Abel J. S. C. Vieira

Antioxidant mechanisms of Quercetin and Myricetin in the gas phase and in solution--a comparison and validation of semi-empirical methods.

Flavonoids have long been recognized for their general health-promoting properties, of which their antioxidant activity may play an important role. In this work we have studied the properties of two flavonols, quercetin and myricetin, using semi-empirical methods in order to validate the application of the recent Parametric Model 6 and to understand the fundamental difference between the two molecules. Their geometries have been optimized and important molecular properties have been calculated. The energetic of the possible antioxidant mechanisms have also been analyzed. The two studied flavonols do not differ significantly in their molecular properties, but the antioxidant mechanisms by which they may act in solution can be rather different. Moreover, we also show that the Parametric Model 6 can produce reliable information for this type of compounds.

Scavenging activity of aminoantipyrines against hydroxyl radical

The pyrazolone derivatives antipyrine and 4-(N,N-dimethyl)-aminoantipyrine (aminopyrine) have long been used as analgesic, antipyretic and anti-inflammatory drugs. However, in spite of its recognized therapeutic benefits, the use of pyrazolones has been associated with agranulocytosis. Though the oxidation of aminopyrine by neutrophil-generated hypochlorous acid (HOCl), leading to the formation of a cation radical, has been considered responsible for the potential bone marrow toxicity, the reaction mechanisms of pyrazolones against other reactive oxygen species (ROS) remains elusive. Thus, the reactions of 4-aminoantipyrine and methylated derivatives with hydroxyl radicals (HO*) were studied as a model of their reactivity against ROS. The results show that 4-(N,N-dimethyl)-aminoantipyrine (aminopyrine) undergoes demethylation when reacting with HO. radical, leading to 4-(N-methyl)-aminoantipyrine, which is further demethylated to 4-aminoantipyrine. In addition, it was also observed that another favorable reaction of 4-aminoantipyrines in these conditions is the hydroxylation on the aromatic ring, a reaction that is common to aminopyrine, 4-(N-methyl)-aminoantipyrine, and 4-aminoantipyrine. Whether these reaction mechanisms give rise to harmful reactive intermediates requires further chemico-biological evaluation.

Dipyrone and aminopyrine are effective scavengers of reactive nitrogen species

Abstract Reactive nitrogen species (RNS), namely nitric oxide (NO•) and peroxynitrite (ONOO−) are produced in the inflammatory sites and may contribute to the deleterious effects of inflammation. The aim of the present study was to evaluate the putative scavenging effect of a particular group of non-steroidal anti-inflammatory drugs (NSAIDs), the pyrazolone derivatives dipyrone, aminopyrine, isopropylantipyrine, and antipyrine against RNS, using in vitro non-cellular screening systems. The results obtained showed that dipyrone and aminopyrine were highly potent scavengers of NO• and ONOO− while antipyrine exerted little effect and isopropylantipyrine no effect whatsoever against these two RNS and that, in the presence of bicarbonate, the scavenging potencies of both dipyrone and aminopyrine were slightly decreased. It could thus be inferred that the observed scavenging effects may be of therapeutic benefit for patients under anti-inflammatory treatment with dipyrone and aminopyrine in the case of overproduction of RNS. On the other hand, the possible depletion of physiological NO• concentrations, namely at the gastrointestinal tract as well as the formation of reactive derivatives of aminopyrine and/or dipyrone, resulting from their reaction with RNS, may otherwise be harmful for these patients.

Mechanism of free radical oxidation of caffeine in aqueous solution

The oxidation of caffeine with persulfate and hydroxyl radicals in aqueous solution has been studied by EPR spectroscopy and HPLC analysis. In both cases the formation of 1,3,7-trimethyluric acid is observed as the main final product. A C8–OH radical adduct is postulated as the intermediate after reaction with OH˙, and leads to the final product after further oxidation. This radical is too short-lived to be observed by EPR. After oxidation of caffeine with SO4˙- its radical cation is detected by EPR. This radical reacts with water to produce the above mentioned C8–OH radical adduct after deprotonation. In the presence of dibasic phosphate a spectrum attributed to the C8-phosphate radical adduct is observed. This radical results from the nucleophilic attack of the buffer on the radical cation of caffeine. Hydrolysis and oxidation of the phosphate radical intermediate results in the formation of 1,3,7-trimethyluric acid.

Demethylation of theophylline (1,3-dimethylxanthine) to 1-methylxanthine: the first step of an antioxidising cascade

Abstract The reaction of theophylline with HO• radical, produced by photolytic methods at pH 7, was studied in aqueous solution and the products characterised by HPLC and GC-MS. In addition to the expected 1,3-dimethyluric acid, the formation of 1-methylxanthine and, to a lesser extent, of 3-methylxanthine was observed. Theoretical calculations confirmed the preferred formation of the former compound. Both demethylated products were also observed upon reaction of theophylline with O•– radical anion at pH∼13, and 1-methylxanthine was consumed faster than 3-methylxanthine after its formation. Molecular oxygen had no significant effect on the formation of the mono-methylxanthine derivatives. A reaction mechanism for the demethylation of theophylline by oxidising radicals is proposed. This demethylation reaction can play an important role in the protection of biological targets against oxidative stress as the first step of an antioxidising cascade.

