Self-Exchange Electron Transfer Kinetics and Reduction Potentials for Anthraquinone Disulfonate

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Electron Hopping Enables Rapid Electron Transfer between Quinone-/Hydroquinone-Containing Organic Molecules in Microbial Iron(III) Mineral Reduction

The mechanism of long-distance electron transfer via redox-active particulate natural organic matter (NOM) is still unclear, especially considering its aggregated nature and the resulting low diffusivity of its quinone- and hydroquinone-containing molecules. Here we conducted microbial iron(III) mineral reduction experiments in which anthraquinone-2,6-disulfonate (AQDS, a widely used analogue for quinone- and hydroquinone-containing molecules in NOM) was immobilized in agar to achieve a spatial separation between the iron-reducing bacteria and ferrihydrite mineral. Immobilizing AQDS in agar also limited its diffusion, which resembled electron-transfer behavior of quinone- and hydroquinone-containing molecules in particulate NOM. We found that, although the diffusion coefficient of the immobilized AQDS/AH₂QDS was 10 times lower in agar than in water, the iron(III) mineral reduction rate (1.60 ± 0.28 mmol L^-1 Fe(II) d^-1) was still comparable in both media, indicating the existence of another mechanism that accelerated the electron transfer under low diffusive conditions. We found the correlation between the heterogeneous electron-transfer rate constant (10^-3 cm s^-1) and the diffusion coefficient (10^-7 cm² s^-1) fitting well with the "diffusion-electron hopping" model, suggesting that electron transfer via the immobilized AQDS/AH₂QDS couple was accomplished through a combination of diffusion and electron hopping. Electron hopping increased the diffusion concentration gradient up to 10⁶-fold, which largely promoted the overall electron-transfer rate during microbial iron(III) mineral reduction. Our results are helpful to explain the electron-transfer mechanisms in particulate NOM.

AQDS and Redox-Active NOM Enables Microbial Fe(III)-Mineral Reduction at cm-Scales

Redox-active organic molecules such as anthraquinone-2,6-disulfonate (AQDS) and natural organic matter (NOM) can act as electron shuttles thus facilitating electron transfer from Fe(III)-reducing bacteria (FeRB) to terminal electron acceptors such as Fe(III) minerals. In this research, we examined the length scale over which this electron shuttling can occur. We present results from agar-solidified experimental incubations, containing either AQDS or NOM, where FeRB were physically separated from ferrihydrite or goethite by 2 cm. Iron speciation and concentration measurements coupled to a diffusion-reaction model highlighted clearly Fe(III) reduction in the presence of electron shuttles, independent of the type of FeRB. Based on our fitted model, the rate of ferrihydrite reduction increased from 0.07 to 0.19 μmol d^-1 with a 10-fold increase in the AQDS concentration, highlighting a dependence of the reduction rate on the electron-shuttle concentration. To capture the kinetics of Fe(II) production, the effective AQDS diffusion coefficient had to be increased by a factor of 9.4. Thus, we postulate that the 2 cm electron transfer was enabled by a combination of AQDS molecular diffusion and an electron hopping contribution from reduced to oxidized AQDS molecules. Our results demonstrate that AQDS and NOM can drive microbial Fe(III) reduction across 2 cm distances and shed light on the electron transfer process in natural anoxic environments.

Redistribution of Electron Equivalents between Magnetite and Aqueous Fe2+ Induced by a Model Quinone Compound AQDS

The complex interactions between magnetite and aqueous Fe²⁺ (Fe²⁺_(aq)) pertain to many biogeochemical redox processes in anoxic subsurface environments. The effect of natural organic matter, abundant in these same environments, on Fe²⁺_(aq)-magnetite interactions is an additional complex that remains poorly understood. We investigated the influence of a model quinone molecule anthraquinone-2,6-disulfonate (AQDS) on Fe²⁺_(aq)-magnetite interactions by systematically studying equilibrium Fe²⁺_(aq) concentrations, rates and extents of AQDS reduction, and structural versus surface-localized Fe(II)/Fe(III) ratios in magnetite under different controlled experimental conditions. The equilibrium concentration of Fe²⁺_(aq) in Fe²⁺-amended magnetite suspensions with AQDS proportionally changes with solution pH or initial AQDS concentration, but independent of magnetite loadings through the solid concentrations that were studied here. The rates and extents of AQDS reduction by Fe²⁺-amended magnetite proportionally increased with solution pH, magnetite loading, and initial Fe²⁺_(aq) concentration, which correlates with the corresponding change of reduction potentials for the Fe²⁺-magnetite system. AQDS reduction by surface-associated Fe(II) in the Fe²⁺-magnetite suspensions induces solid-state migration of electron equivalents from particle interiors to the near-surface region and the production of nonmagnetic Fe(II)-containing species, which inhibits Fe²⁺_(aq) incorporation or electron injection into the magnetite structure. This study demonstrates the significant influence of quinones on reductive activity of the Fe²⁺-magnetite system.

