Chain-breaking antioxidant activity of hydroxylated and methoxylated magnolol derivatives: the role of H-bonds

Chemical modification of magnolol, an uncommon dimeric neolignan contained in Magnolia genus trees, provides a unique array of polyphenols having interesting biological activity potentially related to radical scavenging. The chain-breaking antioxidant activity of four new hydroxylated and methoxylated magnolol derivatives was explored by experimental and computational methods. The measurement of the rate constant of the reaction with ROO˙ radicals (k_inh) in an apolar solvent showed that the introduction of hydroxyl groups ortho to the phenolic OH in magnolol increased the k_inh value, being 2.4 × 10⁵ M⁻¹ s⁻¹ and 3.3 × 10⁵ M⁻¹ s⁻¹ for the mono and the dihydroxy derivatives respectively (k_inh of magnolol is 6.1 × 10⁴ M⁻¹ s⁻¹). The di-methoxylated derivative is less reactive than magnolol (k_inh = 1.1 × 10⁴ M⁻¹ s⁻¹), while the insertion of both hydroxyl and methoxyl groups showed no effect (6.0 × 10⁴ M⁻¹ s⁻¹). Infrared spectroscopy and theoretical calculations allowed a rationalization of these results and pointed out the crucial role of intramolecular H-bonds. We also show that a correct estimation of the rate constant of the reaction with ROO˙ radicals, by using BDE(OH) calculations, requires that the geometry of the radical is as close as possible to that of the parent phenol.