Showing papers on "Redox published in 2010"

Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

Abstract: Many, if not most, redox reactions are coupled to proton transfers. This includes most common sources of chemical potential energy, from the bioenergetic processes that power cells to the fossil fuel combustion that powers cars. These proton-coupled electron transfer or PCET processes may involve multiple electrons and multiple protons, as in the 4 e–, 4 H+ reduction of dioxygen (O2) to water (eq 1), or can involve one electron and one proton such as the formation of tyrosyl radicals from tyrosine residues (TyrOH) in enzymatic catalytic cycles (eq 2). In addition, many multi-electron, multi-proton processes proceed in one-electron and one-proton steps. Organic reactions that proceed in one-electron steps involve radical intermediates, which play critical roles in a wide range of chemical, biological, and industrial processes. This broad and diverse class of PCET reactions are central to a great many chemical and biochemical processes, from biological catalysis and energy transduction, to bulk industrial chemical processes, to new approaches to solar energy conversion. PCET is therefore of broad and increasing interest, as illustrated by this issue and a number of other recent reviews.

Biogeochemical Redox Processes and their Impact on Contaminant Dynamics

Abstract: Life and element cycling on Earth is directly related to electron transfer (or redox) reactions. An understanding of biogeochemical redox processes is crucial for predicting and protecting environmental health and can provide new opportunities for engineered remediation strategies. Energy can be released and stored by means of redox reactions via the oxidation of labile organic carbon or inorganic compounds (electron donors) by microorganisms coupled to the reduction of electron acceptors including humic substances, iron-bearing minerals, transition metals, metalloids, and actinides. Environmental redox processes play key roles in the formation and dissolution of mineral phases. Redox cycling of naturally occurring trace elements and their host minerals often controls the release or sequestration of inorganic contaminants. Redox processes control the chemical speciation, bioavailability, toxicity, and mobility of many major and trace elements including Fe, Mn, C, P, N, S, Cr, Cu, Co, As, Sb, Se, Hg, Tc, a...

Theory of Coupled Electron and Proton Transfer Reactions

Abstract: Coupled electron and proton transfer reactions play a key role in the mechanisms of biological energy transduction.1–3 Such reactions are also fundamental for artificial energy-related systems such as fuel cells, chemical sensors, and other electrochemical devices. Biological examples include, among others, cytochrome c oxidase,4,5 bc1 complex,6,7 and photosynthetic reaction centers.8,9 In such systems, electrons tunnel between redox cofactors of an enzyme, while the coupled protons are transferred either across a single hydrogen bond or between protonatable groups along special proton-conducting channels. In this paper general theories and models of coupled electron transfer/proton transfer (ET/PT) reactions are discussed. Pure electron transfer reactions in proteins have been thoroughly studied in the past, both experimentally10–17 and theoretically.18–25 The coupled reactions are relatively new and currently are gaining attention in the field.6,8,26–43 Two types of coupled reactions can be distinguished. In concerted electron and proton transfer reactions (denoted PCET in Refs. 29,30,43–45, although this term is also used more generally), both the ET and PT transitions occur in one step. Such concerted processes occur in reactions in which proton transfer is typically limited to one hydrogen bond; however, examples with multiple hydrogen bond rearrangements are also known.46 In sequential reactions, the transitions occur in two steps: ET/PT or PT/ET. Typically each individual step is uphill in energy, while the coupled reaction is downhill. A sequential reaction can proceed along two parallel channels: ET then PT (EP) or PT then ET (PE). In each channel the reaction involves two sequential steps: uphill activation, and then downhill reaction to the final product state. The lifetime of the activated complex is limited by the back reaction. The general formula for the rate of such reactions can be easily developed. In the context of bioenergetics issues, however, it is interesting to analyze all of the possible cases separately because each corresponds to a different mechanism: for example, an electron can go first and pull out a proton; alternatively, a proton can go first and pull out an electron; or an electron can jump back and forth between donor and acceptor and gradually pull out a proton. In enzymes involving coupled proton and electron transport, the exact mechanism of the reaction is of prime interest. First we will consider a simple four-state model of reactions where the proton moves across a single hydrogen bond; both concerted and sequential reactions will be treated. Then we will consider models for long-distance proton transfer, also denoted proton transport or proton translocation. Typically, electron transfer coupled to proton translocation in proteins involves an electron tunneling over a long distance between two redox cofactors, coupled to a proton moving along a proton conducting channel in a classical, diffusion-like random walk fashion. Again, separately the electron and proton transfer reactions are typically uphill, while the coupled reaction is downhill in energy. The schematics of this process is shown in Fig. 1. The kinetics of such reactions can be much different from those involving proton transfer across a single hydrogen bond. In this paper, we will discuss the specifics of such long-distance proton-coupled reactions. Fig. 1 Schematics of the electron transfer reaction coupled to proton translocation. In the reaction, an electron is tunneling over a long distance between two redox cofactors, O and R, and a coupled proton is transferred over a proton conducting channel. The ... Following the review of theoretical concepts, a few applications will be discussed. First the phenoxyl/phenol and benzyl/toluene self-exchange reactions will be examined. The phenoxyl/phenol reaction involves electronically nonadiabatic proton transfer and corresponds to a proton-coupled electron transfer (PCET) mechanism, whereas the benzyl/toluene reaction involves electronically adiabatic proton transfer and corresponds to a hydrogen atom transfer (HAT) mechanism. Comparison of these two systems provides insight into fundamental aspects of electron-proton interactions in these types of systems. Next a series of theoretical calculations on experimentally studied PCET reactions in solution and enzymes will be summarized, along with general predictions concerning the dependence of rates and kinetic isotope effects (the ratio of the rate constants for hydrogen and deuterium transfer) on system properties such as temperature and driving force. The final application that will be discussed is cytochrome c oxidase (CcO). CcO is the terminal component of the electron transport chain of the respiratory system in mitochondria and is one of the key enzymes responsible for energy generation in cells. The intricate correlation between the electron and proton transport via electrostatic interactions, as well as the kinetics of the coupled transitions, appear to be the basis of the pumping mechanism in this enzyme.