EPR spectroscopic study of the radical oxidation of hydroxypurines in aqueous solution: acid–base properties of the derived radicals

EPR spectroscopy has been employed to study the reactions of SO4˙– and OH˙ radicals with xanthine, 8-methylxanthine, hypoxanthine and 2-hydroxypurine.Radicals obtained on the reaction of SO4˙– with these hydroxypurines resulted, depending on the pH, from one-electron oxidation of either the parent compound or its anion, followed by deprotonation. For xanthine and 8-methylxanthine the respective radical anion and radical dianion were observed, for hypoxanthine the species detected correspond to the neutral radical and radical anion, whereas for 2-hydroxypurine only its radical anion was observed.The pKa values of the radical anion of xanthine and 8-methylxanthine were estimated as 12.0 ± 0.1 and 12.1 ± 0.5, respectively, whereas the pKa value of the hypoxanthine neutral radical was 7.7 ± 0.1.In the reaction with OH˙, only the radical anion of xanthine was detected.

Structure and redox properties of radicals derived from one-electron oxidised methylxanthines.

Abstract The repair of the one electron oxidised form of a xanthine derivative by another xanthine derivative was studied by reacting aqueous binary mixtures of xanthine derivatives with sulphate radical anion (SO4˙−) and following the concentration of both compounds as a function of time. The relative behaviour observed enabled the establishment of a qualitative order of antioxidant capacities for the several xanthines studied in acidic and neutral media. The same order was confirmed quantitatively by measuring the reduction potentials of the xanthines by cyclic voltammetry. Theoretical DFT calculations were used to calculate the relative stabilities of the tautomers of each xanthine neutral radical. It was also demonstrated that the deprotonation of a xanthine radical cation never occurs from N1, unless no other possibility is available. At high pH values, it was possible to obtain the ESR spectra of the radical anions derived from 1-methylxanthine, 3-methylxanthine and xanthosine. The theoretical calculations also enabled the assignment of the ESR hyperfine coupling constants of the spectra of these radical anions. The coupling constants calculated are in good agreement with the experimental values.

Transient spectroscopy of ninhydrin

The photochemistry of ninhydrin in benzene and water was studied by laser flash photolysis and electron paramagnetic resonance. Its photochemistry was shown to be dependent on the solvent. In benzene, a triplet excited state was observed, which underwent hydrogen abstraction reactions or reduction to the radical anion. In water, the radical anion of ninhydrin was formed within the laser pulse (15 ns) at neutral pH, whereas the neutral ketyl radical was formed by protonation of the radical anion at low pH. A pKa of 0.77 was determined for the protonation equilibrium. The formation of hydrindantin is proposed to occur through the dimerization of the ketyl radical or the radical anion (or both). In addition, ninhydrin was shown to be a poor precursor for the photogeneration of hydroxyl radicals.

Electron spin resonance study of the reaction of SO4˙– with azolinones

The reactions of photolytically generated sulfate radical-anion, SO4˙–, with several nitrogen-containing heterocycles of the azolinone type in aqueous solutions were studied by ESR spectroscopy. SO4˙– reacts by formal hydrogen abstraction from an NH group to yield neutral radicals in acidic and neutral solutions and the respective radical-anions at basic pH. The pKa-values of radicals derived from 2,3-dihydroimidazol-2(1H)-one, 2-oxo-2,3-dihydro-1H imidazole-4-carboxylic acid and 2,3-dihydroindazol-3(1H)-one were determined to be in the range 8.5–10.4. In the cases of 2,3-dihydroimidazol-2(1H)-one and 2,3-dihydropyrazol-3(1H)-one secondary radicals were detected at basic pH. These radicals resulted from the disproportionation of the primary radicals followed by addition of OH– and oxidation of the OH-adduct.

Gamma irradiation of clove: level of trapped radicals and effects on bioactive composition: Gamma irradiation of clove

BACKGROUND Food irradiation is a widely used technique for improving the safety and shelf life of foods, including most spices. However, growing concerns by consumers about this technique require further investigation on the effects of radiation, both on the safety of the food and on its organoleptic properties. In this work, cloves of diverse origins were submitted to different irradiation doses in a 60 Co source. The presence of trapped radicals and their time-dependent decay after irradiation were assessed by electron spin resonance (ESR) spectroscopy. The volatile bioactive composition and the clove oil were evaluated before and after irradiation by gas chromatography time-of-flight mass spectrometry. RESULTS Results show an increase of the amount of volatiles collected after irradiation, especially of caryophyllene oxide and acetic acid, although these are still minor constituents. No new compound was detected after irradiation. Radicals decay fast, and 60 days after irradiation they were undetectable by ESR. CONCLUSION Gamma irradiation showed to be a clean technique for clove decontamination, since no significant change in the aroma or oil compositions was found, and low levels of trapped paramagnetic species, after the initial decay period, were detected upon irradiation. Furthermore, irradiation doses higher than those legally allowed are equally safe.