Hydrogen selenide ion adsorption to colloidal elemental selenium

Hydrogen selenide ion (HSe^?) has an important role in the metabolism of the essential trace element selenium. Several redox reactions of selenide were found to be dominated by the amount of colloidal elemental selenium (Se°) generated during the reaction. The following reaction of selenide with the disulfide, oxidized glutathione (GSSG), was used as an example: HSe^? + GSSG + H⁺ → Se° + 2 GSH. The resulting thiol is reduced glutathione (GSH; γ-glutamylcysteinylglycine). By following this reaction with polarography, it was seen that the ratio of colloidal selenium produced to selenide unreacted was a constant 2.1 ± 0.1, and was the only factor found to determine the extent of oxidation. This is best explained by the hypothesis that freshly generated colloidal selenium adsorbs selenide readily; no evidence for polyselenide formation was found. Adsorption of selenide should be considered in any reaction involving the oxidation of selenide to colloidal selenium.     

Redox reactions of hydrogen selenide ion

Hydrogen selenide ion (HSe^-) is an important product in the metabolism of the essential trace element selenium. Although its role in selenium metabolism is recognized, aspects of the basic chemistry of selenide have been ignored, particularly the tendency of selenide to undergo rapid redox reactions with biological oxidants. Using polarography, we have found that selenide reacts in vitro with a variety of compounds including dehydroascorbic acid, quinones like vitamin K₁ and FAD (flavin adenine dinucleotide), and disulfides such as oxidized glutathione and lipoic acid. The fact selenide reacts readily in vitro suggests similar reactions may also occur in vivo with important biological consequences. Contrary to expectations, selenide was found not to reduce the disulfide bond of oxidized dithiothreitol (trans-4,5-dihydroxyl-1,2-dithiane), indicating the commonly published value for the standard electrode potential of the selenium/hydrogen selenide ion couple is in error. The electrode potential is an important parameter to aid in anticipating possible redox reactions of selenide in vivo.     

Selenium detoxification of heavy metals: a possible mechanism for the blood plasma

Among the activities of the essential trace element selenium is the ability to reduce the toxicity of heavy metal ions like cadmium(II) and mercury(II). Detoxification often depends on the metabolic reduction of selenium to hydrogen selenide; the mechanism generally advanced to explain such selenium/metal interactions is that selenide combines with heavy metal ions to give a metal selenide which is metabolically inert. However, this hypothesis does not consider circumstances where selenide is quickly removed by other reactions. Given the ease with which selenide is oxidized, such conditions are likely to occur in the blood plasma, an environmental rich in oxidizing agents and a site for many selenium/metal interactions. Using polarography to monitor both selenide and cadmium, we have found that selenide reacts rapidly in vitro with the disulfide bonds present in bovine serum albumin in preference to forming cadmium selenide. We hypothesize that a similar reaction occurs in the blood plasma with the disulfide bonds of plasma proteins to generate thiol groups on the protein involved, and that these newly formed thiols are responsible for the observed reduction of metal toxicity through the ability to chelate heavy metal ions.     

Peroxidase-like activities of iron(III)—Porphyrins: kinetics of the reduction of a peroxidatically active derivative of deuteroferriheme by anilines

The kinetics of the reduction by aniline and a series of substituted anilines of a peroxidatically active intermediate, formed by oxidation of deuteroferriheme with hydrogen peroxide, have been studied by stopped-flow spectrophotometry. The reaction with aniline was first order with respect to [intermediate] and showed first-order saturation kinetics with respect to [aniline]. The second-order rate constant was 2.0 ± 0.2 × 10⁵ M^?1 sec^?1 at 25°C (independent of pH in the range 6.60–9.68) compared with the value of 2.4 × 10⁵ M^?1 sec^?1 for the reaction of aniline with horseradish peroxidase Compound I. The effect of aniline substituents upon reactivity towards the heme intermediate closely paralled those reported for reaction with the enzymic intermediate. Anilines bearing electron-donating substituents reacted more rapidly and those bearing electron-withdrawing substituents more slowly than the unsubstituted amine. The rate constants for the heme intermediate reactions (k_dfh)found to be related to those for the enzymic reactions (k_hrp) by the equation:log k_DFH= 0.65log k_HRP+ 1.96 with a correlation coefficient of 0. 98.     

