pH-dependent Oxidation Research Articles

PH and thiosulfate dependent microbial sulfur oxidation strategies across diverse environments.

Sulfur oxidizing bacteria (SOB) play a key role in sulfur cycling in mine tailings impoundment (TI) waters, where sulfur concentrations are typically high. However, our understanding of SOB sulfur cycling via potential S oxidation pathways (sox, rdsr, and S4I) in these globally ubiquitous contexts, remains limited. Here, we identified TI water column SOB community composition, metagenomics derived metabolic repertoires, physicochemistry, and aqueous sulfur concentration and speciation in four Canadian base metal mine, circumneutral-alkaline TIs over four years (2016 - 2019). Identification and examination of genomes from nine SOB genera occurring in these TI waters revealed two pH partitioned, metabolically distinct groups, which differentially influenced acid generation and sulfur speciation. Complete sox (csox) dominant SOB (e.g., Halothiobacillus spp., Thiomonas spp.) drove acidity generation and S2O3 2- consumption via the csox pathway at lower pH (pH ~5 to ~6.5). At circumneutral pH conditions (pH ~6.5 to ~8.5), the presence of non-csox dominant SOB (hosting the incomplete sox, rdsr, and/or other S oxidation reactions; e.g. Thiobacillus spp., Sulfuriferula spp.) were associated with higher [S2O3 2-] and limited acidity generation. The S4I pathway part 1 (tsdA; S2O3 2- to S4O6 2-), was not constrained by pH, while S4I pathway part 2 (S4O6 2- disproportionation via tetH) was limited to Thiobacillus spp. and thus circumneutral pH values. Comparative analysis of low, natural (e.g., hydrothermal vents and sulfur hot springs) and high (e.g., Zn, Cu, Pb/Zn, and Ni tailings) sulfur systems literature data with these TI results, reveals a distinct TI SOB mining microbiome, characterized by elevated abundances of csox dominant SOB, likely sustained by continuous replenishment of sulfur species through tailings or mining impacted water additions. Our results indicate that under the primarily oxic conditions in these systems, S2O3 2- availability plays a key role in determining the dominant sulfur oxidation pathways and associated geochemical and physicochemical outcomes, highlighting the potential for biological management of mining impacted waters via pH and [S2O3 2-] manipulation.

Iodide oxidation by ozone at the surface of aqueous microdroplets.

The oxidation of iodide by ozone occurs at the sea-surface and within sea spray aerosol, influencing the overall ozone budget in the marine boundary layer and leading to the emission of reactive halogen gases. A detailed account of the surface mechanism has proven elusive, however, due to the difficulty in quantifying multiphase kinetics. To obtain a clearer understanding of this reaction mechanism at the air-water interface, we report pH-dependent oxidation kinetics of I- in single levitated microdroplets as a function of [O3] using a quadrupole electrodynamic trap and an open port sampling interface for mass spectrometry. A kinetic model, constrained by molecular simulations of O3 dynamics at the air-water interface, is used to understand the coupled diffusive, reactive, and evaporative pathways at the microdroplet surface, which exhibit a strong dependence on bulk solution pH. Under acidic conditions, the surface reaction is limited by O3 diffusion in the gas phase, whereas under basic conditions the reaction becomes rate limited on the surface. The pH dependence also suggests the existence of a reactive intermediate IOOO- as has previously been observed in the Br- + O3 reaction. Expressions for steady-state surface concentrations of reactants are derived and utilized to directly compute uptake coefficients for this system, allowing for an exploration of uptake dependence on reactant concentration. In the present experiments, reactive uptake coefficients of O3 scale weakly with bulk solution pH, increasing from 4 × 10-4 to 2 × 10-3 with decreasing solution pH from pH 13 to pH 3.

Neutral Phenolic Contaminants Are Not Necessarily More Resistant to Permanganate Oxidation Than Their Dissociated Counterparts: Importance of Proton-Coupled Electron Transfer.

