Structural elucidation and antioxidant activity evaluation of key phenolic compounds isolated from longan (Dimocarpus longan Lour.) seeds.

Polyphenols have been shown to be the main active components responsible for the antioxidant effect of longan seed extract. To figure out the structure–activity relationships of the major phenolic constituents isolated from longan seeds, structural elucidation and antioxidant activities of these compounds were investigated. The detailed structures of three phenolic compounds, namely 9- O -(3-carboxymethyl-4-( p -formylstyryl))hydroxybutanoic acid, 2-hydroxy-3-methoxycaffeic acid 5- O -β- d -glucopyranoside and 3′- O -methyl-4′- O -(4- O -galloyl-α- l -rhamnopyranosyl)ellagic acid, were elucidated for the first time from longan seeds by nuclear magnetic resonance (NMR), Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR–MS) and other spectroscopic methods. The antioxidant activities of the three compounds as well as four previously reported compounds including corilagin, ( S )-flavogallonic acid, 3′- O -methylellagic acid 4′- O -β- d -glucopyranoside and ethyl gallate were compared in three different assays including the Trolox equivalent antioxidant capacity (TEAC) assay, the oxygen radical absorbance capacity (ORAC) assay and the β-carotene–linoleate model assay. Our results confirmed that the previously uncharacterized 2-hydroxy-3-methoxycaffeic acid 5- O -β- d -glucopyranoside and 3′- O -methyl-4′- O -(4- O -galloyl-α- l -rhamnopyranosyl)ellagic acid were the main contributors to the potent activity of longan seed extract. [ABSTRACT FROM AUTHOR]