3'-Caffeoylquercetin Glycosides and 4'-Caffeoylkaempferol Glycosides-Novel Antioxidant Flavonoids Discovered in the Freesia Yellow Flowers.

The petals of flowering plants should retain unique antioxidants that have not been found in the fruits, as the petals need to stay open to attract pollinators against photooxidation and devise a solution to avoid eating attacks. We reported that the yellow petals of freesia cultivars (Freesia x hybrida) accumulated original apocarotenoids, mono- and di-neapolitanosyl crocetin. Here, in the yellow petals, we discovered eight novel flavonoids by their structural determination, including four 3'-caffeoylquercetin 3,7-glycosides, one 3'-caffeoylquercetin 3-glycoside, and three 4'-caffeoylkaempferol 3,7-glycosides. The 3-carbon sugar part was a minor hexose dimer [D-glucosyl-D-glucoside or D-glucosyl-L-rhamnoside] with the β1,2-linkage, while the 7-carbon was usually O-glycosylated with D-glucose, L-rhamnose, or D-glucuronic acid. Such caffeoyl-flavonol glycosides were also present in freesia white petals, regardless of the cultivars and wild species. When dihydroflavonols, the last common precursors between flavonols and anthocyanins, switch to the flavonol route, these caffeoyl-flavonol glycosides are likely to be synthesized via quercetin or kaempferol. All the eight flavonoids exerted in vitro antioxidant activities against both lipid peroxidation and radical generation. Specifically, 3'-caffeoylquercetin 3-sophoroside and its 7-glucuronide showed superior antioxidant activity. Freesia yellow and white flowers have been utilized as edible flowers, indicating the importance of evaluating the human benefits and risks of newly identified flavonoids.