Antioxidant activities of flavonoid aglycoes from Kenyan gardenia ternifolia schum and thonn
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Phytochemical investigation of surface exudates of the leaves of Gardenia ternifolia resulted to characterization of four flavonoids; 3,5,3′-trihydroxy-7,4′-dimethoxyflavone (1), 5,7-trihydroxy-4′methoxyflavone (2), 5,7-dihydroxy-3,4′-dimethoxyflavone (3), 5,4′-dihydroxy-7-methoxyflavanone (4) and two tritepenoids; β-sitosterol (6) and stigmasterol (7). Compound 1 exhibited the highest antioxidant activity with IC50 = 40.3± 1.55 μΜ. The rest of the flavonoids showed minimal activities with IC50 values of 75.5±1.75, 89±0.22, 94±0.11 μΜ for 2-4, respectively. The antioxidant activities of 1 was substantially lower than the standard, quercetin (IC50 = 20.1±1.34 M). Methoxylation of quercetin at 7 and 4′-position in 1 substantially reduced antioxidant potential. Lack of oxygenation at 3′ position, as observed for kaempferol derivatives was responsible for further reduction in the radical scavenging potential as observed for 2 and 3. Furthermore, methylation of 3-OH position in kaempferol derivatives further reduced the antioxidant activities as exhibited by 3 with an oxygenation pattern similar to 2 except for the methylation at 3-position. The results of this study are consistent with previous findings that revealed that flavonols, exhibited better anti-oxidant activities as compared to 3-methoxyflavones. Acetylation of 3 at the 5 and 7 positions resulting to 3,4′dimethoxy-5,7diacetylflavone (5), substantially reduced the activity of this compound. The triterpenoids exhibited were inactive as expected.
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