The role of lattice oxygen on the activity of manganese oxides towards the oxidation of volatile organic compounds

Abstract: A series of manganese oxides differing in the structure, composition, average manganese oxidation state and specific surface area have been used in the total oxidation of volatile organic compounds (VOC). Ethanol, ethyl acetate and toluene were chosen as models of VOC. Among the manganese oxides tested, cryptomelane (KMn8O16) was found to be very active in the oxidation of VOC. The performance of cryptomelane was significantly affected by the presence of other phases, namely, Mn2O3 and Mn3O4. Temperature-programmed experiments combined with X-ray photoelectron spectroscopy (XPS) show that the mobility and reactivity of the oxygen species were significantly affected, explaining the catalytic performances of those samples. Mn3O4 improves the catalytic performance due to the increase of the reactivity and mobility of lattice oxygen, while Mn2O3 has the opposite effect. These results show that there is a correlation between the redox properties and the catalytic performance of the manganese oxides. Temperature-programmed surface reactions (TPSR) after adsorption of toluene or ethanol, in addition to reactions performed without oxygen in the feed, show that lattice oxygen is involved in the VOC oxidation mechanism. The conversion level was found to be influenced by the type of VOC, the reactivity into CO2 increasing in the following order: Toluene

Degradation of microcystin-LR using sulfate radicals generated through photolysis, thermolysis and e− transfer mechanisms

Abstract: This study explores the potential use of sulfate radical-based advanced oxidation technologies (SR-AOTs) for the degradation of the naturally occurring hepatotoxin, microcystin-LR (MC-LR). The generation of sulfate radicals was achieved by activation of the oxidants persulfate (PS) and peroxymonosulfate (PMS) through electrophilic transition metal cations (Ag+ and Co2+, respectively), radiation (UV 300 − has similar redox potential to hydroxyl radical (HO ), to the best of our knowledge, SR-AOTs have not been tested for the degradation of cyanotoxins. In this study, PMS was activated very efficiently with Co2+ at neutral pH and increasing catalyst concentration resulted in dramatic increase of the initial rates of degradation that reached a plateau for CCo(II) ≥ 1 mg. Based on the optimum pH conditions for each system, the efficiency order is Co2+/PMS > Fe2+/H2O2 ≫ Ag+/PS, which we believe is associated with the energy of the lower unoccupied molecular orbital of the oxidants. When UV (300 Since, the UV lamps used in the study emit light at a range of wavelengths (300

Novel electrochemical approach to assess the redox properties of humic substances.

