluble Fe(III), the oxidation of insoluble Cu(I) led to soluble Cu(II). A new iron biochemistry became possible after the advent of oxygen, with the development of chelators of Fe(III), which rendered iron once again accessible, and with the control of the potential toxicity of iron by its storage in a water soluble, non-toxic, bio-available storage protein (ferritin). Biology also discovered that whereas enzymes involved in anaerobic metabolism were designed to operate in the lower portion of the redox spectrum, the arrival of dioxygen created the need for a new redox active metal which could attain higher redox potentials. Copper, now bioavailable, was ideally suited to exploit the oxidizing power of dioxygen. The arrival of copper also coincided with the development of multicellular organisms which had extracellular cross-linked matrices capable of resisting attack by oxygen free radicals. After the initial 'iron age' subsequent evolution moved, not towards a 'copper age', but rather to an 'iron-copper' age. In the second part of the review, this symbiosis of iron and copper is examined in yeast. We then briefly consider iron and copper metabolism in mammals, before looking at iron-copper interactions in mammals, particularly man, and conclude with the reflection that, as in Greek and Roman mythology, a better understanding of the potentially positive interactions between Mars (iron) and Venus (copper) can only be to the advantage of our species.

Redox Reactions of Hemoglobin and Myoglobin: Biological and Toxicological Implications

Abstract: Direct cytotoxic effects associated with hemoglobin (Hb) or myoglobin (Mb) have been ascribed to redox reactions (involving either one- or two-electron steps) between the heme group and peroxides. These interactions are the basis of the pseudoperoxidase activity of these hemoproteins and can be cytotoxic when reactive species are formed at relatively high concentrations during inflammation and typically lead to cell death. Peroxides relevant to biological systems include hydrogen peroxide, lipid hydroperoxides, and peroxynitrite. Reactions between Hb/Mb and peroxides form the ferryl oxidation state of the protein, analogous to compounds I and II formed in the catalytic cycle of many peroxidase enzymes. This higher oxidation state of the protein is a potent oxidant capable of promoting oxidative damage to most classes of biological molecules. Free iron, released from Hb, also has the potential to promote oxidative damage via classical "Fenton" chemistry. It has become increasingly evident that Hb/Mb redox reactions or their by-products play a critical role in the pathophysiology of some disease states. This review briefly discusses the reactions of Hb/Mb with biological peroxides, potential cytotoxicity and the impact of these interactions on modulation of cell signaling pathways regulated by these reactive species. Also discussed in this article is the role of heme-protein chemistry in relation to the toxicity of hemoproteins.

The “Wired” Laccase Cathode: High Current Density Electroreduction of O2 to Water at +0.7 V (NHE) at pH 5