Despite decades of research on phenols oxidation by permanganate, there are still considerable uncertainties regarding the mechanisms accounting for the unexpected parabolic pH-dependent oxidation rate. Herein, the pH effect on phenols oxidation was reinvestigated experimentally and theoretically by highlighting the previously unappreciated proton transfer. The results revealed that the oxidation of protonated phenols occurred via proton-coupled electron transfer (PCET) pathways, which can switch from ETPT (electron transfer followed by proton transfer) to CEPT (concerted electron-proton transfer) or PTET (proton transfer followed by electron transfer) with an increase in pH. A PCET-based model was thus established, and it could fit the kinetic data of phenols oxidation by permanganate well. In contrast with what was previously thought, both the simulating results and the density functional theory calculation indicated the rate of CEPT reaction of protonated phenols with OH- as the proton acceptor was much higher than that of deprotonated phenols, which could account for the pH-rate profiles for phenols oxidation. Analysis of the quantitative structure-activity relationships among the modeled rate constants, Hammett constants, and pKa values of phenols further supports the idea that the oxidation of protonated phenols is dominated by PCET. This study improves our understanding of permanganate oxidation and suggests a new pattern of reactivity that may be applicable to other systems.

Ozone Initiated pH Dependent Oxidation of Cyclohexane Over Fe Supported SiO2 and γ-Al2O3 Catalysts

Ozone Initiated pH Dependent Oxidation of Cyclohexane Over Fe Supported SiO2 and γ-Al2O3 Catalysts

PH-dependent oxidation mechanisms over FeCu doped g-C3N4 for ofloxacin degradation via the efficient peroxymonosulfate activation

In this work, bimetallic iron and copper co-doped graphitic carbon nitride catalysts (FeCu-g-C3N4) exhibited pH-dependency in the activation of peroxymonosulfate (PMS) for ofloxacin (OFX) degradation, and the mechanism of the generation of reactive oxygen species (ROS) was correlated to the pH of reaction system. The degradation efficiency of OFX reached to 91% within 120 min at pH of 3.1, while only 70% OFX was removed at pH of 9.0. Furthermore, the mainly generated ROS were identified with quenching experiments and electron spin resonance (ESR). Under acidic condition (pH 3.1), •OH and 1O2 made a major contribution for OFX degradation, while 1O2 was the dominant ROS at alkaline condition (pH 9.0). The bimetallic Fe and Cu and the cyano groups on g-C3N4 were revealed to play critical roles in the activation of PMS. Also, Fe and Cu species were suppressed for PMS activation at alkaline condition, and cyano groups in g-C3N4 were active at both acidic and alkaline conditions. This work provides useful information for the influence on the performance of PMS activation under different conditions for the removal of recalcitrant organic compounds in water.

Triclosan: Electrochemistry, Spontaneous Degradation and Effects on Double-Stranded DNA

Triclosan (TCS) is an antiseptic agent widely used mainly in personal care products and an important contaminant, which degrades in the environment causing toxicity on health, including negative effects on DNA. In this context, an electrochemical investigation of TCS in aqueous solution was studied by voltammetric techniques. The TCS underwent irreversible oxidation in a pH-dependent process, leading to the formation of two reversibly oxidized and pH-dependent oxidation products. An oxidation mechanism for TCS and its oxidation products in neutral aqueous medium was proposed. Besides that, the TCS spontaneously degraded into supporting eletrolytes with 3.4 ≤ pH ≤ 12.04 over the incubation time and the degraded TCS in solution was detected by electrochemical and spectrophotometric techniques. A higher degradation of TCS was observed in alkaline medium. In addition, the interaction in situ of this anti-microbial with DNA was investigated using dsDNA incubated solutions and dsDNA electrochemical bisosensor, by voltammetry. TCS and degraded TCS interacted with dsDNA causing the condensation of the double helix structure, release of guanine (by TCS and dedraded TCS) and adenine (by degraded TCS) bases from dsDNA and a possible intercalation of degraded TCS in the polynucleotide chain. No dsDNA oxidative damage was detected.

Screen printed diamond electrode as efficient “point-of-care” platform for submicromolar determination of cytostatic drug in biological fluids and pharmaceutical product

This work presents advanced electrochemical platform based on screen printed diamond electrode (SPDE) system for the single drop “point-of-care” testing. Proposed approach was applied for the quantification of doxorubicin hydrochloride (DOX) in biological fluids and pharmaceutical product. Using a single drop (~30 μL) of the tested solution at the electrode surface, DOX showed high electroactivity over a wide range of pHs. In these conditions, single oval shaped, well-defined and pH dependent oxidation peak was observed in the potential range from 0.5 V to 1.3 V. In the reverse scan, two cathodic peaks, were noted – around 0.3 V and – 0.5 V. Similarly, first reduction peak was pH dependent, while second one was independent in the studied range. Experimental conditions for DOX quantification were optimized and natures of the electrode reactions were investigated. Working linear range obtained for DOX detection was from 0.1 to 2.5 μM. Diffusion controlled electrode reaction reveal long life time of the proposed electrode as well high potential for practical application. Developed procedure was successfully applied for the DOX analysis in biological fluids – urine and pharmaceutical formulation. Obtained results clearly indicated that given procedure can be easily implemented for pharmaceutical control and medical analysis, in both, laboratory and field conditions.

