The Role of Chemical Reactions Placed Between Successive Electron Transfer in the Reduction of 2-Methylthio-4,6-di(alkylamino)-1,3,5-triazines on Mercury Electrodes

The polarographic (direct current, dc, and differential pulse, DP) and voltammetric oxidation of hydrogen peroxide on mercury electrodes has been examined. The electrooxidation of H 2 O 2 involves hydroperoxide radical and superoxide anion radical able to interact with radical scavengers (antioxidants), namely 3-hydroxycoumarin, carvacrol, vanillin and gallic acid. For such interaction, theoretical equations are derived using the convective diffusion approximation and the steady-state conditions, as well as solving the differential equations of the Nernst diffusion-layer approximation by the method of dimensionless variables. The experimental results agree with the theoretical predictions and a kinetic parameter is proposed to evaluate the scavenging activity of antioxidants.An antioxidant is defined as a substance that may delay or prevent the oxidation of a substrate, at relatively low concentrations. 1 Primary antioxidants (prooxidants) prevent the formation of free radicals, particularly reactive oxygen species (ROS). These primary antioxidants include vitamin E, 2 polyphenols, 3 antioxidant enzymes and membranes. 4,5 Secondary antioxidants (antioxidants with radical scavenging activity) operate either interrupting the propagation of free radicals or inhibiting the generation of ROS, as well as preventing the metabolic activation of carcinogens. 6,7 Tertiary antioxidants repair the damage from free radicals or eliminate damaged molecules. 8,9 Although different assays have been used to evaluate the antioxidant activity of natural products, there is not a universal method to cover this task. Direct methods have been proposed, based on the determination of the concentration of antioxidant compounds, 10,11 and indirect methods, as the measure of glutathione levels 12 and the reduction of Fe(III) or Cu(II) ions. 13,14 In addition, enzymatic methods measure the activity of antioxidant enzymes. 15 A good review detailing these methods can be found in reference. 16 The electrochemical measurements have advantages for the determination of antioxidant activity such as their use as a rapid proof of the antioxidant capacity of many organic molecules 17,18 and the capacity to study the interaction between the antioxidant and ROS, as was observed for dihydropyridines and electrogenerated superoxide radical. [19][20][21] The oxidation potentials measured by cyclic voltammetry have been used to compare the antioxidant strength of compounds as phenolic acids, flavonoids, cinnamic acids etc., 22-24 being the glassy carbon electrode, GCE, the most frequently used electrode. Low oxidation potentials are associated with a greater facility of a given molecule for the electrodonation and, thus, to act as an antioxidant, with good correlation with other techniques such as the scavenging of the DPPH • radical. 25 The results show that the DPPH • assay can assess radical scavenging for substances that are thermodynamically capable of reacting with agents with a redox potential below a given value, related to the reduction potential o...

Section: Resultsmentioning

confidence: 99%

Analysis of the Interaction of Radical Scavengers with ROS Electrogenerated from Hydrogen Peroxide

Palma

Montoya

Arteaga

et al. 2013

“…This last value is very low but similar to those found in the reductions of other 1,3,5-triazines. 13,[15][16][17]…”

Section: ͓14͔mentioning

confidence: 99%

“…16 In the reduction of the methylthioderivatives the cleavage of the thiomethyl group takes place in more or less extensive manner. 17 As can be seen, the dechlorinated compounds are in most cases intermediates of the four-electron reduction, and in some cases stable enough to yield separate ͑although overlapped͒ reduction waves. It is interesting to elucidate the reduction mechanism of such intermediates to contribute to the knowledge of the overall reaction pathways of the herbicides.…”

mentioning

confidence: 97%

A Contribution to the Study of the Electroreduction of 2,4-Diamino-1,3,5-triazine on Mercury Electrodes

Pérez

Galvín

Mellado

et al. 2002

This paper presents polarographic ͑dc and differential pulse͒ and voltammetric ͑linear-sweep cyclic voltammetry͒ studies of the electroreduction of the s-triazine derivative 2,4-diamino-1,3,5-triazine on mercury electrodes. The studies are performed in the acidity range 2.25 M H 2 SO 4 to pH 11. Above this last pH value no signals were obtained. In both dc DP polarography, two reduction waves/peaks were observed. The overall process corresponded to a two-electron irreversible reduction. For the first wave, the proposed reduction mechanism consists of the two-electron reduction of the reactant protonated in the heterocyclic N3 nitrogen, with reversible electron transfers. The rate-determining step ͑rds͒ is a protonation placed after such transfers. At pH values lower than the protonation pK, the protonation reaction takes place prior to the reduction. For the second wave, the unprotonated reactant is reduced and the rds is a protonation placed between the two reversible one-electron transfers. At high pH values, dissociation of the amino group causes the disappearance of the wave.

Section: G31mentioning

confidence: 99%

Elucidation of the Electrochemical Oxidation Mechanism of the Antioxidant Sesamol on a Glassy Carbon Electrode

Brito

Mellado

Maldonado

et al. 2014

The dissociation constant of Sesamol (5-hydroxy-1,3-benzodioxol) was determined from the dependence of the UV-visible spectra with the medium acidity. A pK value of 10.1 ± 0.1, corresponding to the hydroxyl group, was obtained. The oxidation of sesamol was investigated on carbon electrodes using controlled-potential electrolysis, linear-sweep cyclic voltammetry and measurements at the foot of the first oxidation peak. Up to three oxidation peaks and one reduction peak were found. The dioxol ring of the dissociated sesamol undergoes a cleavage to give 3-substituted 1,4-benzoquinone, being the rate-determining step the second electron transfer. The formation of radicals in the ring cleavage explains in part the ability of sesamol to interact with reactive oxygen species (ROS).