Modeling the peroxyl radical scavenging behavior of Carnosic acid: Mechanism, kinetics, and effects of physiological environments

Carnosic acid (CA), a phenolic diterpene and abietane-type compound, is a potent natural antioxidant with medical benefits. The present paper elucidates, for the first time, the kinetics and the exact mechanism of the peroxyl radical scavenging activity of CA in the gas phase and under physiological conditions. According to the obtained results, the reaction of CA with HOO center dot is significantly faster in aqueous solution than in the gas phase and nonpolar environments. The abstraction of the hydrogen atom from 2-OH is the decisive mechanism in the gas phase and nonpolar media, while both hydrogen abstraction (15%) and electron transfer (85%) mechanisms can take place in aqueous solution. The overall rate coefficient in water (4.73 x 10(6) M-1 s(-1)) is about 36 times higher than that of the reference antioxidant Trolox (1.30 x 10(5) M-1 s(-1)), suggesting that CA is a potent scavenger of peroxyl radicals in polar media.