Electrochemical and spectroscopic characterization of biologically important Schiff bases

Redox behavior of two biologically important triazol derivatives i.e. (E)-1-((4H-1,2,4 triazol -4-ylimino) methyl)naphthalene-2-ol abbreviated as TMN and (E)-2-((4H-1,2,4 triazol -4-ylimino) methyl) 4-chlorophenol abbreviated as TMC was investigated on glassy carbon electrode surface. Experiments were carried over wide ranged concentration, pH, and temperature to evaluate various useful thermodynamic and kinetic parameters including ka, ΔG#, ΔH#, ΔS# along with Ea and diffusion co-efficient D. Thermodynamic data reveals that both compounds follow irreversible, pH dependent and diffusion-controlled oxidation processes which is both entropy and enthalpy opposed. Voltammetric studies help in determining the number of electrons and protons contributing in each oxidation reaction. The acid–base dissociation constant (pKa) value was found both by spectroscopic and electrochemical studies and the results obtained from both techniques were in good agreement. Computational studies give higher value of EHOMO, lower value of ELUMO, smaller orbital gap (ΔE) for TMN than TMB. Both the experimental and theoretical studies agree well in this regard and confirm that TMN is easily oxidizable compared to TMC. Sulforhodamine B assay demonstrated the non-cytotoxic character of the compounds up to 150 μM concentration. The present studies showed low cytotoxicity for these triazoles and their potential to be used as valuable compounds in biological applications.

Electrochemical oxidation of synthetic amino-substituted benzamides with potential antioxidant activity

Amino-substituted benzamide derivatives are very attractive compounds due to their capacity to act as powerful antioxidants by scavenging free radicals. Knowledge on electrochemical oxidation mechanisms plays an important role in understanding the free radical scavenging activity of antioxidants. In this paper, the electrochemical oxidation of four amino-substituted derivatives [N-(4-aminophenyl)-2-hydroxybenzamide (1), N-(4-aminophenyl)-2-hydroxy-4-methoxybenzamide (2), N-(4-aminophenyl)-2-methoxybenzamide (3), N-(4-aminophenyl)-3,4,5-trihydroxybenzamide (4)] in an aqueous buffer solution was studied by means of cyclic and square-wave voltammetry, and probable reaction mechanisms have been proposed. In compounds 1–3, the primary amino group was found to be the main electroactive centre undergoing a complex, pH-dependent oxidation process with the transfer of two electrons and two protons. This reaction occurs in one step or in two separate steps, depending on the pH of the medium. A possible explanation for this electrochemical behaviour is discussed. The final products of initial electrochemical oxidation of amino-substituted benzamides 1–3 are quinonediimine derivatives that undergo further chemical transformation into electroactive forms. Oxidation of compound 4 proceeds through two sequential reactions; the first anodic reaction is the oxidation of the trihydroxyphenyl group to the corresponding o-quinone, followed by the oxidation of a primary amino group. Considering the oxidation potentials, and bearing in mind that lower/less positive potential implies better radical scavenging activity, the order of antioxidant activity of the here studied amino-substituted benzamides is: 4 >2 >1 >3.

Thallium(I) Oxidation by Permanganate and Chlorine: Kinetics and Manganese Dioxide Catalysis.

