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Trolox-C (TXC) is a water-soluble analogue of α-tocopherol that reduces the metal-bound superoxo complex (SOC), [(NH3)5Co(µ-O2)Co(NH3)5]5+ in water–ethanol media. In aqueous-acid media ([H+] = 0.02–0.70 mol/L), TXC quantitatively reduces SOC under the pseudo first-order reaction condition where the concentration of TXC, [TXC], was at least 10 times or more than that of [SOC]. Variations in media ionic strength suggest for the reaction between two cationic species, and a positive kinetic isotope effect (kH/kD < 1) indicates for a pre-equilibrium step of protonation of the pyran-oxygen of TXC. Both TXC and the protonated TXC, i.e., TXCH, are the potential reductants under the experimental range of media acidity. The reduction of SOC takes place through the single-electron transfer and the kinetic analysis yields the value for second-order rate constants for TXC and TXCH as 9.7 and 3.6 × 103 (mol/L)−1s−1 at 15 °C. [ABSTRACT FROM AUTHOR]

In this study, we have chosen a system to study if NADH can reduce the ROS species even in very strongly acidic media. Literature reports that in presence of an acid, NADH undergoes acid-catalyzed water addition across the 5,6 double bond of the nicotinamide ring to produce NADHX and finally is decomposed. However, we found that in acid media NADHX quantitatively reduces the metal-bound superoxo complex, [(NH3)4CoIII(µ-O2,µ-NH2)CoIII(NH3)4]4+ (1) to the corresponding hydroperoxo complex. The redox interactions monitored spectroscopically follow pseudo-first-order kinetics when the analytical concentration of NADH was kept higher than [1]. The observed rate constant, ko values are indifferent to media ionic strength but ko increases linearly with an increase in [H+]. Following the kinetic scheme, we can evaluate both the rate constants, k1 and k2 values for reactions of NADH and NADHX, respectively. It seems that the reaction path of NADH is completely silent and under the experimental condition ([H+] = 1.0–3.5 M) NADHX is the only species that reduces superoxo moiety of 1 (k2 = 1.74 ± 0.03 M−1s−1). Furthermore, the large kinetic isotope effect in presence of D2O (v/v) (kH/kD = 5.0) indicates that the mechanistic pathway must involve a proton-coupled electron transfer (PCET) step in the rate-determining step. The mass analysis of the product mixture shows a peak at m/z = 664 which suggests the conversion of NADHX to the corresponding cyclic product, c(THN)AD over time. In aqueous-acid NADH undergoes acid-catalyzed water-addition to its 5,6 double bond of the nicotinamide ring to produce NADHX which can reduce the metal-bound superoxo complex, [(NH3)4CoIII(µ-O2,µ-NH2)CoIII(NH3)4]4+ (1), a mimic of ROS. The reduction follows pseudo-first-order kinetics and the observed rate constant, ko increase with an increase in [H+]. Evaluation of the rate values suggests that under the reaction condition, the reaction pathway of NADH is completely silent. In acid media, NADH has converted to NADX and the latter alone is responsible for the reduction. Moreover, the large kinetic isotope effect (kH/kD = 5.0) suggests that NADX reduces the superoxo group through a proton-coupled electron transfer (PCET) mechanism. [ABSTRACT FROM AUTHOR]

The kinetics of the oxidation reactions of APAP and Cafn by a Mn(IV)-oxidant, [MnIV3(µ-O)4(phen)4(H2O)2]4+ (1) has been studied at room temperature. Under the acidic reaction condition ([H+] = 0.2–1.0 M), both APAP and Cafn exist in equilibrium with their conjugate acids APAPH+ and CafnH+ and each of the species acts as the potential reductants. Kinetic observations reveal that the observed rate constant, ko, values increase with the increase in media acidity and the ionic strength, I. The second-order rate-constant values for oxidizing the protonated forms of the two reductants (APAPH+ and CafnH+) were evaluated to be 6.04 ± 0.29 and 0.18 ± 0.01 M−1 s−1, respectively. Overall, under acid media, Mn(IV)-complex efficiently oxidizes APAP to a mixture of benzoquinone and acetamide (or, to quinone oxime and acetic acid) and Cafn to 1,3,7-trimethyluric acid. [ABSTRACT FROM AUTHOR]

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Quercetin (Qu), a noted polyphenolic flavonol molecule, exhibits remarkable antioxidant properties against bioenzymes like peroxidase and tyrosinase. However, it was found that under pH = 3.5–5.5 and room temperature, Qu cannot reduce hydrogen peroxide (HP) which is a well-perceived reactive oxygen species (ROS) responsible for oxidative stress. We found that in presence of noble-metal nano-particles, such redox interactions can be accomplished. We thus synthesized homogenous PVP-coated nano-platinum particles (PNP) to catalyze the oxidation of Qu by HP. On reaction, Qu is oxidized by HP to ortho-quinone (QQ) following the first-order kinetics. The observed rate constant, ko, gradually increases and tends to saturate with increasing [HP]T where T represents the analytical concentrations of the reactant. The saturation kinetics indicates that the activities of PNP resemble to that of bioenzymes and similarly, the nano-particles host the reactants on its active surface during the redox interactions. The estimated value of the kcat reveals that the extent of catalysis of even 10−10 M PNP is nearly comparable with the rate constants of oxidation of Qu in presence of enzymes like horseradish peroxidase and tyrosinase. [ABSTRACT FROM AUTHOR]

