Comparative Analysis of Radical Adduct Formation (RAF) Products and Antioxidant Pathways between Myricetin-3--Galactoside and Myricetin Aglycone.

The biological process, 3--galactosylation, is important in plant cells. To understand the mechanism of the reduction of flavonol antioxidative activity by 3--galactosylation, myricetin-3--galactoside (M3OGa) and myricetin aglycone were each incubated with 2 mol α,α-diphenyl-β-picrylhydrazyl radical (DPPH) and subsequently comparatively analyzed for radical adduct formation (RAF) products using ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UPLC-ESI-Q-TOF-MS) technology. The analyses revealed that M3OGa afforded an M3OGa-DPPH adduct (/ 873.1573) and an M3OGa-M3OGa dimer (/ 958.1620). Similarly, myricetin yielded a myricetin-DPPH adduct (/ 711.1039) and a myricetin-myricetin dimer (/ 634.0544). Subsequently, M3OGa and myricetin were compared using three redox-dependent antioxidant analyses, including DPPH-trapping analysis, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide radical (PTIO)-trapping analysis, and O inhibition analysis. In the three analyses, M3OGa always possessed higher IC values than those of myricetin. Conclusively, M3OGa and its myricetin aglycone could trap the free radical via a chain reaction comprising of a propagation step and a termination step. At the propagation step, both M3OGa and myricetin could trap radicals through redox-dependent antioxidant pathways. The 3--galactosylation process, however, could limit these pathways; thus, M3OGa is an inferior antioxidant compared to its myricetin aglycone. Nevertheless, 3--galactosylation has a negligible effect on the termination step. This 3--galactosylation effect has provided novel evidence that the difference in the antioxidative activities of phytophenols exists at the propagation step rather than the termination step.