Abstract
This study used the 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide radical (PTIO•) trapping model to study the antioxidant activities of 16 natural xanthones in aqueous solution, including garcinone C, γ-mangostin, subelliptenone G, mangiferin, 1,6,7-trihydroxy-xanthone, 1,2,5-trihydroxyxanthone, 1,5,6-trihydroxyxanthone, norathyriol, 1,3,5,6-tetrahydroxy-xanthone, isojacareubin, 1,3,5,8-tetrahydroxyxanthone, isomangiferin, 2-hydroxyxanthone, 7-O-methylmangiferin, neomangiferin, and lancerin. It was observed that most of the 16 xanthones could scavenge the PTIO• radical in a dose-dependent manner at pH 4.5 and 7.4. Among them, 12 xanthones of the para-di-OHs (or ortho-di-OHs) type always exhibited lower half maximal inhibitory concentration (IC50) values than those not of the para-di-OHs (or ortho-di-OHs) type. Ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UPLC-ESI-Q-TOF-MS/MS) analysis revealed that most of these xanthones gave xanthone-xanthone dimers after incubation with PTIO•, except for neomangiferin. Based on these data, we concluded that the antioxidant activity of phenolic xanthone may be mediated by electron-transfer (ET) plus H+-transfer mechanisms. Through these mechanisms, some xanthones can further dimerize unless they bear huge substituents with steric hindrance. Four substituent types (i.e., para-di-OHs, 5,6-di-OHs, 6,7-di-OHs, and 7,8-di-OHs) dominate the antioxidant activity of phenolic xanthones, while other substituents (including isoprenyl and 3-hydroxy-3-methylbutyl substituents) play a minor role as long as they do not break the above four types.
Highlights
Natural xanthones can be isolated from edible plants, medicinal plants, and marine-derived fungi (e.g., Talaromyces islandicus EN-501 [2])
22 of of 13 dozens of xanthones have been successfully isolated from the tropical fruits mangosteen and mango, dozens of xanthones been successfully isolated from the tropical fruits mangosteen and mango, and xanthones xanthones have have been and have been considered considered as as the the bioactive bioactive constituents constituents of of these these fruits fruits [3,4,5,6]
PTIO-trapping assaycan canwell wellcharacterize characterize the of an antioxidant and is more suitable to study the antioxidant activity of phenolic xanthones
Summary
Natural xanthones can be isolated from edible plants, medicinal plants (including Chinese herbal medicines [1]), and marine-derived fungi (e.g., Talaromyces islandicus EN-501 [2]). A systematic investigation of the antioxidant activities and mechanisms of the xanthones family has not yet been reported. PTIO-trapping assaycan canwell wellcharacterize characterize the of an antioxidant and is more suitable to study the antioxidant activity of phenolic xanthones. As indicated in the literature [13,14], substituents the antioxidant activities and mechanisms should arise fromthese the effectiveness some specific substituents in xanthones. As indicated in the literature [13,14], these substituents mainly refer to phenolic -OH, isoprenyl, cyclized-isoprenyl, methyl, and glycoside. Isoprenyl5,6-di-OHs, substituent6,7-di-OHs, is considered as a can be further classified several types:The para-di-OHs, 7,8-di-OHs, OHs, single phenolic -OH, and meta-di-OHs. The isoprenyl substituent is considered as a characteristic of-OH, the xanthones family, because about 50%.
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