On the nature of factors determining reactivity of strongly bound surface species as intermediates in heterogeneous catalytic redox reactions

The rate-limiting stage of heterogeneous catalytic redox reactions with participation of strongly bound surface species, such as nitrates, carbonates and sulfites, is the interaction of these heteroatomic oxidants with the reducing agents. It appears to be mainly determined by their bonding strength with the surface, being independent of the nature of the central atom (carbon, sulfur, nitrogen) and the energy of its bond with oxygen.     

Surface effects of monolayer-protected gold nanoparticles on the redox reactions between ferricyanide and thiosulfate

The intense research interest in nanosized particle materials is mainly fueled by their unique physical, chemical, electronic, and magnetic properties[1]. Among these, they have shown great application po-tentialities in the catalytic regulation of elec-t…     

Synthesis, structure, and both cathodic and anodic reversible redox reactions of benzochalcogenophenes containing ferrocene units

2,3-Diferrocenylbenzo[b]thiophene and 1,3-diferrocenylbenzo[c]thiophene have been systematically and selectively synthesized from benzo[b]thiophene and phthaloyl dichloride, respectively. Characterization of the molecules was performed by physical and spectroscopic means and X-ray crystallographic analyses. The cyclic voltammograms of the novel thiophene derivatives containing ferrocene fragments showed a well-defined reversible cathodic step derived from the unusually stable thiophene radical anions and two distinct reversible anodic steps derived from ferrocenium cations separated from each other by a thiophene heterocycle. 1,3-Diferrocenylbenzo[c]selenophene was also synthesized in a similar manner for formation of 1,3-diferrocenylbenzo[c]thiophene by the use of bis(dimethylaluminum) selenide as a selenating reagent.     

The relationship between the redox reaction of camphor‐induced cytochrome p‐450 and its activity

The relationship between the redox reaction of camphor‐induced cytochrome P‐450 (P‐450_cam) and its activity was measured by using cyclic voltammetry. The redox potential of P‐450_cam solution shifted to the lower side of the potential by binding of substrate, and the change was proportional to the amount of the substrate binding to the protein. The substrate binding was inhibited at the low concentration of oxygen in the reaction solution. The reaction product, hydroxycamphor, was observed in the reaction mixture by gas chromatography/mass spectroscopy. However, hydroxycamphor was not observed at an oxygen concentration of about a tenth part of the saturated one. The shift of redox potential of P‐450_cam solution corresponded to the substrate specificity of the activity. These results suggest that the redox reaction of P‐450_cam related to the substrate‐binding to the protein and its activity. Furthermore, the present system was very simple and speedy for the measurement of the activity. Copyright © 1999 John Wiley & Sons, Ltd.     

A novel electrochemical approach to the characterization of oxidoreductase reactions

Electrochemical methods based on enzyme-electrochemical reactions have been developed for studying oxidoreductase reactions. The methods measure a current resulting from an oxidoreductase reaction with an electrode serving as a final electron acceptor (or donor) in the reaction. A theoretical equation for the enzyme-electrochemical reaction, called bioelectrocatalysis, is derived, which enables kinetic analysis of the reaction. In combination with spectrophotometry, the electrochemical method provides a method for determining the redox potentials of proteins and enzymes. An alternative method based on bulk electrolysis in a quartz cell for UV-vis spectroscopy has been developed for the measurements of protein redox potentials on a conventional spectrophotometer. The electrochemical methods are applied to kinetic and thermodynamic analyses for the reactions of a variety of enzymes including a newly discovered enzyme, quinohemoprotein amine dehydrogenase (QH-AmDH), and bilirubin oxidase (BOD) [EC 1.3.3.5, from Myrothecium verrucaria], a copper-containing enzyme useful for bioelectrocatalytic O(2) reduction in biofuel cells. The electrochemical method for kinetic analysis has been successfully applied to the analysis of oxidoreductase reactions in vivo, as demonstrated by the reaction of glucose dehydrogenase in Escherichia coli. The advantages of the electrochemical methods are discussed.     

DFT analysis of the active site in catalytic metabolic redox reactions of mononuclear molybdenum enzymes

Abstract

Model complexes [Mo^VIO₂(S₂C₂Me₂)SMe]^? (A, derived from the X-ray crystal structure of native sulfite oxidase (SO)) and [Mo^VIO₂(mnt)₂]^2? (B, coordination mode similar to the active site of selenate reductase (SeR)) were computed at the COSMO-B3LYP/SDDp//B3LYP/Lanl2DZ(p) energy level of Density Functional Theory in order to study their behavior in oxidation of selenite (Se^IV) and sulfite (S^IV) to selenate (Se^VI) and sulfate (S^VI), respectively. For the oxidation of sulfite, computational model A, which resembles the SO active site, is clearly the best choice (lowest barrier, minor exothermicity). For the reduction of selenate, a smaller activation is computed for model A; however, the reaction is less exothermic with model B, which resembles the SeR active site.     

