Chemical Compounds and Antioxidant Activity of Forsythia suspensa Leaves Black Tea

Forsythia suspensa leaves black tea (FSLBT) is a fermented herbal tea traditionally consumed in Northern China, yet its bioactive constituents and antioxidant basis remain insufficiently understood. In this study, a phytochemical investigation of FSLBT led to the isolation and structural identification of nine compounds, including five lignans and four pentacyclic triterpenic acids, whose structures were elucidated by ¹H-NMR, ¹³C-NMR, and HRESIMS spectral analysis. Notably, epipinoresinol and pinoresinol monomethyl ether were isolated from Forsythia suspensa leaves (FSL) for the first time. Among the isolated lignans, phillygenin, epipinoresinol, pinoresinol monomethyl ether, and pinoresinol were further evaluated for antioxidant activity using DPPH•, ABTS•+, and FRAP assays. The four lignans exhibited concentration-dependent antioxidant activities, with IC₅₀ (half maximal inhibitory concentration) values ranging from 20.32 to 46.40 μg/mL for DPPH• scavenging and 37.29 to 72.71 μg/mL for ABTS•+ scavenging, while FRAP EC₅₀ (half maximal effective concentration) values ranged from 1.53 to 1.90 mg/mL. Quantitative HPLC analysis showed that the contents of phillygenin, epipinoresinol, pinoresinol monomethyl ether, and pinoresinol in FSLBT were 3.48 ± 1.12 wt%, 0.39 ± 0.21 wt%, 0.26 ± 0.20 wt%, and 0.18 ± 0.07 wt%, respectively. These results indicate that FSLBT is enriched in lignan aglycones, particularly phillygenin, and that these major lignans possess measurable chemical antioxidant activities, providing a phytochemical basis for further investigation of the functional properties of this fermented herbal tea.