| Summary: | 碩士 === 國立中正大學 === 化學所 === 97 === Recently alpha-lipoic acid (LA) has been shown to possess highly antioxidant properties, but its antioxidant mechanism is not clear. In order to provide LA antioxidant mechanism, our aim of this study is to propose a mechanism for hydroxyl radical attack on LA to analyze theoretically
the possible pathways.
The interactions between LA and hydroxyl radical were studied by B3LYP method using 6-31G(d), 6-31+G(d, p), and 6-311++G(3df, 2p) basis sets. Six different stable structures and three transition states of LA with hydroxyl radical have been identified. In the first step, B3LYP/6-311++G(3df, 2p) was used to study anti-oxidation activity of LA, then the results were compared with those of phenol compound. The relative values of bond dissociation energy and ionization potential of LA are 19.87 and -20.75 kcal/mol, respectively. It can be seen that the activity of LA mainly donates the electron and does not favor H-atom
transfer. In the second step, the total reaction energies of LA and hydroxyl radical have been calculated for three possible reaction routes: (a) electron transfer; (b) adduct formation, and (c) hydrogen atom transfer. At B3LYP/6-311++G(3df, 2p)// B3LYP/6-31G* level, the reaction energies for route (a), (b), and (c) are 131.25, -21.062, and -11.30 kcal/mol, respectively. It is found that the most favored reaction mechanism is the adduct formation where the hydroxyl radical attacks LA at sulfur.
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