Composition and Antioxidant and Antidiabetic Activities of Different Polyphenol Fractions from the Fruits of Rubus chingii Hu

To elucidate the differences in the antioxidant and antidiabetic activities of different phenolic fractions from Rubus chingii Hu, aqueous-soluble, alcohol-soluble, free and bound polyphenols from the fruits of R. chingii Hu were extracted using different solvents and methods. The difference in the contents of total phenolic compounds, total flavonoids, and hydrolyzable tannins in the different extracts were compared, and the radical scavenging, α-glucosidase inhibitory and antiglycation activity of the extracts were determined. The major chemical components in the extract with the highest activity were determined using mass spectrometry (MS). The results indicated that the alcohol-soluble polyphenol-rich extract had the highest contents of total phenols and hydrolysable tannins (414.95 μg/mg and 182.47 μg/mg (expressed as gallic acid equivalent in per milligram of extract), respectively). Meanwhile, it also presented the strongest ability to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and 2,2’-azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS) cation radical and to inhibit α-glucosidase activity with half maximal inhibitory concentration (IC₅₀) values of 40.68, 23.90 μg/mL and 1.65 μg/mL, respectively. But the highest total flavonoid content and antiglycation activity were observed in the bound polyphenol-rich extract. Totally, 31 compounds were tentatively identified from the bound polyphenol-rich extract, and tannins and flavonoids were the major bioactive constituents. Therefore, the R. chingii Hu residues left after the extraction of free polyphenols and aqueous-soluble constituents can be further utilized to develop bioactive constituents with high antioxidant and antidiabetic activities.