Antiradical Activity of Lignans from Cleistanthus sumatranus : Theoretical Insights into the Mechanism, Kinetics, and Solvent Effects.

Sumatranus lignans ( SL ) isolated from Cleistanthus sumatranus have demonstrated bioactivities, e.g., they were shown to exhibit immunosuppressive properties in previous research. Their structure suggests potential antioxidant activity that has not attracted any attention thus far. Consistently, a comprehensive analysis of the antioxidant activity of these compounds is highly desirable with the view of prospective medical applications. In this work, the mechanism and kinetics of the antiradical properties of SL against hydroperoxyl radicals were studied by using calculations based on density functional theory (DFT). In the lipid medium, it was discovered that SL reacted with HOO • through the formal hydrogen transfer mechanism with a rate constant of 10 1 -10 5 M -1 s -1 , whereas in aqueous media, the activity primarily occurred through the sequential proton loss electron transfer mechanism with rate constants of 10 2 -10 8 M -1 s -1 . In both lipidic and aqueous environments, the antiradical activity of compounds 6 and 7 exceeds that of resveratrol, ascorbic acid, and Trolox. These substances are therefore predicted to be good radical scavengers in physiological environments.