Isolation and Antioxidant Activity of Flavonoids from Coriaria myrtifolia Methanolic Extract

Abstract

where A blank is the absorbance of the blank and A sample is the absorbance of the sample. The air-dried leaves of C. myrtifolia (30 g) were extracted with methanol. The methanolic residue was washed with n-hexane until discoloration to remove fatty acids. The polar fraction (PF1) was extracted first with ethyl acetate, yielding two fractions: ethyl acetate residue (EA; 12.5 g) and the polar fraction (PF2). The latter was extracted with n-butanol to obtain the n-butanol fraction (B; 6.15 g). The four fractions (PF1, EA, PF2, and B) were tested to evaluate their free radical scavenging activities. All fractions tested showed antioxidant activity higher than the standard (BHT, butylated hydroxyltoluene, 5.98 # 0.5 mg/mL) with IC 50 values of 1.68 # 0.5 mg/mL (B), 2.45 # 1.5 mg/mL (PF2), 2.81 # 1.0 mg/mL (PF1), and 3.68 # 1.26 mg/mL (EA). Fraction EA and B were further purified by repeated chromatographies (Sephadex LH 20, SPE, and HPLC in RP8 reverse phase) to obtain four pure flavonol glycosides: kaempferol 3-O-rhamnoside (1, 3.7 mg), quercetin 3-O-rhamnoside (2, 23.2 mg), quercetin 3-O-galactoside (3, 24.5 mg), and quercetin 3-O-glucoside (4, 11.4 mg), known as afzelin, quercitrin, hyperoside, and isoquercitrin, respectively. All these constituents gave yellow-orange fluorescence at 366 nm on TLC after spraying with NTS-PEG reagent. The identification of compounds 1‐4 was established by comparing their 1 H and 13 C NMR chemical shifts and proton coupling constants with those reported in the literature [10‐12]. These pure compounds were screened for their free radical scavenging properties according to the method used for the extracts. The IC 50 of the pure isolated flavonol glycosides 1‐4 ranged between 1.98 # 1.0 mg/mL of kaempferol 3-O-rhamnoside (1) to 3.52 # 1.0 mg/mL of quercetin 3-O-galactoside (3), with similar intermediate values of compounds 4 and 2 (2.45 # 1.5 and 2.68 # 1.0 mg/mL, respectively).