Bioactivity-guided fractionation was performed to evaluate the antioxidant activity of Stevia rebaudiana Bertoni. The methanolic crude (MC) extract of leaves was fractionated using a modified Kupchan partitioning method. The antioxidant activity was measured for the MC and fractions. Free radical scavenging using 2,2-Diphenyl-1-picryl-hydrazyl (DPPH) and total antioxidant activity using phosphomolybdenum complex methods were selected for determination of the antioxidant activity using ascorbic acid (AA) as a reference. Preliminary antioxidant activity measurements revealed that the dichloromethane fraction was of interest. Consequently, it was subjected to chromatographic separation and purification techniques. One major pure compound was isolated and its structure elucidation using NMR and HRMS confirmed it to be 3',5,7-trihydroxy-3,4',6-trimethoxyflavone (centaureidin). The IC50 values of MC, butanol, methanol and dichloromethane fractions and centaureidin (CT) were 120.6±0.58, 66.5±0.49, 79.5±0.44, 37.1±0.23 and 18.4±0.44 µg/mL, respectively. The results were compared to those of synthetic antioxidants, butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) with IC50 values of 13.7±0.09 and 33.1±1.05 µg/mL, respectively; while that of AA was 6.5±0.25 µg/mL. The highest total antioxidant activity evaluated by the phosphomolybdenum complex method was exhibited by the purified compound (CT) with a value of 5028.6±645.87 mg AA equivalent/g sample. The study reveals that components of this plant could be a potential alternative to synthetic antioxidants by virtue of their antioxidant properties and natural origin.
Key words: antioxidant activity; Asteraceae; Stevia rebaudiana; synthetic antioxidant; natural antioxidant; Centaureidin
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