Constituents of Nymphaea caerulea Flowers and their Antioxidant Acivity

As part of an ongoing search for antioxidants from medicinal plants, twenty constituents were isolated from ethanolic extract of Nymphaea caerulea flowers, including two new ones 2S,3S,4S-trihydroxypentanoic acid (5-Deoxyribonic acid) (1), and myricetin 3-O-(3″-O-acetyl)-α-L-rhamnoside (2), along with eighteen known compounds (3–20). The structures were determined by spectroscopic means including 1D- and 2D-NMR, IR, UV, HR-MS. The plant is a new source for compounds 4, 7–20. Previous work on the flowers of this species resulted in the isolation of seven flavonol glycosides [1] and three acylated anthocyanins [2]. The compounds were tested for antioxidant activity and the results indicated that nine compounds 2–7, 11, 12 and 18 were active with IC_50’s of 1.16, 4.1, 0.75, 1.7, 1.0, 0.34, 11.0, 1.7 and 0.95 µg/mL, respectively, while 1 was marginally active (IC₅₀ > 31.25 µg/mL). Vitamin C, the positive control, had an IC₅₀ of 1.95 µg/mL. The promising activity of EtOAc fraction (IC₅₀ 0.2 µg/mL) could be attributed to the synergistic effect of the compounds present in it. Acknowledgements: This work was supported in part by the United States Department of Agriculture, Agricultural Research Service, Specific Cooperative Agreement No 58-6408-2-0009 and Grant Number P20RR021929 from the National Center For Research Resources. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Center for Research Resources or the National Institutes of Health. References: [1] Fossen T, et al. (1999) Phytochemistry 51: 1133–1137. [2] Fossen T, Andersen OM (1999) Phytochemistry 50: 1185–1188.