Antioxidant Activity of Magnolol and Honokiol: Kinetic and Mechanistic Investigations of Their Reaction with Peroxyl Radicals

Magnolol and honokiol, the bioactive phytochemicals contained in Magnolia officinalis, are uncommon antioxidants bearing isomeric bisphenol cores substituted with allyl functions. We have elucidated the chemistry behind their antioxidant activity by experimental and computational methods. In the inhibited autoxidation of cumene and styrene at 303 K, magnolol trapped four peroxyl radicals, with a <i>k</i>_inh of 6.1 × 10⁴ M^–1 s^–1 in chlorobenzene and 6.0 × 10³ M^–1 s^–1 in acetonitrile, and honokiol trapped two peroxyl radicals in chlorobenzene (<i>k</i>_inh = 3.8 × 10⁴ M^–1 s^–1) and four peroxyl radicals in acetonitrile (<i>k</i>_inh = 9.5 × 10³ M^–1 s^–1). Their different behavior arises from a combination of intramolecular hydrogen bonding among the reactive OH groups (in magnolol) and of the OH groups with the aromatic and allyl π-systems, as confirmed by FT-IR spectroscopy and DFT calculations. Comparison with structurally related 3,3′,5,5′-tetramethylbiphenyl-4,4′-diol, 2-allylphenol, and 2-allylanisole allowed us to exclude that the antioxidant behavior of magnolol and honokiol is due to the allyl groups. The reaction of the allyl group with a peroxyl radical (C–H hydrogen abstraction) proceeds with rate constant of 1.1 M^–1 s^–1 at 303 K. Magnolol and honokiol radicals do not react with molecular oxygen and produce no superoxide radical under the typical settings of inhibited autoxidations