Stability of Polyphenols under Alkaline Conditions and the Formation of a Xanthine Oxidase Inhibitor from Gallic Acid in a Solution at pH 7.4

The stability of 20 different types of polyphenols that are constituents of edible plants was evaluated under different pH conditions (in buffer solutions at pH 6.8, 7.4, and 8.3). Carnosol was the most unstable, followed by myricetin, quercetin, carnosic acid, nordihydroguaiaretic acid, baicalein, gallic acid, and hydroxytyrosol. They were more unstable at a higher pH, which may be attributed to their enhanced redox properties in alkaline solutions, determined by their antioxidant and prooxidant activity. The xanthine oxidase (XO) inhibitory activity of the reaction products under alkaline conditions was evaluated. XO activity was significantly inhibited by the product produced from gallic acid in pH 7.4 solution. The structural analysis of this reaction product revealed that a gallic acid dimer, purpurogallin-8-carboxylic acid, was formed and played a role in the resulting XO inhibitory activity.