Determination of Nitrite in Aqueous Solutions using the Linear Sweep Voltammetry Technique

ABSTRACT

An electrochemical method using linear sweep voltammetry techniques was developed to determine nitrite ion in aqueous solution in the presence of nitrate. Nitrite solutions exhibited a well-defined oxidation wave at +1.0V vs SCE at vitreous carbon, while no oxidation process was observed for nitrate solutions. The pH of the nitrite solutions varied from 2.37 to 5.60 and no change was observed in the Ep values, except for the pH 5.60 solution, where little change was verified. The potential also did not vary with change in the nitrite concentration in the 5.0 x 10^?5 to 7.0 x 10^?4 mol L^?1 range. Very good straight lines for plots of current versus nitrite concentration in the 7.0 x 10^?5 to 7.0 x 10^?4 mol L^?1 range were obtained; the correlation coefficient was never worse than 0.990. The nitrite determination was also performed in the presence of 1.0 x 10^?3 mol L^?1 of NO₃ ^? ions. The addition of NO₃ ^? did not change significantly the current values even when it was added in one hundred times molar excess.     

Sensitively probing the cofactor redox species and photo-induced electron transfer of wild-type and pheophytin-replaced photosynthetic proteins reconstituted in self-assembled monolayers

An ultrathin, ordered, and packed protein film, consisting of the 2-mercaptoacetic acid (MAA), polydimethyldiallylammonium chloride (PDDA), and wild-type (WT) photosynthetic reaction center (RC; termed as WT-RC) or its pheophytin (Phe)-replaced counterpart (termed as Phe-RC), was fabricated by self-assembling technique onto gold electrode for facilitating the electron transfer (ET) between RC and the electrode surface. Near-infrared (NIR)-visible (Vis) absorption and fluorescence (FL) emission spectra revealed the influence of pigment substitution on the cofactors arrangement and excitation relaxation of the proteins, respectively. Square wave voltammetry (SWV) and photoelectric tests were employed to systematically address the differences between the WT-RC films and mutant ones on the direct and photo-induced ET. The electrochemical results demonstrated that ET initiated by the oxidation of the primary donor (P) was obviously slowed down, and the formed P⁺ had more population as well as more positive redox potential in the Phe-RC films compared with those in the WT ones. The photoelectrochemical results displayed the dramatically enhanced photoelectric performances of the mutant ones, further suggesting the slow-down formation of final charge-separated state in Phe-RC. The functionalized protein films introduced in this paper provided an efficient approach to sensitively probe the redox cofactors and ET differences resulting from only minor changes in pigment arrangement in the pigment–protein complex. The favored ET process observed for the membrane proteins RC was potentially valuable for a deep understanding of the multi-step biological ET process and development of versatile bioelectronic devices.     

Predicting the redox properties of uranyl complexes using electronic structure calculations

A plethora of chemical reactions is redox driven processes. The conversion of toxic and highly soluble U(VI) complexes to nontoxic and insoluble U(IV) form are carried out through proton coupled electron transfer by iron containing cytochromes and mineral surfaces such as machinawite. This redox process takes place through the formation of U(V) species which is unstable and immediately undergo the disproportionation reaction. Thus, theoretical methods are extremely useful to understand the reduction process of U(VI) to U(V) species. We here have carried out the structures and reduction properties of several U(VI) to U(V) complexes using a variety of electronic structure methods. Due to the lack of experimental ionization energies for uranyl (UO₂(V)‐UO₂(VI)) couple, we have benchmarked the current and popularly used density functionals and cost effective ab initio methods against the experimental electron detachment energies of [UO₂F₄]^1‐/2‐ and [UO₂Cl₄]^1‐/2‐. We find that electron detachment energy of U(VI) predicted by RI‐MP2 level on the BP86 geometries correlate nicely with the experimental and CCSD(T) data. Based on our benchmark studies, we have predicted the structures and electron detachment energies of U(V) to U(VI) species for a series of uranium complexes at the RI‐MP2//BP86 level which are experimentally inaccessible till date. We find that the redox active molecular orbital is ligand centered for the oxidation of U(VI) species, where it is metal centered (primarily f‐orbital) for the oxidation of U(V) species. Finally, we have also calculated the detachment energies of a known uranyl [UO₂]¹⁺ complex whose X‐ray crystal structures of both oxidation states are available. The large bulky nature of the ligand stabilizing the uncommon U(V) species which cannot be routinely studied by present day CCSD(T) methods as the system size are more than 20–30 atoms. The success of our efficient computational strategy can be experimentally verified in the near future for the complex as the structures are stable in gas phase which can undergo oxidation.     

