Kinetics of oxidation of hydrogen peroxide at hemin-modified electrodes in nonaqueous solvents

Hemin adsorbed on graphite electrodes and used to catalyse the reduction of hydrogen peroxide in an aqueous buffer and in a range of nonaqueous solvents has been described. The immobilised hemin is stable in the solvents examined. The rate limiting step involves the reaction between hemin and hydrogen peroxide. Kinetic analysis of the response in nonaqueous solvents showed that I_max / K_m^app increased linearly with the solvent hydrophobicity (log P) in all solvents, a trend that is explained by preferable partitioning of hydrogen peroxide into the polar hemin layer.     

A New Colorimetric Reagent for Carbon Monoxide

Abstract

A new colorimetric reagent for carbon monoxide is described. The reaction sequence apparently involves reduction by carbon monoxide of [PdCl₄]^2-, which then reduces [Fe^III EDTA]^? to [Fe^IIEDTA]^2-. The latter undergoes ligand exchange with 2,2′-dipyridyl or 1,10-phenanthroline to form the more stable colored [FeL₃]²⁺, which is measured spectrophotometrically. Sodium molybdate enhances the overall reaction. The color intensity produced is non-linear but reproducible. The method is unique in that a soluble colored compound is produced rather than a metal sol.     

Electrochemical determination of dioxygen and hydrogen peroxide using Fe3O4@SiO2@hemin microparticles

Iron oxide microparticles were coated with 3-aminopropyltriethoxysilane and then coated with hemin via an amidation reaction. The resulting composite particles were characterized by transmission electron microscopy. FTIR spectroscopy revealed two bands (at 1,701 and 1,634 cm^?1), which were assigned to the carboxy group and the amide linkage, respectively, resulting from the linkage between hemin and the amino-modified Fe₃O₄ particles. In addition, strong Fe-O vibrations can be observed at 563 cm^?1. An electrode was modified with these microparticles and then showed a well-defined redox behavior of the immobilized hemin, with a fast heterogeneous electron transfer process (14.5 s^?1). The electrode is capable of sensing both O₂ and H₂O₂ and displays a wide linear range, high sensitivity, and fast response. The composites reported here also may serve as a support for the immobilization of proteins, which paves the way to potential applications in novel biosensors and bioelectronic devices.

Voltammetric Behavior of Antileukemia Drug Glivec. Part II – Redox Processes of Glivec Electrochemical Metabolite

Glivec is a newly developed drug that belongs to the class of 2‐phenylaminopyrimidine. It is a potent inhibitor of ABL‐kinase, the main clinical manifestation of chronic myelogenous leukemia (CML). Based on its activity on CML, glivec is undergoing extensive evaluation for its activity against other tumor types. Detection and quantitation of glivec in biological fluids or cells is thus very important. The antileukemia drug glivec undergoes oxidation at glassy carbon electrodes and involves the formation of an oxidation product, P_glivec. The adsorption of P_glivec at the GCE surface yields a compact monolayer allowing an electrochemical study of this compound adsorbed at the GCE surface. The reversible redox reaction of the adsorbed P_glivec is pH dependent and occurs with the transfer of 2 electrons and 2 protons. The surface standard potential and the rate constant of the heterogeneous electrochemical reaction were calculated using cyclic voltammetry to be E^θ′=+180 mV and k=15.5 s^?1, respectively. The total surface concentration of adsorbed P_glivec is 2.5×10^?12 mol cm^?2. The analytical determination of glivec was carried out by differential pulse voltammetric measurement of the anodic peak current corresponding to either the oxidation peak of glivec or the oxidation peak of P_glivec adsorbed on the GCE surface. The limits of detection of glivec and adsorbed P_glivec based on three times the noise level are 3.3×10^?8 M and 2.9×10^?10 M, respectively.     