Abstract: Two electrochemical methods to assess the redox properties of humic substances (HS) are presented: direct electrochemical reduction (DER) on glassy carbon working electrodes (WE) and mediated electrochemical reduction (MER) and oxidation (MEO) using organic radicals to facilitate electron transfer between HS and the WE. DER allows for continuous monitoring of electron and proton transfer to HS by chronocoulometry and automated acid titration, respectively, and of changes in bulk HS redox potential Eh. Leonardite Humic Acid (LHA) showed an H+/e− ratio of unity and a decrease in potential from Eh = +0.18 to −0.23 V upon transfer of 822 μmole- gLHA−1 at pH 7, consistent with quinones as major redox-active functional moieties in LHA. MER and MEO quantitatively detected electrons in LHA samples that were prereduced by DER to different extents. MER and MEO therefore accurately quantify the redox state of HS. Cyclic DER and O2-reoxidation revealed that electron transfer to LHA was largely reversible. However, LH...

Fluorescent protein-based redox probes

Abstract: Redox biochemistry is increasingly recognized as an integral component of cellular signal processing and cell fate decision making. Unfortunately, our capabilities to observe and measure clearly defined redox processes in the natural context of living cells, tissues, or organisms are woefully limited. The most advanced and promising tools for specific, quantitative, dynamic and compartment-specific observations are genetically encoded redox probes derived from green fluorescent protein (GFP). Within only few years from their initial introduction, redox-sensitive yellow FP (rxYFP), redox-sensitive GFPs (roGFPs), and HyPer have generated enormous interest in applying these novel tools to monitor dynamic redox changes in vivo. As genetically encoded probes, these biosensors can be specifically targeted to different subcellular locations. A critical advantage of roGFPs and HyPer is their ratiometric fluorogenic behavior. Moreover, the probe scaffold of redox-sensitive fluorescent proteins (rxYFP and roGFPs) is amenable to molecular engineering, offering fascinating prospects for further developments. In particular, the engineering of redox relays between roGFPs and redox enzymes allows control of probe specificity and enhancement of sensitivity. Genetically encoded redox probes enable the functional analysis of individual proteins in cellular redox homeostasis. In addition, redox biosensor transgenic model organisms offer extended opportunities for dynamic in vivo imaging of redox processes.

Proton-Coupled Electron Flow in Protein Redox Machines

Abstract: Electron transfer (ET) reactions are fundamental steps in biological redox processes. Respiration is a case in point: at least 15 ET reactions are required to take reducing equivalents from NADH, deposit them in O_2, and generate the electrochemical proton gradient that drives ATP synthesis. Most of these reactions involve quantum tunneling between weakly coupled redox cofactors (ET distances > 10 A) embedded in the interiors of folded proteins. Here we review experimental findings that have shed light on the factors controlling these distant ET events. We also review work on a sensitizer-modified copper protein photosystem in which multistep electron tunneling (hopping) through an intervening tryptophan is orders of magnitude faster than the corresponding single-step ET reaction.If proton transfers are coupled to ET events, we refer to the processes as proton coupled ET, or PCET, a term introduced by Huynh and Meyer in 1981. Here we focus on two protein redox machines, photosystem II and ribonucleotide reductase, where PCET processes involving tyrosines are believed to be critical for function. Relevant tyrosine model systems also will be discussed.