A novel kinetic determination of nitrite based on the perphenazine-bromate redox reaction

An extremely sensitive and selective kinetic method was developed for the determination of trace levels of nitrite based on its catalytic effect on the oxidation of perphenazine (PPZ) with bromate in a phosphoric acid medium. The reaction rate was monitored spectrophotometrically by tracing the formation of the red-colored oxidized product of PPZ at 525 nm within 30 sec of mixing. The optimum reaction conditions were 4.0 μmol L⁻¹ PPZ, 0.4 mol L⁻¹ H₃PO₄ and 30 mmol L⁻¹ bromate at 25 °C. Using the recommended procedure, nitrite could be determined with a linear calibration graph up to 4.50 ng mL⁻¹ and a detection limit of 0.07 ng mL⁻¹. The method was conveniently applied to the determination of nitrite in samples of rain, polluted well and formulated waste waters. Moreover, the published kinetic spectrophotometric methods for nitrite determination are reviewed.     

Cyclic voltammetric studies of the redox behavior of iron(III)-vitamin B6 complex at carbon paste electrode

Electrochemical redox behavior of Fe-vit B₆ complex is investigated in HEPES buffer in the pH range 5.1–13.1 using cyclic voltammetry. Well-defined anodic and cathodic peaks are observed in the voltammograms at pH 13.1. At pH 8.0, only one cathodic peak and at pH 5.1, only one anodic peak are found. At all the pH values, the peak potential separation is much higher than that of a reversible electrochemical reaction. The peak current ratio (i _pa/i _pc) is less than unity and decreases with the scan rate. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 6, pp. 691–697. The text was submitted by the authors in English.     

Square-wave Voltammetry and Electrochemical Faradaic Spectroscopy of a Reversible Electrode Reaction: Determination of the Concentration Fraction of the Redox Couple

A comparative study conducted with square-wave voltammetry (SWV) and electrochemical faradaic spectroscopy (EFS) is presented for a reversible electrode reaction of dissolved redox couple in the presence of both Ox and Red components. In agreement with previous studies, the net peak current ΔΨ_p of the theoretical SW voltammograms is positioned at the formal potential E^0′ and does not depend on the concentration ratio c(Ox)/c(Red). However the forward-to-backward peak current ratio is sensitive to the redox state. For very low SW amplitudes, theoretical data imply superior features of EFS over SWV. Theoretical and experimental calibration lines are in good agreement within the interval 0.2≤x(Ox)≤0.8.     

A Comparative Study of Functionalized High‐Purity Carbon Nanotubes towards the V(IV)/V(V) Redox Reaction Using Cyclic Voltammetry and Scanning Electrochemical Microscopy

Functionalized high purity carbon nanotubes (CNTs) with various amounts of oxygen containing surface groups were investigated towards the relevant redox reactions of the all‐vanadium redox flow battery. The quinone/hydroquinone redox peaks between 0.0 and 0.7 V vs. Ag|AgCl|KCl_sat. were used to quantifying the degree of functionalization and correlated to XPS results. Cyclic voltammetry in vanadyl sulfate‐containing 3 M H₂SO₄ as a common supporting electrolyte showed no influence of the amount of surface groups on the V(IV)/V(V) redox system. In contrast, the reactions occurring at the negative electrode (V(II)/V(III) and V(III)/V(IV)) are strongly affected by oxygen surface groups. However, under modified experimental conditions, SECM experiments detecting the consumption of VO²⁺ molecules by CNT thin films in pH=2 solution show improved onset potentials with increased surface oxygen content up to ∼ 3 at%. Further increase in surface oxygen up to 8 at% led to minor improvement. These dissimilar results under different experimental conditions are rationalized by suggesting that oxygen functional groups do not form the active site for the V(IV)/V(V) reaction but wetting of the catalyst layer is of high importance.     

