Novel Antioxidant Quercetin Glycosides Conjugated with Phenolic Acids from Young Leaves of Vicia unijuga

Vicia unijuga is an edible wild plant distributed throughout eastern Asia. Its young leaves, when boiled or fried, are palatable and exhibit an aroma and flavor reminiscent of soybeans. In this study, we isolated two previously unreported quercetin glucosides conjugated with phenolic acids (compounds 2 and 3), together with quercitrin (compound 1), from young V. unijuga leaves as potent inhibitors of lipid peroxidation. Their structures were elucidated using high-resolution electrospray ionization mass spectrometry (negative mode), 1D and 2D nuclear magnetic resonance analyses, and enzymatic hydrolysis. Both new compounds shared a common structure in which α-L-rhamnose and β-D-glucuronic acid were attached to quercetin at the 3- and 7-positions, respectively. Compound 2 contained a caffeic acid moiety esterified at the 2-position of β-D-glucuronic acid, whereas compound 3 carried a p-coumaric acid moiety at the same position. Compounds 1-3 inhibited lipid peroxidation, with IC50 values of 11±1.3, 0.21±0.02, and 1.3±0.09 μM, respectively.