Complex Formation of Ferric Protoporphyrin IX From the Reaction of Hemin with Ammonia and Small Aliphatic Amines

Complexes of Fe^III protoporphyrin IX (Fe^IIIPPIX) with the amido anion were obtained from the reaction of Fe^IIIPPIX chloride (hemin) with ammonia and small aliphatic amines under solvent free conditions. The reaction of hemin with gaseous ammonia leads to a pentacoordinated complex at the iron site, PPIX–Fe–NH₂, plus NH₄Cl, while at the peripheral propionic acidic groups ammonium carboxylate is formed. The corresponding stoichiometry (1:4 molar ratio of hemin to ammonia) was confirmed by the adsorption isotherm. Analogous reactions and complex formation were observed with EtNH₂ and Et₂NH. These reactions were monitored using X-ray diffraction (XRD), and i.r. and Mössbauer spectroscopies. The isomer shift and quadrupole splitting values of the resulting complexes are in correspondence with the strong -donor character of the amido anion linked to the iron atom. For comparison, the Mössbauer parameters for hemin complexes with arginine and 2-aminoguanidine, which also have pure interaction with the porphyrin iron, were included and discussed.     

The electrochemistry of proteins and related substances: I. Cystine and cysteine at the mercury electrode

Studies of the electroreduction of cystine (RSSR) and oxidation of cysteine (RSH) at several pH's at a hanging Hg drop electrode by cyclic voltammetry and at a Hg pool by coulometry are described. The proposed mechanism for RSSR involves reduction of an adsorbed monolayer (maximum coverage of 41 μC cm^?2) to form solution phase RSH at the adsorption prewave. A diffusion controlled reduction of RSSR to RSH occurs at more negative potentials. Oxidation of RSH involves formation of an adsorbed organomercury species, e.g., Hg(RS)₂ (maximum coverage of 80 μC cm^?2), which is reducible back to RSH. At higher RSH concentrations, anodic and cathodic current spikes appear on the cyclic voltammograms which are ascribed to formation of a tight or compact film when monolayer coverage of Hg(RS)₂ is attained, corresponding to strong interactions between adsorbed species.     

High‐Quality Covalently Grafting Hemoglobin on Gold Electrodes: Characterization,Redox Thermodynamics and Bio‐electrocatalysis

Herein, we report a versatile surface chemistry methodology to covalently immobilize ligands and proteins to self‐assembled monolayers (SAMs) on gold electrode. The strategy is based on two steps: 1) the coupling of soluble azido‐PEG‐amimo ligand with an alkynyl‐terminated monolayer via click reaction and 2) covalent immobilization hemoglobin (Hb) to the amine‐terminated ligand via carbodiimide reaction. Surface‐enhanced Raman scattering spectroscopy (SERS), atomic force microscopy (AFM), reflection absorption infrared spectroscopy (RAIR) and cyclic voltammetry are used to characterize the model interfacial reactions. We also demonstrate the excellent biocompatibility of the interface for Hb immobilization and reliable application of the proposed method for H₂O₂ biosensing. Moreover, the redox thermodynamics of the Fe³⁺/Fe²⁺ couple in Hb is also investigated.     

Dual Metal Active Sites in an Ir1/FeOx Single-Atom Catalyst: A Redox Mechanism for the Water-Gas Shift Reaction

Herein, we report a theoretical and experimental study of the water-gas shift (WGS) reaction on Ir₁/FeO_x single-atom catalysts. Water dissociates to OH* on the Ir₁ single atom and H* on the first-neighbour O atom bonded with a Fe site. The adsorbed CO on Ir₁ reacts with another adjacent O atom to produce CO₂, yielding an oxygen vacancy (O_vac). Then, the formation of H₂ becomes feasible due to migration of H from adsorbed OH* toward Ir₁ and its subsequent reaction with another H*. The interaction of Ir₁ and the second-neighbouring Fe species demonstrates a new WGS pathway featured by electron transfer at the active site from Fe³⁺−O⋅⋅⋅Ir²⁺−O_vac to Fe²⁺−O_vac⋅⋅⋅Ir³⁺−O with the involvement of O_vac. The redox mechanism for WGS reaction through a dual metal active site (DMAS) is different from the conventional associative mechanism with the formation of formate or carboxyl intermediates. The proposed new reaction mechanism is corroborated by the experimental results with Ir₁/FeO_x for sequential production of CO₂ and H₂.     

