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 thepara-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 liquidchromatography coupled with electrospray ionization quadrupole time-of-flight tandem massspectrometry (UPLC-ESI-Q-TOF-MS/MS) analysis revealed that most of these xanthones gavexanthone-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 dimerizeunless 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 minorrole as long as they do not break the above four types.