Insight into the extractability of metals from soils using an implementation of the linear adsorption isotherm model

This paper deals with an often overlooked artifact in sequential and single extraction of metals from soils, viz. the volume to mass (V/m) ratio as a potential source for inadequate extraction yields. We offer a theoretical framework to get a grip on this intricate parameter and came up with a model based on a linear adsorption isotherm to derive the correct maximal metal extractability for a certain extractant. We verified the model experimentally using 0.1 mol l⁻¹ nitric acid for extraction of seven metals (Cr, Co, Cu, Cd, Pb, Ni and Zn) from an urban soil sample, and concluded that commonly used V/m ratios in the range of 10-40 ml g⁻¹ may give as much as 50% too low extraction yields. Thus, a strong caveat is in place as to be very critical what V/m ratios to use and preferably apply the model derived to obtain the correct maximal extractability using a variable V/m ratio method.     

Rapid determination of reduced and oxidized glutathione levels using a new thiol-masking reagent and the enzymatic recycling method: application to the rat liver and bile samples

A microtiter plate assay for quantitation of reduced (GSH) and oxidized (GSSG) glutathione in the rat liver tissue and bile is described. The assay is based on the established enzymatic recycling method and a new thiol-masking reagent, 1-methyl-4-vinyl-pyridinium trifluoromethane sulfonate (M4VP). Samples were first processed by homogenization with (liver) or addition of (bile) sulfosalicylic acid. The total glutathione and GSSG were then determined before and after rapid (≤2 min) and efficient (100%) masking of the GSH content of the samples with M4VP followed by the enzymatic recycling assay. The percentages of error and coefficient of variation of the assay were within the accepted guidelines, indicating the accuracy and precision of the assay in the range of 6.25–100 pmol GSH per microplate well and 2.17–140 pmol GSSG per well, with lower limit of quantitation of 6.25 and 2.17 pmol per well for GSH and GSSG, respectively. Furthermore, the recoveries of added GSH or GSSG from the liver and bile samples were accurate and precise. The assay was applied to measurement of GSH, GSSG, and GSH:GSSG ratio in the liver and serially collected bile samples in sham-operated and ischemic rat livers, demonstrating a depletion of glutathione and a decrease in the GSH:GSSG ratio as a result of ischemia. The developed assay is rapid, sensitive, accurate, and precise and is suitable for studies of the redox status of liver under physiologic and pathophysiologic conditions.     

On the reactions of methyl radicals with TiO2 nanoparticles and granular powders immersed in aqueous solutions

Methyl radicals react with TiO(2) nanoparticles (NPs) immersed in aqueous solutions to form transients in which the methyls are covalently bound to the particles. The rate constant for this reaction approaches the diffusion-controlled limit and increases somewhat with the number of methyls bound to the particle. The transients decompose to yield ethane. Thus, formally the particles "catalyse" the dimerization of the radicals, a reaction that is diffusion-controlled. Rutile powders behave similarly to the TiO(2) NPs whereas the mechanism for the decomposition of the transients formed in the analogous reaction of the radicals with anatase powders differs. These results are of importance as alkyl radicals are formed near the surface of TiO(2) in a variety of important photocatalytic processes. The results imply that the reactions of alkyl radicals with TiO(2) have to be considered in these processes.     

Enantioselective total synthesis of octalactin a using asymmetric aldol reactions and a rapid lactonization to form a medium-sized ring

Octalactin A, an antitumor agent containing an eight-membered lactone moiety, has been stereoselectively prepared by means of enantioselective aldol reactions of selected silyl enolates with achiral aldehydes, promoted by a chiral Sn(II) complex. The medium-sized lactone part was effectively constructed by way of a new and rapid mixed-anhydride lactonization using 2-methyl-6-nitrobenzoic anhydride (MNBA) with a catalytic amount of 4-(dimethylamino)pyridine (DMAP) or 4-(dimethylamino)pyridine 1-oxide (DMAPO). The use of only 5 mol % of DMAP or 2 mol % of DMAPO rapidly promoted formation of the medium-sized ring of the octalactin, demonstrating the remarkable efficiency of the new lactonization protocol.     

The [Cu]-catalyzed SNAR reactions: direct amination of electron deficient aryl halides with sodium azide and the synthesis of arylthioethers under Cu(II)-ascorbate redox system

A one pot [Cu]-promoted S_NAr reaction of electron-deficient halobenzenes with sodium azide and the reduction of the intermediate aryl azides under the same Cu(II)-ascorbate redox conditions leading to anilines has been documented. Control experiments revealed that both ascorbate and proline play important role in the reaction path way. Further, the use of this catalytic Cu(II)-ascorbate redox system has been explored for the synthesis of arylthioethers.