Theaflavoids A-C, new flavan-3-ols with potent α-glucosidase inhibitory activity from Yunnan black tea 'Jin-Ya'

Black tea, rich in various and complex polyphenols, is one of the most popular beverages in the world due to its pleasant aroma and health benefits, such as alleviate hyperglycemia and antioxidant. The extensive investigation on the chemical constituents of Yunnan black tea 'Jin-Ya', produced from the leaf buds of Camellia sinensis var. assamica, led to the isolation and identification of 16 flavonoids including three new flavan-3-ols, namely theaflavoids A-C (1–3). Most of the isolates displayed obvious inhibitory activity on α-glucosidase with IC50 values ranging from 1.11 to 24.7 μM. Notably, the new theaflavoids 1–3 showed much stronger inhibitory activity (IC50 = 1.34, 1.11, and 4.57 μM, respectively) than those of the positive controls (acarbose and quercetin, IC50 = 228 and 9.38 μM, respectively). In addition, the isolated flavonol glycosides 5–16 were evaluated for their antioxidant activities by DPPH and ABTS + radical scavenging assays. Those with galloyl group in molecular exhibited comparable free radical scavenging activities to the positive controls (ascorbic acid and Trolox), while most of the flavonol glycosides showed only moderate activities. The results indicated that Yunnan black tea 'Jin-Ya' is an ideal drink with hypoglycemic and antioxidant property, and is worthy of regular consumption.