The oxidation of the toxic heavy metal thallium(I) (Tl(I)) is an efficient way to enhance Tl removal from water and wastewater. However, few studies have focused on the kinetics of Tl(I) oxidation in water, especially at environmentally relevant pH values. Therefore, the kinetics and mechanisms of Tl(I) oxidation by the common agents KMnO4 and HOCl under environmentally relevant pH condition were explored in the present study. The results indicated that the pH-dependent oxidation of Tl(I) by KMnO4 exhibited second-order kinetics under alkaline conditions (pH 8-10) with the main active species being TlOH, while the reaction could be characterized by autocatalysis at pH 4-6, and Mn(III) might also play an essential role in the MnO2 catalysis. Furthermore, a two-electron transfer mechanism under alkaline conditions was preliminarily proposed by using linear free energy relationships and X-ray photoelectron spectroscopy (XPS) analysis. Distinctively, the reaction rate of Tl(I) oxidation by HOCl decreased with increasing pH, and protonated chlorine might be the main active species. Moreover, the Tl(I)-HOCl reaction could be regarded as first order with respect to Tl(I), but the order with respect to HOCl was variable. Significant catalysis by MnO2 could also be observed in the oxidation of Tl(I) by HOCl, mainly due to the vacancies on MnO2 as active sites for sorbing Tl. This study elucidates the oxidation characteristics of thallium and establishes a theoretical foundation for the oxidation processes in thallium removal.

Direct Evidence of Local pH Change and the Role of Alkali Cation during CO2 Electroreduction in Aqueous Media.

We report, for the first time, utilizing a rotating ring-disc electrode (RRDE) assembly for detecting changes in the local pH during aqueous CO2 reduction reaction (CO2 RR). Using Au as a model catalyst where CO is the only product, we show that the CO oxidation peak shifts by -86±2 mV/pH during CO2 RR, which can be used to directly quantify the change in the local pH near the catalyst surface during electrolysis. We then applied this methodology to investigate the role of cations in affecting the local pH during CO2 RR and find that during CO2 RR to CO on Au in an MHCO3 buffer (where M is an alkali metal), the experimentally measured local basicity decreased in the order Li+ > Na+ > K+ > Cs+ , which agreed with an earlier theoretical prediction by Singh et al. Our results also reveal that the formation of CO is independent of the cation. In summary, RRDE is a versatile tool for detecting local pH change over a diverse range of CO2 RR catalysts. Additionally, using the product itself (i.e. CO) as the local pH probe allows us to investigate CO2 RR without the interference of additional probe molecules introduced to the system. Most importantly, considering that most CO2 RR products have pH-dependent oxidation, RRDE can be a powerful tool for determining the local pH and correlating the local pH to reaction selectivity.

PH Dependence of Arsenic Oxidation by Rice-Husk-Derived Biochar: Roles of Redox-Active Moieties.

Biochars have demonstrated great potential for water decontamination and soil remediation; however, their redox reactivity toward trace contaminants and the corresponding redox-active moieties (RAMs, i.e., phenolic -OH, semiquinone-type persistent free radicals (PFRs), and quinoid C═O) remain poorly understood. Here we investigated the roles of the RAMs on biochar in oxidation of As(III) under varying pH and O2 conditions. The results showed that the promoted oxidation of As(III) by the RAMs is strongly pH dependent. Under acidic and neutral conditions, only the oxidation of As(III) by •OH and H2O2 produced from activation of O2 by phenolic -OH and semiquinone-type PFRs occurred. In contrast, the oxidation by semiquinone-type PFRs, quinoid C═O, and H2O2 (if O2 was introduced) appeared under alkaline conditions. This pH-dependent oxidation behavior was attributed to the varying redox activities of RAMs, as confirmed by multiple characterization and validation experiments using biochar with tuned RAMs compositions, as well as thermodynamics evaluation. Our findings provide new insights into the roles of the RAMs on biochar in the promoted oxidation of trace As(III) over a broader pH range under both anoxic and oxic conditions. This study also paves a promising way to oxidize As(III) with biochar.

Nivolumab anticancer monoclonal antibody native and denatured direct electrochemistry at a glassy carbon electrode

Nivolumab (NIVO) is a recombinant humanized monoclonal antibody used in the immunotherapeutic treatment of cancer. The electrochemical oxidation of native NIVO, in solution and adsorbed on a glassy carbon electrode, using differential pulse voltammetry, showed two pH-dependent oxidation peaks, the first one corresponding to the cysteine amino acid residues, and the second one corresponding to the tyrosine and tryptophan amino acid residues. The interfacial behaviour and adsorption of native and denatured NIVO thick multilayer film adsorbed on glassy carbon electrode surface was probed by cyclic voltammetry and electrochemical impedance spectroscopy. The denaturation of a NIVO thick multilayer film adsorbed on a glassy carbon electrode surface, with chemical agents: denaturing agents urea and sodium dodecyl sulphate, and reductants tris(2-carboxyethyl)phosphine and dithiothreitol, was investigated electrochemically. The unfolded denatured adsorbed NIVO morphological structure, enhanced the exposure of more electroactive amino acid residues to the electrode surface, and a new additional peak, due to the tryptophan, methionine, and histidine amino acid residues oxidation, was found.