Abstract: We report on the four-electron electrocatalytic reduction of oxygen to water at a current density of 5 mA cm and at +0.7 V (NHE) in pH 5 citrate buffer at 37.5oC. The electroreduction of O2 to water near neutral pH and at ambient temperature has been one of the longest-standing and best-researched problems of electrochemistry. It has been a missing element of a miniature membrane-less biofuel cell of projected respective areal and volumetric power densities of 1 μW mm and 10 μW μL. Unlike a primary lithium battery, the cell, in which glucose is electrooxidized at the anode and dissolved O2 is electroreduced at the cathode, can be miniaturized to sub-millimeter dimensions because it does not require a difficult to miniaturize case or seal. The application of the cell is in powering a chip-based sensor and transmitter, operating in the human body for a week and transmitting to a distance of ~ 1 m. Such a system requires only a few hundred nW of power and its fuel cell will comprise only the electrocatalytic anode and a cathode strips, each of ~1 mm length, deposited on the chip. The novel electrocatalyst was made of hydrophilic carbon cloth coated with a crosslinked electrostatic adduct of laccase from Coriolus hirsutus (a polyanion above pH 4) and an electron-conducting redox polymer (a polycation). The 0.78 V (NHE) redox polymer electrically connected (“wired”) the laccase reaction centers to the carbon fibers. The laccase-“wiring” redox potential polymer was formed of poly-N-vinyl imidazole by coordinating 1/5 of its rings to [Os(tpy)(dme-bpy)], where tpy is 2,2’:6’,2”-terpyridine and dme-bpy is 4,4′-dimethyl-2,2′bipyridine. In the catalytic process the copper-containing reaction centers of laccase are oxidized by O2; the oxidized copper centers oxidize Os centers of the redox polymer to Os; and the Os centers of the redox polymer are electroreduced, at a potential negative of their + 0.78 V (NHE) potential. The crosslinking of the redox polymer-enzyme adduct on the carbon fibers yields a mechanically tough composite, withstanding more than 0.1 N m shear stress generated when the 4 mm diameter cathode is rotated at 1000 RPM. The carbon-cloth composite electrodes were assembled on 3-mm diameter vitreous carbon rotating disk electrodes mounted in Teflon sleeves. The cloth (Toray TGPH030, Japan) had a 78% void fraction consisted of 10 μm diameter fibers and had a nominal thickness of 350 μm. They were made hydrophilic by plasma treatment, coated with the “wired” laccase and were cemented, using conductive carbon paint to the tips of the rotated vitreous carbon rods. At 4000 RPM, where the current density reaches 6 mA cm at 0.6 V (NHE) , the current becomes limited by the kinetics of the electrocatalyst. In a 22 hour long test, a current density of > 2 mA cm was maintained when the rotating electrode was poised at +0.7 V (NHE). Laccase cathodes on which O2 is electroreduced to water were introduced by Tarasevich et al. who formed these by directly adsorbing laccase on TeflonTM-bound high-surface area carbon-black particles. The cathodes operated for days with little or no evidence of degradation, at a current density of 175 μA cm 2 when poised at 1.10 V (NHE) at pH 5. Members of the same group later reported a current density of 10 mA cm at pH 3.5 for an electrode poised at 0.8 V (NHE), but did not provide sufficient information to allow other investigators to reproduce these results. 2 Publications of the past decade do not mention high current density O2 cathodes, all reported current densities being less than . 3-9 The principles of the design of the novel electrocatalyst were the following: Independence of orientation of the enzyme: Use of “wired” laccase. When a redox enzyme is adsorbed on the surface of an electrode, its electroreduction/electrooxidation is orientation dependent, because its redox center must be close to the electrode. In a randomly oriented monolayer of enzyme molecules, fewer than 1% participate in the desired reaction. In contrast, when the electrostatic adduct of polyanionic enzyme and a water soluble electron-conducting redox polymer are crosslinked on an electrode to form a film that swells in water, most of the enzyme molecules electrocatalyze the reaction. The reason is that electrons are exchanged between the enzyme’s reaction centers and the redox polymer through collisions with the tethered, but mobile, redox functions of the hydrated polymer. Electroreduction of multiple layers of laccase. Electron exchange between the mobile but tethered redox functions leads to electron conduction in the hydrogel, allowing the electrical connection of multiple layers of redox enzymes to electrodes. Because unique orientation is not required and because a multiple layers of enzyme are “wired”, the current density exceeds by three orders of magnitude that for an absorbed, randomly oriented, monolayer. Formation of an electrostatic complex of laccase and its connecting redox polymer. Because the increase of entropy is small when two macromolecules are mixed, they phase separate unless they are chemically bound to each other. Electrostatic bonding of the polycationic redox polymer and the polyanionic laccase prevents phase separation.

Molecular machines based on metal ion translocation.

Abstract: Transition metal ions can be moved reversibly between the two coordinatively unequivalent compartments A and B of a ditopic ligand, using as an input the variation of a bulk solution parameter, either pH or redox potential. In a redox-driven translocation, the metal moves reversibly from A to B on cycling between two consecutive oxidation states (e.g., CuII/CuI; FeIII/FeII) by means of auxiliary oxidation and reduction reactions. In a pH-driven process, one compartment displays also acid−base properties (AHn ⇆ An- + nH+), and the Mn+ ion is translocated between B and An- through consecutive addition of base and acid.

Electroreduction of O2 to water on the "wired" laccase cathode

Abstract: Oxygen was electrocatalytically reduced to water at a current density of 5 mA/cm2 and at +0.7 V (NHE) in pH 5 citrate buffer at 37.5 °C. The electrocatalyst was a composite of laccase cross-linked with a redox polymer on a hydrophilic cloth of 10 μm diameter carbon fibers. The redox polymer, PVI−Os(tpy)(dme-bpy)2+/3+, [poly-N-vinyl imidazole with 1/5th of the imidazoles complexed with [Os(tpy)(dme-bpy)]2+/3+ (tpy=terpyridine; dme-bpy= 4,4‘-dimethyl-2,2‘-bipyridine)], electrically connected (“wired”) the laccase reaction centers to the fibers.