The role of interactions between O2, H2O2, .OH,e- and O2- in free radical damage to biological systems

The occurrence of the Haber-Weiss reaction and other interactions between free radicals has been investigated in the effects of mixtures of free radicals on the permeability of resealed erythrocyte ghosts and on the activity of membrane-bound glyceraldehyde-3-phosphate dehydrogenase. The following mixtures were found to induce damage greater than that which could be accounted for by the independent actions of the constituent free radicals: (i) · OH + H₂O₂, and (ii) · OH + H₂O₂ + O₂^?. In contrast, the following mixtures were found to induce less damage than that predicted on the basis of independent actions of constituent free radicals: (i) H₂O₂ + O₂^?, and (ii) oxidizing radicals ( · OH, H₂O₂) + reducing radicals (e^?, H · ). These results suggest a Haber-Weiss-like interaction between H₂O₂ and O₂^?and an interaction between H₂O₂ and · OH to produce a species more potent than either in causing increased permeability. The decrease in damage observed in the simultaneous presence of oxidizing and reducing radicals suggests an antagonistic effect by which each tends to moderate damage by the other. Inactivation of glyceraldehyde-3-phosphate dehydrogenase was found to be more sensitive to radiation than permeability by an order of magnitude, while permeability was more sensitive to the enhancement of damage by oxygen. Comparison of the effectiveness of free radical scavengers in inhibiting the increase in permeability caused by free radicals showed the following order of effectiveness, expressed in terms of percentage protection: formate (90%) > nitrogen (65%) > catalase (60%) > dismutase (32%), and with respect to enzymatic inactivation, nitrogen (100%) > formate (77%) > dismutase (48%) > catalase (44%). The relative rates observed anaerobically and aerobically in the presence and absence of the above scavengers suggest that (at least in the case of radiation damage to the membranes of erythrocyte ghost cells) the “oxygen effect” is due to the interaction of oxygen with e^? and H., producing O₂^? which aggravates damage under conditions which allow consequent Haber-Weiss-like reactions. The further increase in damage when oxygen concentration is raised yet higher is due to the interaction of oxygen with the sites of initial damage.     

The participation of a tryptophan residue in the binding of ferric iron to pyrocatechase

The fast reaction technique of pulse radiolysis was used to produce free radicals in aqueous solution from alcohols, deoxyribose, cytosine, uracil, thymine, dihydrothymine and histidine. The electron transfer reactions from these radicals to p-benzoquinone was observed from the formation kinetics of the semiquinone anion ·BQ^? at 430 nm and the efficiency was found to be as high as 90% or more, with k～5×10⁹ M^?1sec^?1. In acid or neutral solutions in the presence of oxygen the peroxy radicals ·O₂RH formed do not essentially transfer an electron to BQ, and the efficiency is <10%. The significance of these results in the fixation of radiation damage in photobiology and radiation biology are indicated. The reactions of the superoxide ·O₂^? radical with BQ are also presented and discussed.     

Reactions of ruthenium(II) para-substituted tetraphenylporphyrin complexes with carbon monoxide oxygen: A kinetic investigation

The reaction of Ru(XTPP)(DMF)₂, where XTPP is the dianion of para substituted tetraphenylporphyrins and X is MeO, Me, H, Cl, Br, I, F, with O₂ and CO were studied in DMF. The process was found to be first-order in metalloporphyrin, first-order in molecular oxygen and carbon monoxide, and second-order overall. Second-order rate constants for the CO reaction ranged from 0.170 to 0.665 M^?1 s^?1 at 25°C, those for the O₂ reaction from 0.132 to 0.840 M^?1 s^?1 at 25°C. Similar activation parameters (ΔH_CO^± = 87 ± 1 kJ mol^?1, ΔS_CO^± = 22 ± 4 JK^?1 mol^?1; ΔH_O2^± = 81 ± 1 kJ mol^?1, and ΔS_O2^± = 11 ± 5 JK^?1 mol^?1) were found within each series. Reactivities of X substituted metalloporphyrins were found to follow different Hammett σ functions. The CO reactions correlated with σ_? following normal behavior; the O₂ reactions correlated with σ₈° indicating O₂ is π-bonded in the transition states. A dissociative mechanism is postulated for the process.     