Kinetic Modeling of pH-Dependent Oxidation of Dopamine by Iron and Its Relevance to Parkinson's Disease.

Parkinson's disease is the second most common neurodegenerative disease. While age is the most significant risk factor, the exact cause of this disease and the most effective approaches to mitigation remain unclear. It has long been proposed that dopamine may play a role in the pathology of Parkinson's disease in view of its ability to generate both protein-modifying quinones such as aminochrome and reactive oxygen species, especially in the presence of pathological iron accumulation in the primary site of neuron loss. Given the clinically measured acidosis of post-mortem Parkinson's disease brain tissue, the interaction between dopamine and iron was investigated over a pH range of 7.4 to 6.5 with emphasis on the accumulation of toxic quinones and generation of reactive oxygen species. Our results show that the presence of iron accelerates the formation of aminochrome with ferrous iron (Fe[II]) being more efficient in this regard than ferric iron (Fe[III]). Our results further suggest that a reduced aminochrome rearrangement rate coupled with an enhanced turnover rate of Fe[II] as a result of brain tissue acidosis could result in aminochrome accumulation within cells. Additionally, under these conditions, the enhanced redox cycling of iron in the presence of dopamine aggravates oxidative stress as a result of the production of damaging reactive species, including hydroxyl radicals.

Ultrasonic route synthesis, characterization and electrochemical study of graphene oxide and reduced graphene oxide

In this study, an efficient route for graphene oxide (GO) and reduced GO (RGO) synthesis was developed by using an ultrasonic probe and bath alternatively. RGO was prepared in acidic and neutral conditions. The products were characterized via FTIR, XRD, Raman spectroscopy, SEM and AFM. Well-exfoliated and smooth sheets without crumples are observed in GO. The RGO synthesized in acidic media had fewer layers and smaller crystallite size than that derived from neutral condition. Electrochemical investigations revealed higher effective surface area and better electrocatalytic activity for the RGO synthesized in acidic media (RGO1). The electrochemical sensing ability of the glassy carbon electrode modified with RGO1 (RGO1/GCE) towards dopamine (DA) was examined using cyclic and differential pulse voltammetry methods. The evaluation of the pH and scan rate effect on the DA electrochemical response confirmed a reversible electrocatalytic pH-dependent oxidation reaction, controlled by a mixed adsorption–diffusion process at the RGO1/GCE. Retained low disorders on the basal plane and high defect on the edge sites of the layers have provided high electrocatalytic activity for RGO1 that make it suitable for (bio) sensing applications.

Electrochemical behaviour of dipyrone (metamizole) and others pyrazolones

The electrochemical oxidation of dipyrone (MTM) in aqueous medium was characterized using antipyrine (AA), 4-aminoantipyrine (4AA), 4-methyl-aminoantipyrine (MAA) and 4-dimethyl-aminoantipyrine (DMAA) as model molecules for the elucidation of all MTM voltammetric signals.The MTM and the other pyrazolones show up to four oxidation electrochemical processes. The voltammograms obtained in AA solutions presented an irreversible electrochemical oxidation process involving one electron at Eap3, which is common to all pyrazolone derivatives, while the amino pyrazolones present electrochemical oxidation processes at Eap0 or Eap1. The stabilization of the oxidation products depends on different effects: the proton release added to the thermodynamic stability, in the case of the imine formation at Eap0 (4AA and MAA) and the hyperconjugation (σ-stabilization) in the case of iminium formation (DMAA and MTM) at Eap1. The process observed at Epa0 corresponds to the pH-dependent oxidation of the primary and secondary enamines, while the process observed at Eap1 occurs in the tertiary enamines, is pH independent. The oxidation peak potential follows the order: MAA < 4AA < DMAA < MTM and it was demonstrated that DMAA in an aqueous medium can simulate the MTM in an aprotic medium; therefore, the analytical MTM determination can be performed using the DMAA aqueous analytical curve. DMAA and MTM analytical curves, presented a linear range from 10 μmol L−1 to 100 μmol L−1 with a LOD of 1.94 and 2.97 μM for DMAA and MTM, respectively, LOQ of 6.48 and 9.91 μM (n = 10) and, sensitivity of 0.96 μA/μM for DMAA and 0.92 μA/μM; with a recoveries of 95–105% for MTM.

