Isolation, Structural Characterization and Antioxidant Activity of Brown Rice Bound Phenolics
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Abstract:
Brown rice bound phenolics were extracted by alkaline hydrolysis and separated by multiple chromatographic methods. Four monomers were obtained and their structures were identified by high performance liquid chromatography-tandem mass spectrometry (LC-MS) and nuclear magnetic resonance (NMR). The isolated compounds were elucidated as 6-hydroxy-2-oxo-1,2,3,4-tetrahydroquinoline-4-carboxylic acid methyl ester (1), 4-carboxymethoxy-6-hydroxy-2-quinolone (2), 4-carboxyl-6-hydroxy-2-quinolone(3) and 2 - (5-hydroxy-2-oxoindole-3-yl) acetate methyl ester (4). Compound 1 was a new natural product and compound 2 was reported from brown rice for the first time. The contents of four monomers were determined by high performance liquid chromatography (HPLC) and the content of compound 1 was the highest (0.9 mg/g DW). Antioxidant activity of four compounds was evaluated using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, ferric ion reducing antioxidant power (FRAP) assays and oxygen radical absorbance capacity (ORAC) methods, respectively. The strongest DPPH radical scavenging activity was compound 4, with the IC50 value being 72.19 μM. Compound 2 exhibited the highest FRAP reducing activity, which was 249.60 mmol FeE/mol. ORAC method showed that compounds 3 and 4 had the best antioxidant capacity, which were 2.87 and 2.69 μmol TE/μmol, respectively. These results will promote the cognition of brown rice bound phenolics. It can also provide scientific basis for the development and utilization of functional substances in brown rice.
