The important role of benzylic CH bond in the antioxidant behaviours of the xanthones

Abstract Xanthones are the major active substances in the mangosteen pericarp with well-documented antioxidant activities. The antioxidant activity of 13 different xanthones identified from the mangosteen pericarp was examined in the gas, benzene and water phases using density functional theory. We found that the benzylic C groups play great contribution to the antioxidant activity of the xanthones. The thermodynamic preferred free radical scavenging mechanisms vary in different phases. In the gas and benzene phases, hydrogen atom transfer (HAT) was the most preferred mechanism in the free radical scavenging reaction of xanthones and the benzylic C H groups are the most favourable antioxidant sites. In these phases, α-mangostin is the best antioxidant. In water phase, sequential proton loss electron transfer (SPLET) was more favourable than HAT and single electron transfer followed by proton transfer (SET-PT) mechanisms and the phenolic O H groups play the most important role in the free radical trapping progress. Besides, γ-mangostin is better antioxidant than other xanthones in water phase.