Dual Metal Active Sites in an Ir1/FeOx Single‐Atom Catalyst: A Redox Mechanism for the Water‐Gas Shift Reaction

Herein, we report a theoretical and experimental study of the water‐gas shift (WGS) reaction on Ir₁/FeO_x single‐atom catalysts. Water dissociates to OH* on the Ir₁ single atom and H* on the first‐neighbour O atom bonded with a Fe site. The adsorbed CO on Ir₁ reacts with another adjacent O atom to produce CO₂, yielding an oxygen vacancy (O_vac). Then, the formation of H₂ becomes feasible due to migration of H from adsorbed OH* toward Ir₁ and its subsequent reaction with another H*. The interaction of Ir₁ and the second‐neighbouring Fe species demonstrates a new WGS pathway featured by electron transfer at the active site from Fe³⁺?O???Ir²⁺?O_vac to Fe²⁺?O_vac???Ir³⁺?O with the involvement of O_vac. The redox mechanism for WGS reaction through a dual metal active site (DMAS) is different from the conventional associative mechanism with the formation of formate or carboxyl intermediates. The proposed new reaction mechanism is corroborated by the experimental results with Ir₁/FeO_x for sequential production of CO₂ and H₂.     

Synthesis, structure, and properties of a binuclear Fe(III) complex with N-(1-propanol)-N,N-bis(3-tert-butyl-5-methyl-2-hydroxybenxyl)amine

Mannich reaction of 2-Amino propanol, 2-tert-butyl-4-methylphenol, and formaldehyde in the ratio of 1:2:2 provides a new compound, N-(1-propanol)-N,N-bis(3-tert-butyl-5-methyl-2-hydroxybenxyl)amine (H₃L), which has been characterized by X-ray crystallography and elemental analysis. In the presence of Et₃N, the reaction of H₃L and FeCl₃·6H₂O gives a dinuclear Fe(III) complex [Fe₂L₂] 1, which has been characterized by X-ray crystallography, magnetic measurement, and cyclic voltammetry. The value of μ_eff at room temperature (5.97 μ_B) is much less than the expected spin-only value (8.37 μ_B) of two high spin (hs) Fe³⁺ (S = 5/2) ions [μ = g[∑ZS(S + 1)]^1/2], indicating there are strong coupling interactions between Fe³⁺ ions. The magnetic behavior of 1 denotes the occurrence of intramolecular antiferromagnetic interactions (J = −13.35 cm⁻¹ ). CV of 1 reveals two reversible waves at 0.433 and 1.227 V versus AgCl/Ag, which can be ascribed to the successive redox coupling of Fe^IIFe^II/Fe^IIIFe^II and Fe^IIIFe^II/Fe^IIIFe^III, respectively.     

Electrochemical kinetic analysis of a 1,4-hydroxynaphthoquinone self-assembled monolayer

Electroactive 5-hydroxy-3-hexanedithiol-1,4-naphthoquinone (JUG_thio) has been self-assembled on gold. Electrochemical results show the surface coverage is 2.2 × 10⁻¹⁰ mol cm⁻², which is consistent with a dense monolayer. The JUG_thio shows one quasi-reversible and stable voltametric wave in aqueous buffered solution. A kinetic analysis of the redox reactions involving both electron and proton transfer has revealed an unusual behaviour of this molecule due to the presence of the hydroxyl function in the vicinity of the quinone group. The apparent kinetic rate constant and the anodic coefficient transfer of this reaction depend on the pH. In acid medium, a classical concerted 2e⁻/2H⁺ mechanism is obtained. In basic medium (pH > 7), strong intramolecular hydrogen interactions between the quinone and the hydroxyl function have a strong influence on the redox kinetics. These results show that JUG_thio electroactivity is very sensitive to hydrogen interactions in neutral and basic pH solution and is able to act as sensitive layer for electrochemical biosensing.     

