Isolation, characterization, antioxidant activity, and protein-precipitating capacity of the hydrolyzable tannin punicalagin from pomegranate yellow peel (Punica granatum)

Abstract This work describes the isolation and characterization of the hydrolysable tannin punicalagin, obtained from the yellow peel of pomegranate ( Punica granatum , belonging to the family Lythraceae ). The natural product was present as a mixture of α and β anomers, rapidly interconverting under acidic pH conditions. A fast and efficient purification method was established using semi-preparative high performance liquid chromatography (HPLC). The chemical structure of the molecule was confirmed as punicalagin based on IR spectroscopy, 1 H and 13 C NMR, and MALDI-TOF mass spectrometry studies. The lowest energy conformations of the α and β anomers were calculated by quantum chemical methods, and their ratio compared to the experimental value (experimental: α/β = 2.2 to 2.5; calculated α/β = 2.4, in aqueous solution at acidic pH). The antioxidant activity of pure punicalagin was determined with a DPPH radical scavenging assay (IC 50  = 1.9 ± 0.2 μg/mL) and was comparable to that of tannic acid (IC 50  = 1.3 ± 0.2 μg/mL). Finally, we demonstrated that punicalagin, when used above a threshold concentration, was able to precipitate bovine serum albumin (BSA), although less efficiently than tannic acid.