Voltammetry and Growth Physiology of Geobacter sulfurreducens Biofilms as a Function of Growth Stage and Imposed Electrode Potential

Abstract: The ability of Geobacter sulfurreducens to utilize electrodes as electron acceptors provides a system for monitoring mechanisms of electron transfer beyond the cell surface. This study examined the physiology of extracellular electron transfer during many stages of growth, and in response to short- and long-term changes in electron acceptor potential. When G. sulfurreducens was grown on planar potentiostat-controlled electrodes, the magnitude of early cell attachment increased with initial cell density. However, the first cells to attach did not demonstrate the same electron transfer rates as cells grown on electrodes. For example, following initial attachment of fumarate-grown cells, the electron transfer rate was 2 mA/mg protein, but increased to nearly 8 mA/mg protein within 6 h of growth. Once attached, all biofilms grew at a constant rate (doubling every 6 h), and sustained a high specific electron transfer rate and growth yield, while current density was below 300 μA/cm2. Beyond this point, the rate of current increase slowed and approached a stable plateau. At all phases, slow scan rate cyclic voltammetry of G. sulfurreducens showed a similar well-defined sigmoidal catalytic wave, indicating the general model of electron transfer to the electrode was not changing. Short-term exposure to reducing potentials (3 h) did not alter these characteristics, but did cause accumulation of electrons which could be discharged at potentials above −0.1 V. Sustained growth at lower potentials (−0.16 V) only slightly altered the pattern of detectable redox species at the electrode, but did eliminate this pattern of discharge from the biofilm. Single-turnover voltammetry of colonized electrodes showed at least 3 redox couples at potentials similar to other recent observations, with redox protein coverage of the electrode on the order of ca. 1 nmol/cm2. The consistent electrochemistry, growth rate, and growth yield of the G. sulfurreducens biofilm at all stages suggests an initial phase where cells must optimize attachment or electron transfer to a surface, and that after this point, the rate of electron production by cells (rate electrons are delivered to the external surface) remains rate limiting compared to the rate electrons can be transferred between cells, and to electrodes.

Redox compartmentalization and cellular stress

Abstract: Mammalian cells are highly organized to optimize function. For instance, oxidative energy-producing processes in mitochondria are sequestered away from plasma membrane redox signalling complexes and also from nuclear DNA, which is subject to oxidant-induced mutation. Proteins are unique among macromolecules in having reversible oxidizable elements, 'sulphur switches', which support dynamic regulation of structure and function. Accumulating evidence shows that redox signalling and control systems are maintained under kinetically limited steady states, which are highly displaced from redox equilibrium and distinct among organelles. Mitochondria are most reducing and susceptible to oxidation under stressed conditions, while nuclei are also reducing but relatively resistant to oxidation. Within compartments, the glutathione and thioredoxin systems serve parallel and non-redundant functions to maintain the dynamic redox balance of subsets of protein cysteines, which function in redox signalling and control. This organization allows cells to be poised to respond to cell stress but also creates sites of vulnerability. Importantly, disruption of redox organization is a common basis for disease. Research tools are becoming available to elucidate details of subcellular redox organization, and this development highlights an opportunity for a new generation of targeted antioxidants to enhance and restore redox signalling and control in disease prevention.

Novel Thiosemicarbazones of the ApT and DpT Series and Their Copper Complexes: Identification of Pronounced Redox Activity and Characterization of Their Antitumor Activity

Abstract: The novel chelators 2-acetylpyridine-4,4-dimethyl-3-thiosemicarbazone (HAp44mT) and di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (HDp44mT) have been examined to elucidate the structure-activity relationships necessary to form copper (Cu) complexes with pronounced antitumor activity. Electrochemical studies demonstrated that the Cu complexes of these ligands had lower redox potentials than their iron complexes. Moreover, the Cu complexes where the ligand/metal ratio was 1:1 rather than 2:1 had significantly higher intracellular oxidative properties and antitumor efficacy. Interestingly, the 2:1 complex was shown to dissociate to give significant amounts of the 1:1 complex that could be the major cytotoxic effector. Both types of Cu complex showed significantly more antiproliferative activity than the ligand alone. We also demonstrate the importance of the inductive effects of substituents on the carbonyl group of the parent ketone, which influence the Cu(II/I) redox potentials because of their proximity to the metal center. The structure-activity relationships described are important for the design of potent thiosemicarbazone Cu complexes.

Cellobiose Dehydrogenase: A Versatile Catalyst for Electrochemical Applications.