Probing the Role of the Conserved Arg174 in Formate Dehydrogenase by Chemical Modification and Site-Directed Mutagenesis

The reactive adenosine derivative, adenosine 5′-O-[S-(4-hydroxy-2,3-dioxobutyl)]-thiophosphate (AMPS-HDB), contains a dicarbonyl group linked to the purine nucleotide at a position equivalent to the pyrophosphate region of NAD⁺. AMPS-HDB was used as a chemical label towards Candida boidinii formate dehydrogenase (CbFDH). AMPS-HDB reacts covalently with CbFDH, leading to complete inactivation of the enzyme activity. The inactivation kinetics of CbFDH fit the Kitz and Wilson model for time-dependent, irreversible inhibition (K_D = 0.66 ± 0.15 mM, first order maximum rate constant k₃ = 0.198 ± 0.06 min⁻¹). NAD⁺ and NADH protects CbFDH from inactivation by AMPS-HDB, showing the specificity of the reaction. Molecular modelling studies revealed Arg174 as a candidate residue able to be modified by the dicarbonyl group of AMPS-HDB. Arg174 is a strictly conserved residue among FDHs and is located at the Rossmann fold, the common mononucleotide-binding motif of dehydrogenases. Arg174 was replaced by Asn, using site-directed mutagenesis. The mutant enzyme CbFDHArg174Asn was showed to be resistant to inactivation by AMPS-HDB, confirming that the guanidinium group of Arg174 is the target for AMPS-HDB. The CbFDHArg174Asn mutant enzyme exhibited substantial reduced affinity for NAD⁺ and lower thermostability. The results of the study underline the pivotal and multifunctional role of Arg174 in catalysis, coenzyme binding and structural stability of CbFDH.     

A Polyaniline-Deposited Nonplatinum Metal as a Potentiometric Sensor—An Inexpensive Alternate to Conventional Platinum for Some Potentiometric Redox Reactions

For the purpose of employing an inexpensive alternative to conventional platinum for use by upper-division as well as graduate students, polyaniline (PANI)-deposited stainless steel (SS) and mild steel (MS) electrodes are described as indicator electrodes for potentiometry and potentiometric titrations of some redox reactions. PANI is deposited on the nonplatinum metal by electrochemical polymerization of aniline using cyclic voltammetric technique. Alternate methods to produce the PANI electrodes are also suggested. The electrodes respond to concentration changes of hydroquinone (H₂O), Fe²⁺/Fe³⁺, and [Fe(CN)₆]^4–/[Fe(CN)₆]^3– in HCL electrolytes, and the potential variation with concentration follows the Nernst relationship. Under identical experimental conditions, the response time of the PANI/SS, PANI/MS, and Pt electrodes for a change in concentration of Fe³⁺ in a mixed electrolyte of Fe²⁺ and Fe³⁺ is found to be about 20 s. Neutralization reaction of HC1 versus NaOH, redox reaction of Fe²⁺ and Ce⁴⁺, and redox reaction of Fe²⁺ and KMnO₄ in several concentrations in the range from 1 mM to 100 mM are carried out using the PANI/SS, PANI/MS, and Pt indicator electrodes. The performance of the PANI/SS and PANI/MS electrodes is as good as that of the Pt at all concentration levels of the titrations. The electrodes can be reused for several titrations by storing them in an acid electrolyte for a long period of time. Thus, the conventional inert Pt or Au can be substituted for by using a PANI-deposited nonplatinum reactive metal as a potentiometric sensor for redox titrations.     

Effect of the Length of the Hydrocarbon Chain of Adsorbed Aliphatic Acids and Alcohols on the Electron Transfer Probability