The Colorful World of Extracellular Electron Shuttles

Descriptions of the changeable, striking colors associated with secreted natural products date back well over a century. These molecules can serve as extracellular electron shuttles (EESs) that permit microbes to access substrates at a distance. In this review, we argue that the colorful world of EESs has been too long neglected. Rather than simply serving as a diagnostic attribute of a particular microbial strain, redox-active natural products likely play fundamental, underappreciated roles in the biology of their producers, particularly those that inhabit biofilms. Here, we describe the chemical diversity and potential distribution of EES producers and users, discuss the costs associated with their biosynthesis, and critically evaluate strategies for their economical usage. We hope this review will inspire efforts to identify and explore the importance of EES cycling by a wide range of microorganisms so that their contributions to shaping microbial communities can be better assessed and exploited.

Theoretical estimation of redox potential of biological quinone cofactors

Redox potentials are essential to understand biological cofactor reactivity and to predict their behavior in biological media. Experimental determination of redox potential in biological system is often difficult due to complexity of biological media but computational approaches can be used to estimate them. Nevertheless, the quality of the computational methodology remains a key issue to validate the results. Instead of looking to the best absolute results, we present here the calibration of theoretical redox potential for quinone derivatives in water coupling QM + MM or QM/MM scheme. Our approach allows using low computational cost theoretical level, ideal for long simulations in biological systems, and determination of the uncertainties linked to the calculations. © 2017 Wiley Periodicals, Inc.

Quantifying TEMPO Redox Polymer Charge Transport toward the Organic Radical Battery

To design new and better organic active battery materials in a rational fashion, fundamental parameters of the charge transport must be studied. Herein we report on the electronic conductivity by electron diffusion in a TEMPO-containing redox polymer, and the reorganization energy of the TEMPO self-exchange in an organic solvent is determined for the first time. The electronic conductivity was 8.5 μS/cm at E⁰ and corresponded to a redox hopping mechanism. The apparent electron diffusion coefficient was 1.9 × 10^-9 cm²/s at room temperature, and at short times the ion diffusion was limiting with a diffusion coefficient of 6.5 × 10^-10 cm²/s. The reorganization energy was determined to be 1.01 eV, indicating a rather polar chemical environment for the TEMPO groups. The implications for the usage of this type of materials in organic energy storage are discussed. As conductivity through 10 μm was demonstrated, we show that, if sufficient swellability can be ensured, charge can be transported through several micrometer thick layers in a battery electrode without any conducting additive.

Tuning Interfacial Electron Transfer in Nanostructured Cuprous Oxide Photoelectrochemical Cells with Charge-Selective Molecular Coatings

The coating of nanostructured films of cuprous oxide with electroactive molecules strongly affects their photoelectrochemical performance in nonaqueous photocells, with photocurrent density increased up to an order of magnitude relative to bare cuprous oxide films or almost completely suppressed, depending on the choice of molecular adsorbant. Among adsorbants that enhance photocurrent, a strong variance of photoelectrochemical behavior is observed with changes in the molecular structure of the sensitizer, associated with differences in the reorganization energy and molecular size, which are interpreted to enhance forward electron transport and impede electrolyte/photocathode recombination, respectively. These results demonstrate that nanostructured cuprous oxide is a promising cathode material for p-type dye-sensitized solar cells.