H2O2− and alkyl hydroperoxide-supported para hydroxylation of aniline by alkaline hematin

Under alkaline conditions and in the presence of reductant, NADH or NADPH, and molecular oxygen, hemin catalyzes the regiospecific para-hydroxylation of aniline to form p-aminophenol [P.A. Adams, D.A. Baldwin, and M.C. Berman, J. Chem. Soc. (Lond) Chem. Commun. 856 (1979); P.A. Adams and M.C. Berman, J. Inorg. Biochem.17, 1 (1982)]. The reaction has now been studied in the presence of H₂O₂ and alkyl hydroperoxides and in the absence of oxygen and reductant. Results indicate that the H₂O₂^? and alkyl hydroperoxide-supported processes proceed via different mechanisms involving, on the one hand, the hydroperoxide anion (HO₂^?) and on the other, the undissociated alkyl hydroperoxide molecule (ROOH). The addition of superoxide dismutase to the reaction had no effect, unlike the NADH/O₂ supported reaction where the enzyme completely inhibits reaction. Similarities between the hemin-mediated peroxide-supported reactions reported here, and the cytochrome P-450-mediated peroxide-supported reactions reinforce our earlier contentions that the alkaline hemin system appears to be a good model for the in vivo activation of oxygen by hemoproteins.     

Light-induced electron-transfer reactions. Part 2. Kinetics of the intra-molecular redox decomposition of tris(oxalato)-cobaltate(III) by irradiation with visible light of aqueous solution containing tris(2,2′-bipyridine)ruthenium(II) ion

The rate of the intra-molecular redox decomposition of the tris(oxalato)cobaltate(III) ion [Co(Ox)₃]^3? is greatly accelerated by irradiation with visible light of aqueous acidic solutions containing the tris(2,2′-bipyridine)ruthenium(II) ion [Ru(bpy)₃]²⁺. The rate of the light-induced reaction in hydrochloric acid with an acidity range 0.05–0.18 mol dm^?3 is of zero-order with respect to the [Co(Ox)₃]^3? ion concentration and is proportional to the light-intensity irradiated and also essentially to the [Ru(bpy)₃]²⁺ ion concentration. Moreover, the rate is independent not only of the oxalate ion concentrations, but also of the acidity over the range 0.05–0.18 mol dm^?3 hydrochloric acid. The ionic-strength dependence, as well as temperature dependence, were extremely small. The [Ru(bpy)₃]²⁺ concentration does not change during the occurrence of the reaction and the tris(2,2′-bipyridine)ruthenium(II) ion acts as a homogeneous catalyzer. However, a dramatic indication that the situation was rather different was found in the stronger acid solutions of 0.5 or 1.0 mol dm^?3 hydrochloric acid, in which the [Ru(bpy)₃]²⁺ concentration decreased greatly immediately after the initiation of reaction and then increased up to the initial concentration. Such a decrease at the initial stage of the reaction disappeared by addition of oxalate before the start of the reaction. A chain mechanism of reaction is proposed to account for these results.     

Hemin-mediated para-hydroxylation of aniline: A potential model for oxygen activation and insertion reactions of mixed function oxidases

The activation of molecular oxygen by alkaline hemin (ferriprotoporphyrin IX) has been studied. In the presence of reductant nicotineamide adenine dinucleotide (NADH) or nicotineamide adenine dinucleotide phosphate (NADPH) and organic substrate, aniline, hemin activates oxygen to the hydroperoxide anion (HO₂^?) and subsequently mediates insertion of active oxygen into the benzene ring of the substrate to form p-aminophenol, with a high degree of regiospecificity. Oxygen activation does not occur in the absence of aniline. Stoichiometry of the reaction indicates that two electrons are required per molecule of oxygen activated or atom of oxygen inserted into the substrate aromatic ring system. Direct measurements of H₂O₂ and of the pK_a for maximum rate of p-aminophenol formation (11.7 ± 0.1) indicate participation of the hydroperoxide anion as the active oxygen species in the rate-determining step of the insertion reaction. Powerful scavengers of the hydroxyl radical (OH′) have little effect on the formation of H₂O₂ or p-aminophenol by the system. Superoxide dismutase (10^?7 mol dm^?3) inhibited both p-aminophenol and H₂O₂ formation, when added to the system immediately prior to initiation of the reaction. Studies involving N-phenylhydroxylamine indicate that aromatic ring hydroxylation is occurring directly and not by rearrangement of an N-hydroxylated intermediate. Implications of hemin-mediated hydroxylation reactions for those of enzymatic mixed function oxidase activity are discussed.     

