As part of an ongoing search for antioxidants from medicinal plants, 20 constituents were isolated from the Nymphaea caerulea flowers, including two 2S,3S,4S-trihydroxypentanoic acid (1), and myricetin 3-O-(3''-O-acetyl)-alpha-l-rhamnoside (2), along with the known myricetin 3-O-alpha-l-rhamnoside (3), myricetin 3-O-beta-d-glucoside (4), quercetin 3-O-(3''-O-acetyl)-alpha-l-rhamnoside (5), quercetin 3-O-alpha-l-rhamnoside (6), quercetin 3-O-beta-d-glucoside (7), kaempferol 3-O-(3''-O-acetyl)-alpha-l-rhamnoside (8), kaempferol 3-O-beta-d-glucoside (9), naringenin (10), (S)-naringenin 5-O-beta-d-glucoside (11), isosalipurposide (12), beta-sitosterol (13), beta-sitosterol palmitate (14), 24-methylenecholesterol palmitate (15), 4alpha-methyl-5alpha-ergosta-7,24(28)-diene-3beta,4beta-diol (16), ethyl gallate (17), gallic acid (18), p-coumaric acid (19), and 4-methoxybenzoic acid (20). The structures were determined by spectroscopic means. Compounds were tested for antioxidant activity and nine compounds 2-7, 11, 12 and 18 were considered active with IC(50) of 1.16, 4.1, 0.75, 1.7, 1.0, 0.34, 11.0, 1.7 and 0.95mug/ml, respectively, while 1 was marginally active (IC(50)>31.25mug/ml). The most promising activity was found in the EtOAc fraction (IC(50) 0.2mug/ml). This can be attributed to the synergistic effect of the compounds present in it.
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