- Oxidation reactions of thymol: A pulse radiolysis and theoretical study
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The reactions of ?OH and O?-, with thymol, a monoterpene phenol and an antioxidant, were studied by pulse radiolysis technique and DFT calculations at B3LYP/6-31+G(d,p) level of theory. Thymol was found to efficiently scavenge OH radicals (k = 8.1 × 109 dm3 mol-1 s-1) to produce reducing adduct radicals, with an absorption maximum at 330 nm and oxidizing phenoxyl radicals, with absorption maxima at 390 and 410 nm. A major part of these adduct radicals was found to undergo water elimination, leading to phenoxyl radicals, and the process was catalyzed by OH- (or Na2HPO4). The rate of reaction of O?- with thymol was found to be comparatively low (k = 1.1 × 109 dm3 mol -1 s-1), producing H abstracted species of thymol as well as phenoxyl radicals. Further, these phenoxyl radicals of thymol were found to be repaired by ascorbate (k = 2.1 × 108 dm3 mol -1 s-1). To support the interpretation of the experimental results, DFT calculations were carried out. The transients (both adducts and H abstracted species) have been optimized in gas phase at B3LYP/6-31+G(d,p) level of calculation. The relative energy values and thermodynamic stability suggests that the ortho adduct (C6-OH adduct) to be most stable in the reaction of thymol with OH radicals, which favors the water elimination. However, theoretical calculations showed that C4 atom in thymol (para position) can also be the reaction center as it is the main contributor of HOMO. The absorption maxima (λmax) calculated from time-dependent density functional theory (TDDFT) for these transient species were close to those obtained experimentally. Finally, the redox potential value of thymol?/ thymol couple (0.98 V vs NHE) obtained by cyclic voltammetry is less than those of physiologically important oxidants, which reveals the antioxidant capacity of thymol, by scavenging these oxidants. The repair of the phenoxyl radicals of thymol with ascorbate together with the redox potential value makes it a potent antioxidant with minimum pro-oxidant effects.
- Venu,Naik,Sarkar,Aravind, Usha K.,Nijamudheen,Aravindakumar
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p. 291 - 299
(2013/03/14)
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- Interconversion between dehydro-L-ascorbic acid and L-ascorbic acid.
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L-Ascorbic acid (AA) plays an important role in biological systems as an electron donor. Erythorbic acid (EA) is the epimer of AA and has chemical characteristics very similar to those of AA. It is demonstrated in the present study by 1H-NMR that dehydro-L-ascorbic acid (DAA) was reduced by EA under neutral conditions but not acidic, and that dehydroerythorbic acid (DEA) was also reduced by AA under the same conditions. These reactions also occurred at a low concentration close to the concentration of AA in such biological tissue as the liver. Furthermore, the interconversion of DAA and AA at neutral pH and low concentration was also confirmed by radioluminography. These results suggest the interconversion between DAA and AA in vivo.
- Nishikawa,Kurata
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p. 476 - 483
(2007/10/03)
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- Outer-Sphere Electron-Transfer Reactions of Ascorbate Anions
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Rate constants for the one-electron oxidation of ascorbate dianion (A2-) by bis(terpyridine)cobalt(III) ion (8.5*106 dm3 mol-1 s-1) and pentaammine(pyridine)ruthenium(III) ion (6.0*109 dm3 mol-1 s-1), and of the monoanion (HA-) by tetraammine(bipyridine)ruthenium(III) ion (2.1*105 dm3 mol-1 s-1) have been determined in aqueous solution at 25 deg C and ionic strength 0.1 (NaNO3 or NaClO4).It is shown that these rate constants, and other published rate constants for oxidation of HA- and A2-, are consistent with the Marcus cross relation, on the assumption that the self-exchange rate constants for both the HA-/HA. and A2-/A-. couples are 106 dm3 mol-1 s-1.One-electron redox potentials for the ascorbate/dehydroascorbate system have been derived from scattered literature data.
- Williams, Nicola H.,Yandell, John K.
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p. 1133 - 1144
(2007/10/02)
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- Electron Transfer Reactions of Halogenated Aliphatic Peroxyl Radicals: Measurement of Absolute Rate Constants by Pulse Radiolysis
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The peroxyl radicals Cl3OO., Cl2CHOO., ClCH2OO., CH3OO., .OOCCl2CO2-, .OOCHClCO2-, .OOCH2CO2-, .OOCF2CO2-, and (CH3)2C(OH)CH2OO. have been generated in aqueous alcohol solutions by pulse radiolysis using the Brunel and Hahn-Meitner accelerators.The absolute rate constants of the one-electron transfer reactions of related aliphatic peroxyl radicals with ascorbate, promethazine, phenol, and tyrosine have been found to increase with increasing substitution of chlorine atoms.For the reaction of the radical Cl3CO2. and promethazine, at pH 6, k 6.0 +/- 0.4 x 108 l mol-1 s-1 has been measured.For the analogous reaction of the radical ClCH2O2., k 3.3 +/- 0.2 x 107 l mol-1 s-1 has been determined.
- Packer, John E.,Willson, Robin L.,Bahnemann, Detlef,Asmus, Klaus-Dieter
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p. 296 - 299
(2007/10/02)
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