The kinetics of redox reactions of hemin adsorbed over a monolayer of long-chain aliphatic alcohols, such as cetyl alcohol (C₁₆) and stearyl alcohol (C₁₈), and stearic (C₁₈), behenic (C₂₂), and melissic (C₃₀) acids is studied. The reaction inhibition on an alcohol monolayer (0.34 orders of magnitude per methylene group) is close to that reported in the literature. The inhibition on an acid monolayer is substantially smaller (0.2 to 0.07 orders), which is attributed to irregularity of the structure of the monolayer of carboxylic acids caused by the dissociation of carboxyl groups, especially in alkaline solutions. Violation of a regular structure of an acid monolayer is confirmed by a considerable irreversible increase in the electrode capacitance in alkaline solutions and by a decrease in the ohmic resistance of a monolayer of the longest saturated acid, i.e. triacontanoic acid. For a regular monolayer structure, when the length of an extended hydrocarbon chain defines the barrier thickness (alcohols), experimental data accord with predictions of the superexchange theory, which holds that the probability of a long-distance electron transfer depends on the length of the chain of covalent bonds over which the superexchange occurs. If the film structure is irregular (film thickness decreases with increasing hydrocarbon chain length (acids)), the packing of these bonds becomes essential, which allows the electron not to pass over all the bonds but to hop onto a nonbonded chain link nearby. The obtained data indicate that the electron transfer probability is defined by the electron's path, rather than by the barrier's geometrical thickness.     

Direct electrochemistry of recombinant tobacco peroxidase on gold

Direct electron transfer (DET) reactions of recombinant tobacco peroxidase (rTOP), namely direct electroreduction of Compound I/Compound II and heme Fe^3+/2+ conversion, were studied on gold electrodes. rTOP of wild type, non-glycosylated, was produced using an Escherichia coli expression system. At pH 5.0, the redox potential for direct electrochemical transformation of the Fe^3+/2+ of the peroxidase heme was −143 mV vs. AgAgCl, and 0.26 ± 0.07 pmol of the adsorbed rTOP were in DET contact with the gold electrode. The total amount of the adsorbed rTOP estimated from QCM data was 53 ± 5 pmol/cm² or 1.67 pmol when referred to the surface area of the electrodes used for electrochemical measurements. Of 1.67 pmol of adsorbed rTOP, only 0.76 pmol were catalytically active. DET between Au and the enzyme was also studied in the reaction of the bioelectrocatalytic reduction of H₂O₂ by cyclic voltammetry and amperometric detection of H₂O₂ at +50 mV with rTOP-modified Au electrodes placed in a wall-jet flow-through electrochemical cell. Maximal bioelectrocatalytic current response of the rTOP-modified gold electrodes to H₂O₂ was observed at pH 5.0 and stemmed from its bioelectrocatalytic reduction based on DET between Au and the active site of rTOP. Kinetic analysis of the DET reactions gave 52% of the adsorbed rTOP molecules active in DET reactions (0.4 pmol of adsorbed catalytically active rTOP, correspondingly), which correlated well with the non-catalytic-voltammetry data. DET was characterised by a heterogeneous ET rate constant of 13.2 s⁻¹, if one takes into account the QCM data, and 19.6 s⁻¹, if the amount of rTOP estimated from the data on DET transformation of Fe^3+/2+ couple of rTOP is considered. The sensitivity for H₂O₂ obtained for the rTOP-modified Au electrodes was 0.7 ± 0.1 A M⁻¹ cm⁻². These are the first ever-reported data on DET reactions of anionic plant peroxidases on bare gold electrodes.     

Activation of nitrite ion by biomimetic iron(III) complexes

The rate of reaction of NO ₂ ⁻ ion with various Fe^III porphyrins in the presence of PPh₃ is shown to depend on the redox potential of the Fe^III center. There is a linear relationship between the ease of reduction of the Fe^III to Fe^II and the kinetics for the formation of the Fe^II porphyrin nitrosyl adduct, with concomitant oxidation of PPh₃ to PPh₃O. Cyclic voltammograms show reversible one-electron reductions that can be ascribed to the Fe^III/Fe^II couple ranging from E_1/2 = −343 to −145 mV (versus Ag/AgCl). The order of increasing half-wave reduction potentials for the Fe^III/Fe^II porphyrin redox centers studied is octaethylporphyrin > etioporphyrin I > deuteroporphyrin IX dimethyl ester > protoporphyrin IX dimethyl ester > α,β,γ,δ-tetraphenylporphyrin. This sequence of redox potentials complements the pseudo first-order kinetics ( to m s ⁻¹) for the oxidation of PPh₃ and subsequent Fe^II porphyrin nitrosyl adduct formation. The rates of reaction of biomimetic Fe^III porphyrins with NO ₂ ⁻ ion demonstrate how metal center redox properties are influenced by the surrounding ligand. In this paper we have elucidated a possible mechanistic control for the rate of this reaction.     

