Thermal stability of the functional ingredients, glucosylated benzophenones and xanthones of honeybush (Cyclopia genistoides), in an aqueous model solution.

Thermal stability of the benzophenones, 3-β-d-glucopyranosyl-4-β-d-glucopyranosyloxyiriflophenone (1), 3-β-d-glucopyranosylmaclurin (2) and 3-β-d-glucopyranosyliriflophenone (3), and the xanthones, mangiferin (4) and isomangiferin (5), was assessed separately in an aqueous model solution (pH 5) to delineate their major degradation products and mechanism(s). Degradation followed first-order reaction kinetics and the temperature-dependence of the respective reaction rate constants complied with the Arrhenius equation. The stability of the compounds increased in the order 2>4>3>5>1. 4-O-Glucosylation significantly stabilised 1 against degradation compared to 3, enediol B-ring functionality of 2 decreased stability compared to 3 and position of glucosylation affected the stability of the xanthones with 5 being more stable than 4. The xanthone nucleus (C-ring) conferred higher stability to 4 and 5 compared to their benzophenone analogue 2. Cyclisation of 2 to 4 and 5 would underestimate their degradation in mixtures. Other reactions were isomerisation, dimerisation, acetylation and hydrolysis.