Abstract: Cellobiose dehydrogenase catalyses the oxidation of aldoses--a simple reaction, a boring enzyme? No, neither for the envisaged bioelectrochemical applications nor mechanistically. The catalytic cycle of this flavocytochrome is complex and modulated by its flexible cytochrome domain, which acts as a built-in redox mediator. This intramolecular electron transfer is modulated by the pH, an adaptation to the environmental conditions encountered or created by the enzyme-producing fungi. The cytochrome domain forms the base from which electrons can jump to large terminal electron acceptors, such as redox proteins, and also enables by that path direct electron transfer from the catalytically active flavodehydrogenase domain to electrode surfaces. The application of electrochemical techniques to the elucidation of the molecular and catalytic properties of cellobiose dehydrogenase is discussed and compared to biochemical methods. The results lead to valuable insights into the function of this cellulose-bound enzyme, but also form the basis of exciting applications in biosensors, biofuel cells and bioelectrocatalysis.

The Role of One- and Two-Electron Transfer Reactions in Forming Thermodynamically Unstable Intermediates as Barriers in Multi-Electron Redox Reactions

Abstract: In the aquatic geochemical literature, a redox half-reaction is normally written for a multi-electron process (n > 2); e.g., sulfide oxidation to sulfate. When coupling two multi-electron half-reactions, thermodynamic calculations indicate possible reactivity, and the coupled half-reactions are considered favorable even when there is a known barrier to reactivity. Thermodynamic calculations should be done for one or two-electron transfer steps and then compared with known reactivity to determine the rate controlling step in a reaction pathway. Here, thermodynamic calculations are presented for selected reactions for compounds of C, O, N, S, Fe, Mn and Cu. Calculations predict reactivity barriers and agree with one previous analysis showing the first step in reducing O2 to O2− with Fe2+ and Mn2+ is rate limiting. Similar problems occur for the first electron transfer step in these metals reducing NO3−, but if reactive oxygen species form or if two-electron transfer steps with O atom transfer occur, reactivity becomes favorable. H2S and NH4+ oxidation in a one-electron transfer step by O2 is also not favorable unless activation of oxygen can occur. H2S oxidation by Cu2+, Fe(III) and Mn(III, IV) phases in two-electron transfer steps is favorable but not in one-electron steps indicating that (nano)particles with bands of orbitals are needed to accept two electrons from H2S. NH4+ oxidation by Fe(III) and Mn(III, IV) phases is generally not favorable for both one- and two-electron transfer steps, but their reaction with hydroxylamine and hydrazine to form N2O and N2, respectively, is favorable. The anammox reaction using hydroxylamine via nitrite reduction is the most favorable for NH4+ oxidation. Other chemical processes including photosynthesis and chemosynthesis are considered for these element–element transformations.

Syngas production from methane and air via a redox process using Ce-Fe mixed oxides as oxygen carriers

Abstract: CeO2, Fe2O3, Fe2O3/Al2O3 and Ce–Fe mixed oxides with different Ce/Fe ratios were prepared and characterized using XRD, Raman, XPS, and H2–TPR techniques. The selective oxidation of methane to syngas using a gas–solid reaction was investigated at 850 °C. For binary Ce–Fe oxides, only small amounts of iron ions could be incorporated into the CeO2 lattice with the superfluous Fe2O3 remaining on the surface of the molecule. Chemical interactions between surface iron sites and the Ce–Fe solid solution strongly enhanced the reducibility of materials. Methane was found to adsorb and activate on the surface iron sites as carbonaceous species and hydrogen. Carbon deposition was selectively oxidized to CO by the release of activated oxygen from the CeO2 lattice. The activation rate of methane was dependent on the quality of dispersion of surface Fe species, while the oxygen mobility of the material dominated the CO formation rate. Hydrothermally prepared Ce0.7Fe0.3O2−δ showed high activity and selectivity during the successive production of syngas using repetitive redox processes (methane reduction/air re-oxidation). Both the dispersion of surface Fe2O3 and the formation of the Ce–Fe solid solution were enhanced by the redox treatment, which made the oxygen carrier more stable.

