Novel proanthocyanidin dimer analogues with the C-ring-opened diaryl-propan-2-gallate structural unit and enhanced antioxidant activities

Abstract

This study aimed to establish an approach for preparing new representative proanthocyanidin oligomers from persimmon for further structure−activity relationship study. Persimmon tannins (with a mean degree of polymerization of 23.7) were depolymerized, using 0.1 M methanolic HCl, with (+)-catechin and (−)-epigallocatechin gallate as chain breakers. Normal phase high-performance liquid chromatography (NP−HPLC) coupled with reversed phase high-performance liquid chromatography mass spectrometry (RP−HPLC−MS) method was used to analyse the degradation products. Four B-type and one A-type dimers were tentatively identified. In addition, three dimer analogues, 1-(3′,4′-dihydroxyphenyl)-3-(2′′,4′′,6′′-trihydroxyphenyl)propan-2-ol-(epi)catechin dimer (B-type) and 1-(3′,4′,5′-trihydroxyphenyl)-3-(2′′,4′′,6′′-trihydroxyphenyl)-propan-2-gallate-(epi)gallocatechin-3-gallate dimers (A- and B-type), were reported for the first time. Three different assays were employed to evaluate the antioxidant capacities of the new dimer analogues. Our results revealed that the formation of the C-ring-opened diaryl-propan-2-gallate structural unit on dimer analogues enhanced their antioxidant abilities in aqueous systems for the first time. This research provided a workable approach for preparing novel dimeric proanthocyanidins for further structure–activity relationship study.