Decomposition of water by cerium oxide of δ-phase

Reduced cerium dioxide (CeO_2?x) can reduce water, producing hydrogen at ?298 K. Kinetic studies were focused on the stoichiometric reaction of δ-phase cerium oxide (CeO_1.818) with water vapor. Different activation energies of 18.1 and 33.4 kJ mol^?1 were observed for the reactions at the temperature ranges above and below ca. 453 K, respectively. Rate equations observed in the two temperature ranges were also different. These results strongly suggest that the rate-determining steps are different between the two temperature ranges. Rapid oxygen exchange observed between H₂¹⁸O and lattice oxygen in cerium oxide of δ- phase at ? 298 K indicated that neither the adsorption of water molecules not the diffusion of oxygen ions in the bulk of the oxide can be the rate-determining step. H₂D₂ exchange occurred rapidly at 373 K compared to the rate of water decomposition, suggesting that the recombination of hydrogen atoms on the surface is not rate- determining either. A tentative reaction mechanism was proposed to explain the results of the kinetic studies. The rate-determining step at high temperatures (>453 K) is the reduction of OH^? by the six-coordinated Ce³⁺ which is present in the nonstoichiometric cerium oxide, while that at low temperatures (<453 K) is the subsequent reduction of H⁺ by the seven-coordinated Ce³⁺.     

Chemiluminescence Emission during Reactions between Superoxide and Selected Aliphatic and Aromatic Halocarbons in Aprotic Media

The reactions between superoxide free radical anion (.O⁻₂) with the halocarbons CCl₄, CHCl₃, BrCH₂CH₂Br(EDB), decachloro-biphenyl (DCBP), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in dimethyl sulphoxide (DMSO) results in the emission of chemiluminescence (CL). The chemiluminescence reactions are characterized as having biphasic second order kinetics, CL wavelengths between 350 nm and 650 nm, and exhibiting perturbation by chemicals reactive with singlet oxygen. These data suggest that singlet oxygen species are the excited state responsible for the light emissions. Polarographic studies confirm .O⁻₂ consumption and halide release in the reactions, while gas liquid chromatography and NBT reduction demonstrate the decomposition of the halocarbons into products. A chemiluminescent reaction mechanism is proposed involving reductive dehalogenation of the halocarbons and the generation of singlet oxygen. The significance of singlet oxygen generation is discussed with respect to a general mechanism for explaining the rapid initiation of lipid peroxidative membrane damage in halocarbon toxigenicity in animal and plant tissues.     

Photon emission from chemically perturbed yeast cells

Photon emission (PE) from yeast cells Saccharomyces cerevisiae strain SP-4 in normal conditions and in conditions perturbed by the addition of formaldehyde was investigated using single-photon counting equipment. PE from yeast cells, growing in a standard nutrient medium (YPG) then centrifuged and resuspended in a phosphate buffer (pH = 6.5), was measured in the presence of oxygen or argon. The solution of formaldehyde (2%) was injected into the sample. The intensity of PE increased and reached a maximum, then slowly decreased to a level which was higher than the PE level without the perturbing factor. The kinetics of PE was found to be strongly dependent upon the presence of oxygen. The model of formation and recombination of free radicals was tested. The results indicate that PE can arise during the recombination reactions of free radicals like R^? + R^?, RO^? + RO^?, RO^?₂ + RO which are formed in the enzymatic oxidative reactions.     

Hierarchical NiCo2S4@NiO Core–Shell Heterostructures as Catalytic Cathode for Long‐Life Li‐O2 Batteries