Electrochemical analysis of proton and electron transfer equilibria of the reducible moieties in humic acids

Humic substances play a key role in biogeochemical and pollutant redox reactions. The objective of this work was to characterize the proton and electron transfer equilibria of the reducible moieties in different humic acids (HA). Cyclic voltammetry experiments demonstrated that diquat and ethylviologen mediated electron transfer between carbon working electrodes and HA. These compounds were used also to facilitate attainment of redox equilibria between redox electrodes and HA in potentiometric E(h) measurements. Bulk electrolysis of HA combined with pH-stat acid titration demonstrated that electron transfer to the reducible moieties in HA also resulted in proton uptake, suggesting decreasing reduction potentials E(h) of HA with increasing pH. This was confirmed by potentiometric E(h)-pH titrations of HA at different redox states. E(h) measurements of HA samples prereduced to different redox states by bulk electrolysis revealed reducible moieties in HA that cover a wide range of apparent standard reduction potentials at pH 7 from E(h)(0)* = +0.15 to -0.3 V. Modeling revealed an overall increase in the relative abundance of reducible moieties with decreasing E(h). The wide range of HA is consistent with its involvement in numerous environmental electron transfer reactions under various redox conditions.

The role of biomass, electron shuttles, and ferrous iron in the kinetics of Geobacter sulfurreducens-mediated ferrihydrite reduction

This work presents a new framework for describing biologically mediated reduction of thin layers of poorly crystalline iron oxides. The research here explores the nature of the biomass to surface area relationship and the role of biogenic ferrous iron during Geobacter sulfurreducens-mediated ferrihydrite reduction, with and without an electron shuttle, through experiments and a mathematical model. The results indicate that a saturating function of biomass most accurately describes the rate of iron reduction without electron shuttles, based on the principle of electron transfer via direct contact. This study also finds that the most appropriate model of iron reduction in the presence of electron shuttles includes both a saturating function of biomass for electron transfer via direct contact and a first-order electron transfer to ferrihydrite via the electron shuttle, strongly supporting the idea that G. sulfurreducens uses both pathways simultaneously. In all experiments, G. sulfurreducens reduced less than 60% of the total ferric iron, a phenomenon that has often been explained through the inhibitory effects of biogenic ferrous iron in the dissolved phase. However, through experiments with spikes of ferrous sulfate, this study suggests that the role of dissolved ferrous iron is passive in this case, and does not directly inhibit the extent of iron reduction in ferrihydrite coated sand. These experiments find that solid phase ferrous iron is the most probable primary product of ferrihydrite reduction, and that the conversion of solid ferric iron to solid ferrous iron depletes a fixed pool of bioavailable ferric iron, thereby accounting for the incomplete reduction of ferric iron observed here. This is the first reported model that explicitly treats solid ferrous iron as the primary product of reduction, with aqueous ferrous iron as a passive byproduct. This simple mathematical model readily translates to other systems of microbially mediated iron reduction.

Oxidation-reduction transformations of chromium in aerobic soils and the role of electron-shuttling quinones

Oxidation of Cr(III) and reduction of Cr(VI) can occur simultaneously in aerobic soils, but the mechanisms involved are not well-understood, especially how electron shuttling by redox-active organic acids is involved. A and B soil horizons from three topohydrosequences from the Coastal Plain and Piedmont physiographic provinces of Maryland were chosen to investigate oxidation-reduction transformations of Cr under field moist conditions. Reduction of added Cr(VI) to Cr(III) was observed in all 18 samples, and 11 demonstrated enhanced reduction with added anthraquinone-2,6-disulfonate (AQDS) acting as an electron shuttle in 24 h quick tests under aerobic conditions. Oxidation of Cr(III) to Cr(VI) was observed in 12 samples, with 7 demonstrating diminished oxidation with AQDS added. Cr(VI) was undetectable after 11 d of incubation when lactic acid was added as a reductant for Cr(VI) to the Watchung soil A horizon. This reduction occurred in the presence of AQDS and a high salt background to suppress microbial growth, suggesting abiotic reduction was the dominant pathway. The results of this study demonstrate that in field-moist, aerobic soils, the electron shuttle, AQDS, enhanced Cr(VI) reduction and inhibited Cr(III) oxidation. This suggests redox-active organic C amendments and electron shuttles can be important in enhancing rates and extent of Cr(VI) reduction, while inhibiting Cr(III) oxidation in the in situ remediation of Cr(VI)-contaminated soils.

