Oxidation Reactions of Thymol: A Pulse Radiolysis and Theoretical Study

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 117; no. 2; pp. 291 - 299
• Main Authors: Venu, S, Naik, D. B, Sarkar, S. K, Aravind, Usha K, Nijamudheen, A, Aravindakumar, C. T
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 17-01-2013
• Subjects: Adducts; Antioxidants; Antioxidants - chemistry; Atomic and molecular physics; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); Chemistry; Density-functional theory; Electrochemical Techniques; Electrode potentials; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; General and physical chemistry; Hydroxyl Radical - chemistry; Kinetics; Mathematical analysis; Maxima; Molecular Dynamics Simulation; Oxidants; Oxidation-Reduction; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Physics; Pulse Radiolysis; Radiation chemistry; Radicals; Reactive Oxygen Species - chemistry; Thermodynamics; Thymol; Thymol - chemistry; Antioxidants; Theoretical study; Density functional method; Monoterpene; Free radical reactions; Kinetics; Gas phase; Thymol
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp3082358

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.