Identification of flavonols in the leaves of daikon (Raphanus sativus L.)

Radish ( Raphanus sativus L.) is a widely used cruciferous vegetable. Japanese radish, daikon, is one of the most consumed vegetables in Japan. Its large white roots are mainly used as root vegetables. The leaves are not widely used but are known to possess high nutritional value, and contain flavonoids, the plant-derived food components with various human health benefits. In general, flavonoids need to be hydrolyzed to aglycones before absorption and the biological efficacy of flavonoids has been mainly examined in terms of the type of aglycone present. However, it has been reported that flavonoids can also exert various effects in their native state before absorption. In this study, we used HPLC analysis and detected multiple flavonoid components in daikon leaves, but not in the roots. In order to clarify which flavonoids are present in daikon leaves, we isolated them from the leaves and elucidated their structures by HR-MS and NMR analyses. We identified two new flavonols, kaempferol 3- O -β-arabinopyranoside-7- O -[3- O -(4- O -β-glucopyranosyl-6- O - trans -caffeoyl-β-glucopyranosyl)-α-rhamnopyranoside] and kaempferol 3- O -α-rhamnopyranoside-7- O -[3- O -(4- O -β-glucopyranosyl-6- O - trans -caffeoyl-β-glucopyranosyl)-α-rhamnopyranoside] along with two known quercetin glycosides and four known kaempferol glycosides. A unique characteristic we observed in them was their glycosylation pattern with rhamnose at the 7-position and with sugars of different kinds (galactose, glucose, arabinose, or rhamnose) at the 3-position of aglycone. These glycosylation patterns and types of sugars were different from those identified in members of the genus Brassica that are used as vegetables, indicating different mechanisms of action for these compounds in daikon. Flavonols identified from leaves of daikon ( Raphanus sativus L.) • Flavonoids of daikon leaves were isolated and identified by HR-FAB-MS and NMR. • Two new kaempferol glycosides were identified along with six known flavonols. • A characteristic of the flavonols was the attachment of rhamnose at the 7-position. • A 4-glucosyl-6- trans -caffeoyl-glucosyl moiety bound to 7-rhamnose in new flavonols. • Various kinds of sugars were attached to the 3-position of aglycone.