The critical challenges of Li‐O₂ batteries lie in sluggish oxygen redox kinetics and undesirable parasitic reactions during the oxygen reduction reaction and oxygen evolution reaction processes, inducing large overpotential and inferior cycle stability. Herein, an elaborately designed 3D hierarchical heterostructure comprising NiCo₂S₄@NiO core–shell arrays on conductive carbon paper is first reported as a freestanding cathode for Li‐O₂ batteries. The unique hierarchical array structures can build up multidimensional channels for oxygen diffusion and electrolyte impregnation. A built‐in interfacial potential between NiCo₂S₄ and NiO can drastically enhance interfacial charge transfer kinetics. According to density functional theory calculations, intrinsic LiO₂‐affinity characteristics of NiCo₂S₄ and NiO play an importantly synergistic role in promoting the formation of large peasecod‐like Li₂O₂, conducive to construct a low‐impedance Li₂O₂/cathode contact interface. As expected, Li‐O₂ cells based on NiCo₂S₄@NiO electrode exhibit an improved overpotential of 0.88 V, a high discharge capacity of 10 050 mAh g^?1 at 200 mA g^?1, an excellent rate capability of 6150 mAh g^?1 at 1.0 A g^?1, and a long‐term cycle stability under a restricted capacity of 1000 mAh g^?1 at 200 mA g^?1. Notably, the reported strategy about heterostructure accouplement may pave a new avenue for the effective electrocatalyst design for Li‐O₂ batteries.     

The enthalpy of oxidation of flavin mononucleotide

The oxidation enthalpy of reduced flavin mononucleotide at pH 7.0 in 0.2 m phosphate buffer has been studied by determining the heat associated with the reaction: FMNH₂ + 2 Fe(CN)^?3₆ ? FMN + 2 Fe(CN)^?4₆ + 2 H⁺. (a) (The quinone, semiquinone, and hydroquinone forms of FMN are represented as FMN, FMNH, and FMNH₂, respectively.) Calorimetric experiments were performed in a flow microcalorimeter which was modified to prevent sample contamination by oxygen. The enthalpy observed for reaction (a), after correction for dilution and buffer effects, was ?39.2 ± 0.4 kcal (mole FMNH₂)^?1 at 25 °C. The potential difference, ΔE′, developed by reaction (a) was determined potentiometrically and corresponded to a free energy change, ΔG′, of ?30.3 kcal (mole FMNH₂)^?1. The resulting entropy change, ΔS′, was thus calculated to be ?29.8 e.u. Reaction (a) was also studied at temperatures of 7 °C and 35.5 °C. ΔC_p′ for the reaction was calculated as ?155 ± 18 cal deg^?1 (mole FMNH₂)^?1 at 20 °C. ΔH′ for the reaction (b), FMNH₂ ? FMN + H₂, (b) was calculated as +14.2 ± 0.7 kcal mole^?1 at 25 °C, relative to the enthalpy of the hydrogen electrode being identically equal to zero at all values of pH and temperature. The free energy at pH 7.0 for reaction (b), calculated from the potential was found to be ?9.7 kcal mole^?1, which resulted in an entropy for reaction (b) of 80.2 e.u. A thermal titration of reaction (a) was used to calculate the thermodynamic parameters for the formation of semiquinone dimer according to the reaction FMNH₂ + FMN ? (·FMNH)₂. (c) The free energy, enthalpy, and entropy changes for reaction (c) were estimated to be ?6.1 kcal mole^?1, ?7 kcal mole^?1, and ?3 e.u., respectively.     

Toxins of the sea anemone Epiactis prolifera

Fe³⁺-EDTA chelates react with the superoxide radical at physiological pH values (k = 1.3 × 10⁶M^?1 s^?1 at pH 7 but is lower at more alkaline pH values) but do not appear to catalyze O₂^? dismutation at a significant rate. Complexes of Fe³⁺ with desferrioxamine, bathophenanthroline, or diethylenetriaminepentaacetic acid react much more slowly, if at all. Fe²⁺ complexes of EDTA, ATP, and diethylenetriaminepentaacetic acid also react with O₂^? at alkaline pH values. The significance of these reactions in the mechanism of the “iron-catalyzed Haber-Weiss reaction” is discussed.     

Hydrothermal Synthesis of Monolithic Co3Se4 Nanowire Electrodes for Oxygen Evolution and Overall Water Splitting with High Efficiency and Extraordinary Catalytic Stability