Accurate estimation of the one-electron reduction potentials of various substituted quinones in DMSO and CH3CN

The one-electron reduction potentials of 116 important p- and o-quinones in DMSO and CH(3)CN were predicted for the first time by using the B3LYP/DZP++ method and the PCM cluster continuum model. The calculated gas-phase electron affinities and one-electron reduction potentials agree well with the available experimental observations, respectively. The study showed the one-electron reduction potentials of the 116 quinones range from -0.949 to 1.128 V in DMSO and from -0.904 to 0.971 V in CH(3)CN. The one-electron reduction potentials of p-quinones are generally smaller than those of the o-quinones by about 0.132 V. For quinones with aromatic properties, 2-substituted-1,4-naphthquinones have the largest one-electron reduction potentials, followed by substituted-1,4-anthraquinones and then by substituted-9,10-anthraquinones. The study also showed that the one-electron reduction potentials of quinones in DMSO are linearly dependent on the sum of the Hammett substituent parameters sigma(p): E(NHE)(p-Q/p-Q(*-)) = 0.45Sigma sigma(p) - 0.194 (V) and E(NHE)(o-Q/o-Q(*-)) = 0.45Sigma sigma(p) - 0.059 (V). Combined with the hydride affinities of quinones in the former paper [DeltaG(H)-(A)(p-Q) = -16.0Sigma sigma(p) - 70.5 (kcal/mol) and DeltaG(H)-(A)(o-Q) = -16.2Sigma sigma(p) - 81.5 (kcal/mol)] and the one-electron reduction potentials of quinones estimated in this work, we obtained the homolytic bond dissociation energies of the hydroquinone anions (QH(-)) and found that these thermodynamic parameters also have linear correlations against the sum of the Hammett substituent parameters sigma(p) if only the substituents have no larger electrostatic inductive force and no large steric hindrance: BDE(p-QH(-)) = 5.05Sigma sigma(p) + 63.18 (kcal/mol) and BDE(o-QH(-)) = 5.33Sigma sigma(p) + 71.30 (kcal/mol). Knowledge about the redox potentials of the quinones should be of great value for the understanding of the nature of chemical reactions of quinones, the designing of new electronic materials of quinones, and the examining of biological activities of quinones.

Redox potentials and pKa for benzoquinone from density functional theory based molecular dynamics

The density functional theory based molecular dynamics (DFTMD) method for the computation of redox free energies presented in previous publications and the more recent modification for computation of acidity constants are reviewed. The method uses a half reaction scheme based on reversible insertion/removal of electrons and protons. The proton insertion is assisted by restraining potentials acting as chaperones. The procedure for relating the calculated deprotonation free energies to Brønsted acidities (pK(a)) and the oxidation free energies to electrode potentials with respect to the normal hydrogen electrode is discussed in some detail. The method is validated in an application to the reduction of aqueous 1,4-benzoquinone. The conversion of hydroquinone to quinone can take place via a number of alternative pathways consisting of combinations of acid dissociations, oxidations, or dehydrogenations. The free energy changes of all elementary steps (ten in total) are computed. The accuracy of the calculations is assessed by comparing the energies of different pathways for the same reaction (Hess's law) and by comparison to experiment. This two-sided test enables us to separate the errors related with the restrictions on length and time scales accessible to DFTMD from the errors introduced by the DFT approximation. It is found that the DFT approximation is the main source of error for oxidation free energies.

Voltammetric characterization of DNA intercalators across the full pH range: anthraquinone-2,6-disulfonate and anthraquinone-2-sulfonate

The use of anthraquinone and its derivatives, notably the sulfonate and disulfonate salts, for the detection of DNA via electrochemical techniques, has been the focus of a number of recent articles. This study provides a quantitative model of the two redox systems of anthraquinone-2,6-disulfonate and anthraquinone-2-sulfonate, over the full aqueous pH range (0-13); the model is based upon the theoretical "scheme of squares" for a 2H(+), 2e(-) system, as first proposed by Jacq (Jacq, J. J. Electroanal. Chem. 1971, 29, 149-180). The effect of pH and ionic strength on the observed cyclic voltammetry was investigated experimentally. The variation of the electrochemical response with proton concentration was modeled through use of the commercially available simulation software, DIGISIM; the system was successfully fitted with attention to voltammetric peak height, position, width, and shape. The model demonstrates how the pK(a) values of the anthraquinone intermediates dominate the observed pH dependence of the voltammetry. At high pH (above pH 12), a simple EE process is found to occur. As the pH decreases, the formation of other protonated species becomes possible; this not only causes a Nernstian shift in the measured electrochemical potential for the redox couple but also results in changes in the mechanistic pathway. At pH 10, an EECC process dominates, as the pH is further lowered into the range 4-7, the overall mechanism is an ECEC process, and finally a CECE mechanism operates at around pH 1 and below. This work provides physical insight into the complex mechanistic pathways involved and will aid the future development of more sophisticated and accurate anthraquinone based DNA sensors.

