Ultrasound-assisted extraction of bound phenolic compounds from the residue of Apocynum venetum tea and their antioxidant activities

Abstract

Bound phenolic compounds from the residue of Apocynum venetum (A. venetum) tea were released by alkaline digestion with sonication, followed by extraction using ethyl acetate (EtOAc). The fragment ions of compounds in free and bound forms were identified by liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) in both positive and negative modes. Further investigation of chemical constituents in the EtOAc extract led to the isolation of eleven individual compounds, and their chemical structures were identified as (−)-epicatechin (1), (−)-loliolide (2), (−)-syringaresinol-4-O-β-D-glucopyranoside (3), alloside of benzyl alcohol (4), 3-[(6-O-hexopyranosylhexopyranosyl)oxy]-2-(palmitoyloxy)propyl (9Z,12Z,15Z)-9,12,15-octadecatrienoate (5), apocynoside I (6), quercetin-3-O-β-D-glucopyranoside (7), kaempferol-3-O-β-D-glucopyranoside (8), [1-acetyloxy-3-[3,4,5-trihydroxy-6-[[3,4,5-trihydroxy-6-(hydroxymethyl)oxan-2-yl]oxymethyl]oxan-2-yl]oxypropan-2-yl] hexadecanoate (9), 1-stearoyl-2-palmitoyl-sn-glycerol (10), and 3-hexene-l-O-β-D-glucoside (11) using a series of spectroscopic methods. Compounds 2–5 and 9–11 were isolated for the first time from A. venetum, while antioxidant experiments showed that compounds 1, 7, and 8 displayed strong DPPH and ABTS + radical scavenging activities. This study represents the first investigation of bound phenolic compounds with potential health benefits in the residue of A. venetum tea.