Cobalt selenide has been proposed to be an effective low‐cost electrocatalyst toward the oxygen evolution reaction (OER) due to its well‐suited electronic configuration. However, pure cobalt selenide has by far still exhibited catalytic activity far below what is expected. Herein, this paper for the first time reports the synthesis of new monoclinic Co₃Se₄ thin nanowires on cobalt foam (CF) via a facile one‐pot hydrothermal process using selenourea. When used to catalyze the OER in basic solution, the conditioned monolithic self‐supported Co₃Se₄/CF electrode shows an exceptionally high catalytic current of 397 mA cm^?2 at a low overpotential (η) of 320 mV, a small Tafel slope of 44 mV dec^?1, a turnover frequency of 6.44 × 10^?2 s^?1 at η = 320 mV, and excellent electrocatalytic stability at various current densities. Furthermore, an electrolyzer is assembled using two symmetrical Co₃Se₄/CF electrodes as anode and cathode, which can deliver 10 and 20 mA cm^?2 at low cell voltages of 1.59 and 1.63 V, respectively. More significantly, the electrolyzer can operate at 10 mA cm^?2 over 3500 h and at 100 mA cm^?2 for at least 2000 h without noticeable degradation, showing extraordinary operational stability.     

The kinetics and mechanism of oxidation of maltose and lactose by Tl(III) in acidic media

The kinetics of the cleavage of d-arabino-hexos-2-ulose (1) and of glyoxal (2) with hydrogen peroxide in alkaline water and in 44% (w/w) ethanol-water solutions (pOH 0.5-5) were studied over a temperature range of ?25 to +25°. The relative rate of the competing reactions of 1 with the cleavage in 0.03-1M sodium hydroxide was determined from the rate of formation of hydrogen peroxide in the oxidation of d-glucose to 1 with 2-anthraquinonesulfonic acid in the presence of oxygen at 25 and 40°. The cleavages of both 1 and 2 were first-order with respect to hydrogen peroxide, and also to hydroxyl ion at low alkalinities. The rate of cleavage of 1 reached a maximum at pOH ～2.5, whereas the competing reactions of 1 and the cleavage of 2 were constantly accelerated with increasing hydroxyl-ion concentration. Unlike 2, compound 1 was cleaved more rapidly in ethanol-water than in water. The activation energies of the cleavage of 1 and 2, and the competing reactions of 1, were 49, 57, and 65 kJ.mol^?1, respectively.     

Anion regulation of lupin asparagine synthetase: Chloride activation of the glutamine-utilizing reactions

Small monovalent anions strongly activate glutamine-dependent asparagine synthesis and glutamine hydrolysis catalysed by highly purified asparagine synthetase (EC 6.3.5.4) from cotyledons of Lupinus luteus seedlings. Cl^? and Br^? are most effective, but F^?, I^?, NO₃^? and CN^? also stimulate both reactions. The synthetase reactions with NH₃, or NH₂OH are only slightly stimulated by Cl^? and Br^?, indicating that the anions selectively accelerate the reactions involving glutamine cleavage. In asparagine synthesis Cl^? is a competitive activator vs glutamine and a noncompetitive activator vs MGATP and aspartate. Addition of Cl^? changes the substrate saturation kinetics of glutamine from negatively cooperative to normal hyperbolic and causes a 50-fold increase in the affinity for glutamine. The inherent glutaminase activity of the enzyme is enhanced up to 30-fold by addition of Cl^?, MgATP and aspartate. Thus, ligands of the synthetase reaction act as allosteric activators of the glutaminase step in the enzyme mechanism.     

The reactions of plant hormones with reactive oxygen species: chemical insights at a molecular level

The reactions of two plant hormones, namely jasmonic acid (JA) and methyl jasmonate (MJ), with different reactive oxygen species (ROS) were investigated using the density functional theory. Different reaction sites and mechanisms were explored, as well as solvents of different polarity, and pH in aqueous solution. The thermochemical viability and kinetics of the investigated reaction pathways were found to be strongly influenced by the reacting ROS. All the investigated pathways were found to be exergonic, both in aqueous and lipid solution and for both JA and MJ, when the reactions involve ^?OH and ^?OCH₃. On the contrary, for the reactions with peroxy radicals (^?OOH and ^?OOCH₂CHCH₂) only a few hydrogen transfer pathways were found to be thermochemically viable. The reactions involving ^?OH were found to be diffusion-controlled, with both JA and MJ, regardless of the polarity of the solvent. This led to the hypothesis that the direct ^?OH scavenging activity of JA and MJ might play a role in the beneficial effects of the jasmonate family regarding the antioxidant defense of plants against metal-induced oxidative stress. The deprotonated fraction of JA is, to some extent, more reactive than the neutral fraction toward ROS. This, together with the acid-base equilibria inherent to some ROS, make the pH an influential environmental factor on the overall reactivity of JA toward ROS.