Oxidation of the FAD cofactor to the 8-formyl-derivative in human electron-transferring flavoprotein

The heterodimeric human (h) electron-transferring flavoprotein (ETF) transfers electrons from at least 13 different flavin dehydrogenases to the mitochondrial respiratory chain through a non-covalently bound FAD cofactor. Here, we describe the discovery of an irreversible and pH-dependent oxidation of the 8α-methyl group to 8-formyl-FAD (8f-FAD), which represents a unique chemical modification of a flavin cofactor in the human flavoproteome. Furthermore, a set of hETF variants revealed that several conserved amino acid residues in the FAD-binding pocket of electron-transferring flavoproteins are required for the conversion to the formyl group. Two of the variants generated in our study, namely αR249C and αT266M, cause glutaric aciduria type II, a severe inherited disease. Both of the variants showed impaired formation of 8f-FAD shedding new light on the potential molecular cause of disease development. Interestingly, the conversion of FAD to 8f-FAD yields a very stable flavin semiquinone that exhibited slightly lower rates of electron transfer in an artificial assay system than hETF containing FAD. In contrast, the formation of 8f-FAD enhanced the affinity to human dimethylglycine dehydrogenase 5-fold, indicating that formation of 8f-FAD modulates the interaction of hETF with client enzymes in the mitochondrial matrix. Thus, we hypothesize that the FAD cofactor bound to hETF is subject to oxidation in the alkaline (pH 8) environment of the mitochondrial matrix, which may modulate electron transport between client dehydrogenases and the respiratory chain. This discovery challenges the current concepts of electron transfer processes in mitochondria.

Catechol oxidation: considerations in the design of wet adhesive materials.

Catechols are found as functional groups in Nature as the 3,4-dihydroxy isomer, generally with electron donating substituents and as the 2,3-dihydroxy isomer, with an electron withdrawing substituent. Adhesive properties of catechol materials rely on the vicinal diol configuration, yet conversely, cohesive properties of catechol materials depend on catechol oxidation that promote subsequent crosslinking reactions. While the pH-dependent oxidation of catechol by dioxygen is well recognized, a better understanding of substituent effects on catechol redox chemistry is important in the design of new catechol-containing functional materials. The pH-dependent oxidation kinetics of catechol and substituted catechols by O2 was investigated with a Clark-type oxygen electrode. The results are consistent with a mechanism in which O2 oxidizes both mono-deprotonated and fully deprotonated catechol anions. A linear Hammett correlation for the pH-independent second order rate constants for catechol oxidation by O2 establishes that catechols functionalized with electron withdrawing groups have slower rates of oxidation by O2, whereas catechols with electron donating groups have faster rates of oxidation by O2. The Hammett correlation allows for selection of functionalized catechols with redox properties ideally suited for adhesive or crosslinking applications.

A Soluble Cu(II) Complex of Sodium 1,4-Dihydroxy-9,10-Anthraquinone-2-Sulphonate Electrocatalyzes Water Oxidation with a Large Turn over Frequency

