Antioxidant activity of magnolol, honokiol, and related phenolic compounds

The antioxidant activity of 10 Japanese and Chinese crude drugs (Kampo drugs) was determined in vitro. Extract of Magnolia cortex, which had the highest antioxidant activity, contained phenolic compounds magnolol and honokiol. However, inhibitory effects of these compounds on lipid oxidation were weaker than that of α‐tocopherol as measured by thiobarbituric acid assay. The structure‐activity relationship of phenolic compounds showed that antioxidant activities were in the order 4‐allyl‐2,6‐dimethoxyphenol ≥ p,p′‐biphenol > eugenol > 2‐allyl‐6‐methylphenol > honokiol > magnolol > caffeic acid > p‐ethylphenol > guaiacol. As expected, these results showed that an electron donor and/or bulky groups at the ortho‐ or para‐position of the phenol were required for inhibition of lipid oxidation. Electron spin resonance spin trapping experiments showed that phenol compounds with an allyl substituent on their aromatic rings directly scavenged superoxide (O2−), and that only eugenol trapped hydroxyl radicals. These findings suggest that phenolic compounds that contain allyl groups may be effective antioxidants because of the scavenging ability of O2− or hydroxyl radical, whereas other phenols, without an allyl moiety such as α‐tocopherol, may play a role in the termination of free radical chain reactions.