Anaerobic sulfur oxidation in the absence of nitrate dominates microbial chemoautotrophy beneath the pelagic chemocline of the eastern Gotland Basin, Baltic Sea

Oxic-anoxic interfaces harbor significant numbers and activity of chemolithoautotrophic microorganisms, known to oxidize reduced sulfur or nitrogen species. However, measurements of in situ distribution of bulk carbon dioxide (CO(2)) assimilation rates and active autotrophic microorganisms have challenged the common concept that aerobic and denitrifying sulfur oxidizers are the predominant autotrophs in pelagic oxic-anoxic interfaces. Here, we provide a comparative investigation of nutrient, sulfur, and manganese chemistry, microbial biomass distribution, as well as CO(2) fixation at the pelagic redoxcline of the eastern Gotland Basin, Baltic Sea. Opposing gradients of oxygen, nitrate, and sulfide approached the detection limits at the chemocline at 204 m water depth. No overlap of oxygen or nitrate with sulfide was observed, whereas particulate manganese was detected down to 220 m. More than 70% of the bulk dark CO(2) assimilation, totaling 9.3 mmol C m(-2) day(-1), was found in the absence of oxygen, nitrite, and nitrate and could not be stimulated by their addition. Maximum fixation rates of up to 1.1 mumol C L(-1) day(-1) were surprisingly susceptible to altered redox potential or sulfide concentration. These results suggest that novel redox-sensitive pathways of microbial sulfide oxidation could account for a significant fraction of chemolithoautotrophic growth beneath pelagic chemoclines. A mechanism of coupled activity of sulfur-oxidizing and sulfur-reducing microorganisms is proposed.

A spectroscopic study of the effect of ligand complexation on the reduction of uranium(VI) by anthraquinone-2,6-disulfonate (AH2DS)

In this paper, the reduction rate of uranyl complexes with hydroxide, carbonate, EDTA, and desferriferrioxamine B (DFB) by anthraquinone-2,6-disulfonate (AH2DS) is studied by stopped-flow kinetic technique under anoxic atmosphere. The apparent reaction rates varied with ligand type, solution pH, and U(VI) concentration. For each ligand, a single largest pseudo −1st order reaction rate constant, k obs, within the studied pH range was observed, suggesting the influence of pH-dependent speciation on the U(VI) reduction rate. The maximum reaction rate found in each case followed the order of OH−>CO3 2−>EDTA>DFB, in reverse order of the trend of the thermodynamic stability of the uranyl complexes and ionic sizes of the ligands. The pH-dependent rates were modeled using a second-order rate expression that was assumed to be dependent on a single U(VI) complex and an AH2DS species. By quantitatively comparing the calculated and measured apparent rate constants as a function of pH, species AHDS3− was suggested as the primary reductant in all cases examined. Species UO2CO3(aq), UO2HEDTA−, and (UO2)2(OH)2 2+ were suggested as the principal electron acceptors among the U(VI) species mixture in each of the carbonate, EDTA, and hydroxyl systems, respectively.