The electro- and photo- catalytic generations of H2 and O2 from water have been explored as inexpensive ways of producing carbon-neutral fuel. Though various metal complexes have shown significant catalytic activity, however, efforts are still on to find out cheap, efficient and robust water soluble catalysts in producing H2 and O2 from water. In the present study we reported a soluble and inexpensive copper(II) complex of sodium 1,4-dihydroxy-9,10-anthraquinone-2-sulphonate (CuA2) as an electro-catalyst for water oxidation. Cyclic voltammetry measurements were carried out by using Digi-Ivy Potentiostat (Model DY2312, made in USA) using three- electrode system at 298.15 K. A glassy carbon electrode was used as the working electrode; a platinum wire was used as counter electrode while Ag/AgCl, saturated KCl was used as reference electrode. The cyclic voltammogram of CuA2 in aqueous phosphate buffer or NaF solutions showed two purely diffusion controlled reduction peaks at +0.635 and -0.265 V which are due to one –electron reduction of Cu(II) and one-electron reduction of one of the two free quinone sites of CuA2, respectively. The cyclic voltammetry measurement of CuA2 in such solutions at pH range 11.4 - 12.5 showed a large, irreversible and pH-dependent oxidation wave which appeared in the range between +1.34 and +1.74 V. A comparison of the present result with earlier clearly showed that the large irreversible anodic wave in the present study was exclusively due to the electrocatalytic evolution of oxygen. The generation of such oxygen was confirmed by suitable analytical cyclic voltammetry method. It is interesting to note that after the electrocatalytic evolution of oxygen, the reduction peak corresponding to Cu(II)/Cu(I) couple disappeared completely though the corresponding oxidation peak of this couple was found. The disappearance of this reduction peak is indicative of the fact that during the electrocatalytic oxidation of OH– to molecular oxygen, Cu(II) of CuA2is reduced to Cu(I) which is in turn oxidized electrochemically in the returned path of the cyclic voltammetry and hence catalyst regains its activity as shown below. 4OH– → O2 + 2H2O + 4e– CuIIA2 + e– = CuIA2 CuIA2 – e– = CuIIA2(Electrochemical oxidation) A cyclic voltammogram of aqueous phosphate buffer or NaF solutions as background showed that oxygen formation at the applied potential in the absence of catalyst was negligibly small which suggests that the chance for the oxidation of the working electrode is negligible. In order to get kinetic information, cyclic voltammograms of CuA2at alkaline pH were recorded at various scan rates. Since the reduction of Cu(II)/Cu(I) in the present study was found to be diffusion controlled and reversible in nature, therefore, the diffusive current may be represented as follows. id= 0.4633ndFAc√(ndFvD/RT) (1) where, nd = number of electrons transferred, F = Faraday’s constant, A = area of the electrode, c = bulk concentration of CuA2, D = diffusion coefficient, R = molar gas constant, and T = absolute temperature. The diffusive peak current in the absence of catalytic process was estimated using the diffusion-controlled Cu(II)/Cu(I) peak. The Cu(II)/Cu(I) peak is reversible in both lower and higher scan rates though reversibility decreases with the increase in scan rate as described above. The ratio of the catalytic current (ic) to the diffusive peak current (id), ic/id, matches the trend from a pure diffusion region to a pure kinetic region with the decrease in scan rate. In the kinetic region, the catalytic peak current in cyclic voltammetry is given as: ic = ncFACcat√(Dkcat) (2) where, k cat = pseudo first-order rate constant, Ccat = bulk concentration of catalyst, nc = number of electrons transferred = 4, F = Faraday’s constant, A = area of the electrode, D = diffusion coefficient of the catalyst, nc = number of electrons transferred in the catalytic wave = 4 and v= scan rate. It is frequently suitable to divide equation (2) by the peak current (id) in the absence of the catalyst (equation (1)), which generates equation (3). ic/id = 0.359(nc/nd 3/2)√(kcat/v) (3) (here, nd = 1). By using equation (3) and plot of ic /id versus v −1/2, the pseudo-first-order rate constant for the catalytic oxidation of water or hydroxide at alkaline pH, k cat, usually known as turnover frequency of the catalyst, was determined as 2927 s-1 at pH 11.90. The overpotential was observed in the present study as ~972 mV vs. NHE. Comparing the turn over frequency of CuA2 with earlier for soluble metal complexes, it may be said that CuA2 acts as plausibly robust catalyst for water oxidation. However, our efforts are still on to decrease the pH and high overpotential.

Electrochemical oxidation of the antitumor antibiotic mitomycin C and in situ evaluation of its interaction with DNA using a DNA-electrochemical biosensor

The electrochemical behaviour of the antitumor drug mitomycin C (MMC) was investigated on carbon paste electrode over a wide pH range and using voltammetric techniques. The MMC undergoes diffusion-controlled irreversible oxidation in two different processes, one pH-independent for 2.2˂pH˂4.5 and one pH-dependent for 4.5˂pH˂12.0, and does not involve the formation of any electroactive oxidation product. Upon incubation in different pH electrolytes, chemical degradation of MMC was electrochemically detected by the appearance of a new oxidation peak at a lower potential. The chemically degraded MMC undergoes an irreversible, pH-dependent oxidation for 3.4˂pH˂5.4, and its redox products are reversibly oxidised. The spontaneous degradation of MMC in aqueous solution was confirmed by UV–Vis spectrophotometry. Moreover, a multilayer dsDNA-electrochemical biosensor was used to evaluate the interaction between MMC and DNA. The results have clearly proven that MMC interacts and binds to dsDNA strands immobilized onto a glassy carbon electrode surface and its metabolite(s) cause oxidative damage to DNA.