Xanthones and benzophenones from Cyclopia genistoides (honeybush) : chemical characterisation and assessment of thermal stability

[摘要] ENGLISH ABSTRACT: Numerous health-promoting benefits may be derived from the consumption of honeybush tea, a herbal infusion prepared from the leaves and fine stems of the endemic Cape fynbos genus, Cyclopia. These health-promoting benefits are attributed to its phenolic constituents and therefore insight into the nature, quantities and biological activities of individual compounds are required. Information regarding the thermal stability of these compounds is also crucial, as the plant material is subjected to a high-temperature chemical oxidation process ('fermentation) to develop the sought-after characteristic sensory attributes of the herbal tea product. In this study, the phenolic composition of Cyclopia genistoides, a commercially important species, was comprehensively characterised by high-performace liquid chromatography (HPLC) coupled with diode-array and mass spectrometric detection. A species-specific HPLC method was developed for the qualitative analysis of aqueous extracts prepared from 'unfermented and 'fermented C. genistoides plant material and was subsequently validated for the quantification of 18 phenolic compounds in these types of extracts. The major phenolic constituents included the C-glucosyl xanthone mangiferin (1) and its regio-isomer isomangiferin (2), and the benzophenone 3-β-D-glucopyranosyliriflophenone (3). The presence of novel benzophenone and xanthone derivatives in C. genistoides was demonstrated for the first time, including an iriflophenone-di-O,C-hexoside derivative, present in large quantities. This compound was isolated and unambiguously identified by nuclear magnetic resonance spectroscopy as 3-β-D-glucopyranosyl-4-β-D-glucopyranosyloxyiriflophenone (4) – a novel benzophenone unique to Cyclopia. 3-β-D-Glucopyranosylmaclurin (5), present in small quantities, was also isolated. The isolated benzophenones (4 and 5) exhibited mammalian α-glucosidase inhibitory activity, while 4 and 3 were also marginally effective in increasing glucose uptake in vitro. Compound 4 was ineffective as antioxidant in the DPPH assay, but the most effective in the ORAC assay, compared to the other compounds tested (1, 2, 3, 5). Degradation of compounds 1-4 in C. genistoides plant material under simulated fermentation conditions (80 °C/24 h and 90 °C/16 h) followed first-order degradation kinetics and their thermal stability decreased in the order 4 > 2 > 3 > 1. An increase in the degree of glucosylation significantly increased the thermal stability of the benzophenones, whereas glucosylation at C-4 of the dibenzo-γ-pyrone structure, as opposed to C-2, increased the stability of the tetrahydroxyxanthones in the plant material matrix. This was also confirmed for individual compounds (1-5) in aqueous model solutions (pH 5). Inclusion of 5 in the model systems provided additional insight into structure-stability relationships. Increased B-ring hydroxylation significantly increased the first-order degradation rate constants of the benzophenones. Oxidative coupling of the polyhydroxybenzophenone 5 with the formation of its corresponding xanthones (1 and 2) led to substantial increases in the thermal stability of 1 and 2 compared to that of 5. Increased temperatures increased the degradation rates of all compounds in both the plant material matrix and model solutions. The thermal stability of 1, tested at pH 3-7, was found to be pH-dependent, with increased degradation rates observed at higher pH. Thermally-induced reactions principally included isomerisation, dimerisation and cleavage of O-linked sugar moieties; conversion of all benzophenones to the xanthones occurred to varying degrees. Of special interest was the rapid and predominant conversion of 5 to 1 and 2.

[发布机构] Stellenbosch University