Antioxidant and Tyrosinase-Inhibitory Activities and Biological Bioactivities of Flavonoid Derivatives from Quercus mongolica Pollen

Flavonoids, present in plants as enriched secondary metabolites, prevent various stresses such as temperature fluctuations, acidity, and insect predation, are commonly found in leaves, stems, and flowers, and serve as important bioactive components. In this study, a total of eighteen different flavonoids, including one newly identified flavonoid glycoside, were successfully isolated from the pollen of Quercus mongolica. The structure of the novel compound was determined using nuclear magnetic resonance, mass spectrometry, and infrared spectroscopy. Additionally, GC analysis was conducted to determine the sugar moiety in the new compound, confirming the specific type of disaccharide present. The 18 compounds were classified as flavonoid glycosides (1–10), flavonoids (11–17), and isoflavone (18). All the isolated compounds were evaluated for their tyrosinase inhibitory and antioxidant activities, and their structure–activity relationships (SARs) were also evaluated. Compounds 12 and 16 showed higher tyrosinase inhibitory activities compared to kojic acid as positive control. Compounds 2, 5, 8, 12, 13, 14, and 16 demonstrated potent antioxidant activities. Among these compounds, 5 and 16 showed even higher antioxidant activity than the ascorbic acid. Structure–activity relationship analysis revealed that tyrosinase-inhibitory and antioxidant activities were enhanced in compounds with a hydroxy group of C-3 or C-3′t in flavonoid aglycones compared to their glycosides. These findings indicate that flavonoids and/or extracts from the pollen of Q. mongolica are valuable natural resources with applications in the pharmaceutical and cosmetic industries.