High-Valent Manganese Corroles and the First Perhalogenated Metallocorrole Catalyst.

Abstract: On pyrrole! The pyrrole-based corrole ligands can offer an alternative to porphyrin systems. The manganese corroles 1-4 are readily synthesized, undergo metal- not ligand-based redox chemistry, and 4 in particular shows impressive catalytic activity in the oxygenation of styrene with iodosylbenzene.

Proton-coupled electron transfer of cytochrome c.

Abstract: Cytochrome c (Cyt-c) was electrostatically bound to self-assembled monolayers (SAM) on an Ag electrode, which are formed by ω-carboxyl alkanethiols of different chain lengths (Cx). The dynamics of the electron-transfer (ET) reaction of the adsorbed heme protein, initiated by a rapid potential jump to the redox potential, was monitored by time-resolved surface enhanced resonance Raman (SERR) spectroscopy. Under conditions of the present experiments, only the reduced and oxidized forms of the native protein state contribute to the SERR spectra. Thus, the data obtained from the spectra were described by a one-step relaxation process yielding the rate constants of the ET between the adsorbed Cyt-c and the electrode for a driving force of zero electronvolts. For C16- and C11-SAMs, the respective rate constants of 0.073 and 43 s-1 correspond to an exponential distance dependence of the ET (β = 1.28 A-1), very similar to that observed for long-range intramolecular ET of redox proteins. Upon further decreasing th...

Electron Transfer Effects in Ozone Decomposition on Supported Manganese Oxide

Abstract: Manganese oxide catalysts supported on Al2O3, ZrO2, TiO2, and SiO2 supports were used to study the effect of support on ozone decomposition kinetics. In-situ laser Raman spectroscopy, temperature-programmed oxygen desorption, surface area measurements, and extended and near-edge X-ray absorption fine structure (EXAFS and NEXAFS) showed that the manganese oxide was highly dispersed on the surface of the supports. The EXAFS spectra suggested that the manganese active centers on all of the surfaces were surrounded by five oxygen atoms. These metal centers were found to be of a monomeric type for the Al2O3-supported catalyst and multinuclear for the other supports. The NEXAFS spectra for the catalysts showed a chemical shift to lower energy, and an intensity change in the L-edge features which followed the trend Al2O3 > ZrO2 > TiO2 > SiO2. The trends provided insights into the positive role of available empty d-states required in the reduction step of a redox reaction. The catalysts were tested for their ozon...

Electron-transfer reactivity and enzymatic activity of hemoglobin in a SP Sephadex membrane.

Abstract: Hemoglobin can exhibit a direct electron-transfer reaction after being entrapped in a SP Sephadex membrane A pair of stable and well-defined redox waves are obtained at a hemoglobin−SP sephadex mo

Equilibrating metal-oxide cluster ensembles for oxidation reactions using oxygen in water.

Abstract: Although many enzymes can readily and selectively use oxygen in water-the most familiar and attractive of all oxidants and solvents, respectively-the design of synthetic catalysts for selective water-based oxidation processes utilizing molecular oxygen remains a daunting task. Particularly problematic is the fact that oxidation of substrates by O2 involves radical chemistry, which is intrinsically non-selective and difficult to control. In addition, metallo-organic catalysts are inherently susceptible to degradation by oxygen-based radicals, while their transition-metal-ion active sites often react with water to give insoluble, and thus inactive, oxides or hydroxides. Furthermore, pH control is often required to avoid acid or base degradation of organic substrates or products. Unlike metallo-organic catalysts, polyoxometalate anions are oxidatively stable and are reversible oxidants for use with O2 (refs 8,9,10). Here we show how thermodynamically controlled self-assembly of an equilibrated ensemble of polyoxometalates, with the heteropolytungstate anion [AIVVW11O40]6- as its main component, imparts both stability in water and internal pH-management. Designed to operate at near-neutral pH, this system facilitates a two-step O2-based process for the selective delignification of wood (lignocellulose) fibres. By directly monitoring the central Al atom, we show that equilibration reactions typical of polyoxometalate anions keep the pH of the system near 7 during both process steps.