Antioxidant activity of a number of small (low molecular weight) natural compounds found in spices, condiments or drugs (gallic acid, sesamol, eugenol, thymol, carvacrol, vanillin, salicylaldehyde, limonene, geraniol, 4-hexylresorcinol, etc.) has been evaluated using electrochemical and DPPH• radical scavenging measurements. Structural analysis of the tested compound suggest a remarkable activity for phenol derivatives and the importance of the -R groups located on the phenolic ring in the molecule's ability to act as free radical scavenging as well as their influence in the electrochemical behavior. The voltammetric method can be used for the determination of the antioxidant capability in the same manner as the DPPH • radical scavenging because of the correlation found between oxidation potentials and anti-radical power (ARP = 1/EC 50 ). Such electrochemical determination is fast and cheap and allows making measurements under a variety of experimental conditions. The accuracy of the electrochemical measurements is the same for all the compounds, irrespective of their scavenging activity, the opposite of what occurs in the DPPH • test.

The anodic oxidation of mercury in presence of hydrogen peroxide in DPV 8 (Differential Pulse Voltammetry) was used to determine the antioxidant (AO) character of 9 radical scavengers. Hydroperoxide radical is formed at the potentials of the oxidation peak on 10 mercury electrodes, such radical reacting with the antioxidants in different extension. The 11 parameter C 10 (antioxidant concentration at which the peak area decreases in a 10%) is used to 12 measure the scavenging activity of the individual antioxidants. To establish the scavenging 13 activity of antioxidant mixtures as a whole it was selected the parameter, µ 10 as the reverse 14 V 10 , being V 10 the volume necessary to decrease the peak area in DPV in a 10%. The higher

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...

No abstract

Graphical abstract