The roles of outer membrane cytochromes of Shewanella and Geobacter in extracellular electron transfer

As key components of the electron transfer (ET) pathways used for dissimilatory reduction of solid iron [Fe(III)] (hydr)oxides, outer membrane multihaem c-type cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 and OmcE and OmcS of Geobacter sulfurreducens mediate ET reactions extracellularly. Both MtrC and OmcA are at least partially exposed to the extracellular side of the outer membrane and their translocation across the outer membrane is mediated by bacterial type II secretion system. Purified MtrC and OmcA can bind Fe(III) oxides, such as haematite (α-Fe2 O3 ), and directly transfer electrons to the haematite surface. Bindings of MtrC and OmcA to haematite are probably facilitated by their putative haematite-binding motifs whose conserved sequence is Thr-Pro-Ser/Thr. Purified MtrC and OmcA also exhibit broad operating potential ranges that make it thermodynamically feasible to transfer electrons directly not only to Fe(III) oxides but also to other extracellular substrates with different redox potentials. OmcE and OmcS are proposed to be located on the Geobacter cell surface where they are believed to function as intermediates to relay electrons to type IV pili, which are hypothesized to transfer electrons directly to the metal oxides. Cell surface-localized cytochromes thus are key components mediating extracellular ET reactions in both Shewanella and Geobacter for extracellular reduction of Fe(III) oxides.

Arsenic redox changes by microbially and chemically formed semiquinone radicals and hydroquinones in a humic substance model quinone

Arsenic is a redox-active metalloid whose toxicity and mobility strongly depends on its oxidation state, with arsenite (As(III)) being more toxic and mobile than arsenate (As(V)). Humic substances (HS) are also redox-active and can potentially react with arsenic and change its redox state. In this study we show that semiquinone radicals produced during microbial or chemical reduction of a HS model quinone (AQDS, 9,10-anthraquinone-2,6-disulfonic acid) are strong oxidants. They oxidize arsenite to arsenate, thus decreasing As toxicity and mobility. This reaction depends strongly on pH with more arsenite (up to 67.3%) being oxidized at pH 11 compared to pH 7 (12.6% oxidation) and pH 3 (0.5% oxidation). In addition to As(III) oxidation by semiquinone radicals, hydroquinones that were also produced during quinone reduction reduced As(V) to As(III) at neutral and acidic pH values (less than 12%) but not at alkaline pH. In order to understand redox reactions between arsenite/arsenate and reduced/oxidized HS, we quantified the radical content in reduced quinone solutions and constructed Eh-pH diagrams that explain the observed redox reactions. The results from this study can be used to better predict the fate of arsenic in the environment and potentially explain the occurrence of oxidized As(V) in anoxic environments.

Kinetics of reduction of Fe(III) complexes by outer membrane cytochromes MtrC and OmcA of Shewanella oneidensis MR-1

Because of their cell surface locations, the outer membrane c-type cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 have been suggested to be the terminal reductases for a range of redox-reactive metals that form poorly soluble solids or that do not readily cross the outer membrane. In this work, we determined the kinetics of reduction of a series of Fe(III) complexes with citrate, nitrilotriacetic acid (NTA), and EDTA by MtrC and OmcA using a stopped-flow technique in combination with theoretical computation methods. Stopped-flow kinetic data showed that the reaction proceeded in two stages, a fast stage that was completed in less than 1 s, followed by a second, relatively slower stage. For a given complex, electron transfer by MtrC was faster than that by OmcA. For a given cytochrome, the reaction was completed in the order Fe-EDTA > Fe-NTA > Fe-citrate. The kinetic data could be modeled by two parallel second-order bimolecular redox reactions with second-order rate constants ranging from 0.872 microM(-1) s(-1) for the reaction between MtrC and the Fe-EDTA complex to 0.012 microM(-1) s(-1) for the reaction between OmcA and Fe-citrate. The biphasic reaction kinetics was attributed to redox potential differences among the heme groups or redox site heterogeneity within the cytochromes. The results of redox potential and reorganization energy calculations showed that the reaction rate was influenced mostly by the relatively large reorganization energy. The results demonstrate that ligand complexation plays an important role in microbial dissimilatory reduction and mineral transformation of iron, as well as other redox-sensitive metal species in nature.

Substituent effect on a family of quinones in aprotic solvents: an experimental and theoretical approach

In this work a comparison between redox potentials, obtained by constructing current-potential plots from chronoamperometric measurements, and the parameter sigma(x), as proposed by Zuman in terms of the Hammett substituent parameters, was performed for several quinone compounds. This study shows the limitations of this approach and proves that methods based on quantum chemistry can be used to study the substituent effect in quinone systems. By using the Density Functional Theory, in the Kohn-Sham context with three exchange-correlation functionals, BLYP, B3LYP, and BHLYP, it was found that the electron affinity is good enough to give a useful relationship with experimental redox potentials of quinone systems. This conclusion is reached when the basis set functions involve diffuse functions, and also when the Hartree-Fock exchange energy is included in the exchange-correlation functional. The Fukui function, to describe preferential sites involved at initial stages of a system that bind an electron, is analyzed when electron donor and electron acceptor groups are present as substituents in quinone systems. The methods applied in this work are valid for any kind of quinone compound and will be used in further analysis of the electron reorganization in semiquinone species.