XANES Evidence for Rapid Arsenic(III) Oxidation at Magnetite and Ferrihydrite Surfaces by Dissolved O2 via Fe2+-Mediated Reactions

Abstract: To reduce the adverse effects of arsenic on humans, various technologies are used to remove arsenic from groundwater, most relying on As adsorption on Fe-(oxyhydr)oxides and concomitant oxidation of As(III) by dissolved O2. This reaction can be catalyzed by microbial activity or by strongly oxidizing radical species known to form upon oxidation of Fe(II) by dissolved O2. Such catalyzed oxidation reactions have been invoked to explain the enhanced kinetics of As(III) oxidation in aerated water, in the presence of zerovalent iron or dissolved Fe(II). In the present study, we used arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy to investigate the role of Fe(II) in the oxidation of As(III) at the surface of magnetite and ferrihydrite under oxygenated conditions. Our results show rapid oxidation of As(III) to As(V) upon sorption onto magnetite under oxic conditions at neutral pH. Moreover, under similar oxic conditions, As(III) oxidized upon sorption onto ferrihydrite only after additi...

Facile Synthesis of N- and S-Incorporated Nanocrystalline TiO2 and Direct Solar-Light-Driven Photocatalytic Activity

Abstract: Sulfur- and nitrogen-incorporated mesoporous TiO2 (SNT) nanocomposites have been synthesized by a template-free homogeneous coprecipitation technique. The above nanocomposites have been thoroughly characterized by physicochemical and spectroscopy methods to explore the structural, electronic, and optical properties. The photocatalytic activities of the catalysts were evaluated for the degradation of methyl orange and phenol under direct solar light. SNT shows about a 2-fold higher photocatalytic activity than singly N-doped or S-doped mesoporous TiO2 and 3-fold higher than Degussa P25. The higher activity might be attributed to the synergetic interaction of sulfate and nitrogen with the TiO2 lattice. N−Ti−O and O−Ti−N−O environments are responsible for a red shift, and the sulfate group on TiO2 acts as a cocatalyst, for increasing surface acidity as well as for sustaining the redox cycles for high stability.

Understanding the Shifts in the Redox Potentials of Olivine LiM1−yMyPO4 (M = Fe, Mn, Co, and Mg) Solid Solution Cathodes

Abstract: A facile, high-energy mechanical milling (HEMM) approach has been developed to synthesize carbon-coated olivine LiM1−yMyPO4 (M = Fe, Mn, Co, and Mg) solid solution nanoparticles. A systematic structural and electrochemical characterization of the solid solution series has been carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), charge−discharge measurements, and galvanostatic intermittent titration technique (GITT). The discharge capacity, voltage profile, and cycle performance of the LiM1−yMyPO4 solid solution cathodes are found to be dependent on the different redox couples involved in the reaction. Equilibrium potentials obtained from GITT and dQ/dV plot reveal a systematic shift in the redox potential of Fe2+/3+, Mn2+/3+, and Co2+/3+ couples in the LiM1−yMyPO4 solid solution compared to their pristine end members (LiMPO4). The shifts in the redox potential are explained on the basis of the changes in the covalency of the M−O bond and M−O−M interaction, and the consequent change...

Tetraaza‐macrocyclic cobalt(II) and nickel(II) complexes as electron‐transfer agents in the photo(electro)chemical and electrochemical reduction of carbon dioxide

Abstract: The potential of a number of tetraaza‐macrocyclic complexes of cobalt and nickel to mediate the photoreduction of carbon dioxide has been studied. Carbon monoxide and hydrogen are the main products, which result from illumination of an aqueous solution containing Ru(2, 2′‐bipyridine)2+ 3 as sensitizer, ascorbic acid as sacrificial eletron donor and a tetraaza‐macrocyclic complex as relay. Their ratio strongly depends upon the relay compound used. The complexes can be used as electrocatalysts for CO2 reduction at a mercury electrode since they lower the overpotential for CO2 reduction showing turnover numbers/hour of approximately 3. A minimum potential is required to induce reaction of electrocatalysts and CO2 (and H2O). Finally, the redox properties of the macrocyclic metal complexes at p‐GaP and p‐GaAs photocathodes in acetonitrile are compared with those observed at Pt.