The chlorophenols (CPs), 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP) and 2,4,6-trichlorophenol (2,4,6-TCP), are potent environmental hazards. They can be dechlorinated to safer products by reacting them with the Co(III)-bound superoxo complex [(NH3)5Co(µ-O2)Co(NH3)5]5+ (1). In acidic media, the redox reactions between 1 and all three CPs follow a first-order process and the observed rate constant ko values increase with increasing concentrations of CPs and pH of the medium. Our observations suggest that the deprotonated forms of the CPs are ~ 105 times more reactive than the protonated forms, and the reactions proceed via a common free radical mechanism. Initially, the CP molecules undergo 2-e− oxidations by complex 1 to generate phenolic radicals, which then form unstable quinones. Through the aromatic ring opening, the latter intermediates finally degrade to different products. The reactivity order for these CPs is 4-CP ≈ 2,4,6-TCP > 2,4-DCP. [ABSTRACT FROM AUTHOR]

In acid-media ([H+] = 0.01–0.06 M), each of the thiol compounds, D-penicillamine (PEN, LPH2) and captopril (CAP, LCH2) exist in several proton-dependent forms which can reduce the superoxo complex [(en)(dien)CoIII(O2)CoIII(en)(dien)]5+(1) to the corresponding peroxo [(en)(dien)CoIII(O2)CoIII(en)(dien)]4+(2) or the hydroperoxo complex [(en)(dien)CoIII(OOH)CoIII(en)(dien)]5+(3). The observed first-order rate constants,ko,Pandko,Cfor PEN and CAP increase with the increase in [TPEN] and [TCAP] (which are the analytical concentrations of the respective thiols) but decrease with the increase in the media-acidity ([H+]) and the media ionic strength (I). The protolytic equilibria in aqueous solution allow several potentially reducing forms to coexist for both PEN (LPH3+, LPH2, LPH−, and LP2−) and CAP (LCH2, LCH−, LC2−) but the kinetic analyses reveal that the order of reactivity for the species are LPH3+~ LPH2 <<< LPH−and LCH2 < LCH− <<< LC2−, respectively. The predominance and higher reactivities of the anionic species, LPH−and LC2−are supported by the negative slopes of the plots ofko,Porko,Cversus I. Moreover, a large value ofkH/kDfor PEN suggests an inner-sphere electroprotic reaction pathway while the absence of such effect for CAP strongly supports an outer-sphere electron transfer reaction. These propositions are supported by the structural features of LPH−and LC2−. [ABSTRACT FROM AUTHOR]

In acetate buffer media (pH 4.5-5.4) thiosulfate ion (S2O32-) reduces the bridged superoxo complex, [(NH3)4CoIII(μ-NH2,μ-O2)CoIII(NH3)4]4+ (1) to its corresponding μ-peroxo product, [(NH3)4CoIII(μ-NH2,μ-O2)CoIII(NH3)4]3+ (2) and along a parallel reaction path, simultaneously S2O32- reacts with 1 to produce the substituted μ-thiosulfato-μ-superoxo complex, [(NH3)4CoIII(μ-S2O3,μ-O2)CoIII(NH3)4]3+ (3). The formation of μ-thiosulfato-μ-superoxo complex (3) appears as a precipitate which on being subjected to FTIR shows absorption peaks that support the presence of Co(III)-bound S-coordinated S2O32- group. In reaction media, 3 readily dissolves to further react with S2O32- to produce μ-thiosulfato-μ-peroxo product, [(NH3)4CoIII(μ-S2O3,μ-O2)CoIII(NH3)4]2+ (4). The observed rate (k0) increases with an increase in [TThio] ([TThio] is the analytical concentration of S2O32-) and temperature (T), but it decreases with an increase in [H+] and the ionic strength (I). Analysis of the log At versus time data (A is the absorbance of 1 at time t) reveals that overall the reaction follows a biphasic consecutive reaction path with rate constants k1 and k2 and the change of absorbance is equal to {a1 exp(-k1t) + a2 exp(-k2t)}, where k1 > k2. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Chemistry is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

ABSTRACT In alkaline media, well-characterized gelatin-stabilized palladium (GPd) nanoparticles catalyze the reduction of the azo group containing pollutant dye, Acid Orange 7 (AO7) by sodium borohydride (NaBH4) to 1-amino-2-napthol and sulfanilic acid. Kinetic observations and detailed FTIR studies suggests that the reaction follows Langmuir-Hinshelwood kinetic model, where during the reaction both AO7 and borohydride are adsorbed on the GPd surface. Plots of ln ko versus ln[AO7] or ln[NaBH4] show that the order of reaction with respect to AO7 and NaBH4 remains almost same over different molar ratios of [NaBH4]/[AO7]. The catalyzed reaction shows an initial induction period ( t0) due to a surface-restructuring process of GPd nanoparticles, and (1/ t0) can be defined as the rate of surface restructuring. The activation energy of the catalyzed reaction and energy of the surface-restructuring process of GPd are estimated as 22 ± 3 and 25 ± 7 kJ M−1, respectively. [ABSTRACT FROM AUTHOR]