Quantum chemical modeling of the reduction of quinones

A systematic study of the redox properties of six parent quinones has been carried out using quantum chemistry methods. The reduction of the ortho (o-) and para (p-) isomers of benzoquinone and naphthoquinone, 9,10- anthraquinone and 9,10-phenantrenequinone to the corresponding hydroquinones and semiquinone radicals was investigated at the B3LYP/6-311+G(d,p) level of theory. Thermodynamic functions in the gas-phase were calculated for all the reduction reactions. Gibbs energies of reaction and standard potentials in water for the reductions were determined using the IEF-PCM model and an empirical correction to the calculations based on the limited thermodynamic data available for the quinones. Potentials were calculated both for the direct reduction to the quinols, and for the two-step reduction via the neutral semiquinones. The calculated potentials for the 2e-, 2H+ reductions were found in good agreement with experiment and to display the same trends as gas-phase enthalpies and energies, i.e., to correlate with the number of C=C double bonds, as well as on the relative position of the C=O groups. The small deviations between experiment and theoretically predicted standard potentials were found to originate from basis set incompleteness and the shortcomings in the B3LYP exchange correlation functional rather than the models used for the thermochemical calculations or description of solvation. Accurate theoretical shifts in standard potentials for the p-/o- pairs of Q <--> HQ and HQ <--> H2Q reactions are presented and compared to experiment. Reliable standard potentials and shifts for the neutral semiquinones are predicted for the first time.

Theoretical Prediction of the Hydride Affinities of Various p- and o-Quinones in DMSO

The hydride affinities of 80 various p- and o-quinones in DMSO solution were predicted by using B3LYP/6-311++G (2df,p)//B3LYP/6-31+G* and MP2/6-311++G**//B3LYP/6-31+G* methods, combined with the PCM cluster continuum model for the first time. The results show that the hydride affinity scale of the 80 quinones in DMSO ranges from -47.4 kcal/mol for 9,10-anthraquinone to -124.5 kcal/mol for 3,4,5,6-tetracyano-1,2-quinone. Such a long scale of the hydride affinities (-47.4 to -124.5 kcal/mol) indicates that the 80 quinones can form a large and useful library of organic oxidants, which can provide various organic hydride acceptors that the hydride affinities are known for chemists to choose in organic syntheses. By examining the effect of substituent on the hydride affinities of quinones, it is found that the hydride affinities of quinones in DMSO are linearly dependent on the sum of the Hammett substituent parameters sigma: DeltaGH-(Q) approximately -16.0Sigmasigmai - 70.5 (kcal/mol) for p-quinones and DeltaGH-(Q) approximately -16.2Sigmasigmai - 81.5 (kcal/mol) for o-quinones only if the substituents have no large electrostatic inductive effect and large ortho-effect. Study of the effect of the aromatic properties of quinone on the hydride affinities showed that the larger the aromatic system of quinone is, the smaller the hydride affinity of the quinone is, and the decrease of the hydride affinities is linearly to take place with the increase of the number of benzene rings in the molecule of quinones, from which the hydride affinities of aromatic quinones with multiple benzene rings can be predicted. By comparing the hydride affinities of p-quinones and the corresponding o-quinones, it is found that the hydride affinities of o-quinones are generally larger than those of the corresponding p-quinones by ca. 11 kcal/mol. Analyzing the effect of solvent on the hydride affinities of quinones showed that the effects of solvent (DMSO) on the hydride affinities of quinones are mainly dependent on the electrostatic interaction of the charged hydroquinone anions (QH-) with solvent (DMSO). All the information disclosed in this work should provide some valuable clues to chemists to choose suitable quinones or hydroquinones as efficient hydride acceptors or donors in organic syntheses and to predict the thermodynamics of hydride exchange between quinones and hydroquinones in DMSO solution.