Separation, UPLC-QTOF-MS/MS analysis, and antioxidant activity of hydrolyzable tannins from water caltrop (Trapa quadrispinosa) pericarps

Trapa quadrispinosa, commonly known as water caltrop, has long been consumed as a food in Asian countries and its pericarp is discarded as waste after the fruit is harvested. In this study, a reliable method was established using UPLC-QTOF-MS/MS to identify the main compounds in Trapa quadrispinosa pericarps (TQP). The UPLC parameters including column, mobile phase, and temperature were optimized. The extraction parameters including solvents, temperatures and solid-to-liquid ratios were also optimized. A total of 35 compounds were tentatively assigned. The QTOF-MS/MS fragmentation pathways of these compounds were proposed. The most abundant compound (1,2,3,6-tetra-O-galloyl-beta-D-glucose) was further purified by a semi-preparative HPLC and confirmed by H-1- and C-13-NMR. Hydrolyzable tannins (gallotannins and ellagitannins) consisting of almost 90% polyphenols were obtained from the TQP extracts. Three of the main hydrolyzable tannins were 1,2,3,6-tetra-Ogalloyl-beta-D-glucose, tri-O-galloyl-hexahydroxydiphenoyl-D-glucose, and tri-O-galloyl-D-glucose. The TQP extracts exhibited better antioxidant activity in the DPPH assay than that of the ascorbic acid standard. This study provides valuable information for the utilization of TQP as an alternative source of plant-derived antioxidants.