Identification of two polyphenolic compounds with antioxidant activities in longan pericarp tissues

Longan fruits contain a significant amount of polyphenols. In the present study, polyphenols were extracted from longan pericarp tissues, and then two representative polyphenols were separated and purified by polyamide column chromatography, Sephadex LH-20 column chromatography, and silica gel column chromatography. On the basis of 1H NMR, 13C NMR, and electrospray ionization mass spectrometric (ESI-MS) data, the two compounds were identified as 4-O-methylgallic acid and (-)-epicatechin, respectively. In terms of reaction with longan polyphenol oxidase (PPO), (-)-epicatechin was further identified as the PPO substrate that caused longan fruit to brown. The results of antioxidant activity showed that 4-O-methylgallic acid had higher reducing power and 2,2-diphenyl-1-picrylhydrazyl- (DPPH-), hydroxyl radical-, and superoxide radical-scavenging activities than (-)-epicatechin.