In the present work we synthesized a series of hydroxy-3-arylcoumarins (compounds 1-9), some of them previously described as MAO-B selective inhibitors, with the aim of evaluating their antioxidant properties. Theoretical evaluation of ADME properties of all the derivatives was also carried out. From the ORAC-FL, ESR and CV data it was concluded that these derivatives are very good antioxidants, with a very interesting hydroxyl, DPPH and superoxide radicals scavenging profiles. In particular compound 9 is the most active and effective antioxidant of the series (ORAC-FL=13.5, capacity of scavenging hydroxyl radicals=100%, capacity of scavenging DPPH radicals=65.9% and capacity of scavenging superoxide radicals=71.5%). Kinetics profile for protection fluorescein probe against peroxyl radicals by addition of antioxidant molecule 9 was also performed. Therefore, it can operate as a potential candidate for preventing or minimizing the free radicals overproduction in oxidative-stress related diseases.

A series of novel hydroxy-coumarin-chalcone hybrid compounds 2a-i has been synthesized by employing a simple and efficient methodology. An electrochemical characterization using cyclic voltammetry and ESR spectroscopy were carried out to characterize the oxidation mechanism for the target compounds. The antioxidant capacity and reactivity were determined by ORAC and ESR assays, respectively. Biological assays were assessed to evaluate the cytotoxicity and cytoprotection capacity against ROS/RNS on BAEC. The results revealed that all tested compounds present ORAC values that are much higher than other well-known antioxidant compounds such as quercetin and catechin. Compound 2e showed the highest ORAC value (14.1) and also presented a low oxidation potential, good scavenging capacity against hydroxyl radicals, low cytotoxicity, and high cytoprotective activity.

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The study of antioxidants and radicals has always been a complex task due to the special characteristics of these species such as reactions at low concentrations and short half-lives. Current techniques do not always produce good results and in some cases they can only be applied in chemical models. From this point of view, the development of electron spin resonance (ESR) has allowed the study of the antioxidant capacity of a wide variety of compounds and the detection of radicals in the reactions in which they are involved. The DPPH technique allows only the study of antioxidants in pure chemical models. The ORAC-ESR assay, based on the spin trapping technique, emerges as an interesting tool for identifying and quantifying the antioxidant capacity of different samples. Furthermore, the spin trapping technique allows us to characterize radicals in in vivo/ex vivo models. The present review discusses the current available techniques associated with ESR for the study of antioxidants and radical species.

Hypochlorite is a strong oxidant able to induce deleterious effects in biological systems. The goal of this work was to investigate the use of PGR and PYR as probes in assays aimed at evaluating antioxidant activities towards hypochorite and apply it to plant extracts employed in Chilean folk medicine. The consumption of PGR and PYR was evaluated from the decrease in the visible absorbance and fluorescence intensity, respectively. Total phenolic content was determined by the Folin Ciocalteau assay. PGR and PYR react with hypochlorite with different kinetics, being considerably faster the consumption of PGR. Different stoichiometric values were also determined: 0.7 molecules of PGR and 0.33 molecules of PYR were bleached per each molecule of added hypochlorite. Both probes were protected by antioxidants, but the rate of PGR bleaching was too fast to perform a kinetic analysis. For PYR, the protection took place without changes in its initial consumption rate, suggesting a competition between the dye and the antioxidant for hypochlorite. Plant extracts protected PYR giving a PYR-HOCl index that follows the order: Fuchsia magellanica ≈ Marrubium vulgare ≈ Tagetes minuta > Chenopodium ambrosoides ≈ Satureja montana > Thymus praecox. Based on both the kinetic data and the protection afforded by pure antioxidants, we selected PYR as the best probe. The proposed methodology allows evaluating an antioxidant capacity index of plant extracts related to the reactivity of the samples towards hypochlorite.

A set of seven hydroxycoumarin derivatives (1-7) were tested to evaluate their antioxidant properties. These molecules were selected on the basis of the number of hydroxyl groups and the presence of electron-donating and electronwithdrawing groups in their scaffolds. Electrochemical studies showed that oxidation process occurs at low potentials, especially with the catechol-containing coumarins. Electron spin resonance (ESR) experiments corroborated the formation of semiquinone radicals in all compounds. The antioxidant properties against peroxyl and hydroxyl radicals were evaluated using two different assays, namely oxygen radical antioxidant capacity (ORAC) and non-catalytic Fenton system by ESR measurements. Our results indicated that monohydroxycoumarin derivatives (1 and 2) displayed better ORAC values relative to the standard molecule Trolox. These values are comparable with known flavonoids such as quercetin and catechin. On the other hand, compounds bearing catechol and resorcinol groups, including derivative 4, gave high hydroxyl radical-scavenging properties. Antioxidant properties were further rationalized using density functional theory calculations. The theoretical results showed good trends among the obtained experimental data, calculated bonding dissociation enthalpy, highest occupied molecular orbital values, free energies of oxidation (ΔG ox ) and spin density distribution. Finally, in vitro studies revealed low toxicity for most of the coumarins studied. The cytoprotection assays in bovine aortic endothelial cells indicated that compounds 1 and 2 showed better protective properties against peroxynitrite and hydrogen peroxide-induced cell death than dihydroxycoumarin derivatives. Using confocal microscopy, we have observed intracellular compartmentalization. These results confirm that 1 or 2 would be a good target for future in vivo studies against oxidative stress.