Redox behaviors of Fe(II/III) and U(IV/VI) studied in synthetic water and KURT groundwater by potentiometry and spectroscopy

The redox behaviors of Fe(II/III) and U(IV/VI) in both synthetic samples and natural groundwater were investigated with potentiometry, UV/VIS absorption spectroscopy, and time-resolved laser fluorescence spectroscopy. Total dissolved Fe(II/III) concentration along with presence of mixed redox couples of Fe²⁺/Fe³⁺ and Fe²⁺/Fe₂O₃(s) were revealed to be the major factors influencing on the redox potentials. Considerable discrepancies between redox potentials obtained with quantitative analysis and chemical speciation of Fe(II/III) and U(IV/VI) ions were identified in the KAERI Underground Research Tunnel groundwater. Chemical speciation of U(IV) in natural groundwater without considering relevant complexation reaction might cause relatively large uncertainties in redox potential calculations.     

Electrochemical behaviour of zinc gluconate complexes

Voltammetric studies revealed that under transient conditions in the pH range 3.7 to 5.0, the deposition of zinc from ZnSO₄ solutions involves the formation of adsorbed monovalent zinc. The conversion of divalent zinc to monovalent is a slow step. In the presence of gluconate, the reduction of divalent complex involves the monovalent zinc complex and the second electron transfer is slow. In the pH range 10 to 12.5, the zinc complex may be [(Zn(GH₄)₄]2^- and is found to vary with gluconate and OH^- ions. The conversion of [Zn(GH₄)(OH)_abs ^-] to Zn(OH)₂ or Zn(GH₄)₂ is the slow step in the reduction of the complexes. In strong alkali solutions sodium gluconate forms zinc hydroxy gluconate complexes. [Zn(OH)₃(GH₄)]^2- to adsorbed [Zn(OH)(GH₄)]^- is the slow step in the reduction.     

MoO42-取代对Nasicon型Li3Fe2(PO4)3正极材料电化学性能的影响

以MoO₄^2-部分取代Li₃Fe₂（PO₄）₃中的PO₄^3-,研究表明：加入的MoO₄^2-离子主要以固溶形式存在于Li₃Fe₂（PO₄）₃中,起到了显著改善其电化学性能的作用。其中,MoO₄^2-掺杂浓度为0.3的样品表现出最佳的电化学性能,其在0.5C倍率下的首次放电容量为113.7 mAh·g^-1,这一数值比未掺杂的提高了20.7%;经过60次循环充放电,容量保持率为94%。将放电倍率从0.5C逐步增大至5C,再降至初始的0.5C,并在每个倍率循环10次,这一材料的最终放电容量可达首次0.5C的95%。这些优异的性能应归因于MoO₄^2-掺杂使材料的氧化还原能力增强,氧化还原电对的电势差减小,电池内部的电荷转移电阻减小,以及Li⁺扩散系数增加。     

MoO42-取代对Nasicon型Li3Fe2(PO4)3正极材料电化学性能的影响

以MoO_4~(2-)部分取代Li3Fe2(PO4)3中的PO_4~(3-),研究表明:加入的MoO_4~(2-)离子主要以固溶形式存在于Li3Fe2(PO4)3中,起到了显著改善其电化学性能的作用。其中,MoO_4~(2-)掺杂浓度为0.3的样品表现出最佳的电化学性能,其在0.5C倍率下的首次放电容量为113.7 m Ah·g~(-1),这一数值比未掺杂的提高了20.7%;经过60次循环充放电,容量保持率为94%。将放电倍率从0.5C逐步增大至5C,再降至初始的0.5C,并在每个倍率循环10次,这一材料的最终放电容量可达首次0.5C的95%。这些优异的性能应归因于MoO_4~(2-)掺杂使材料的氧化还原能力增强,氧化还原电对的电势差减小,电池内部的电荷转移电阻减小,以及Li+扩散系数增加。