Eumitrin L: a new xanthone dimer from the lichen Usnea baileyi

Eumitrin L (1), a new dimeric xanthone, was isolated from the crude acetone extract of the lichen U. baileyi, together with 13 known compounds, including one dimeric xanthone (2), five depsidones (3-7), two depsides (8-9), one dibenzofuran (10) and four mono phenolics (11-14). Spectroscopic analyses (1D, 2D NMR), compared to related references, were employed for structural elucidation, while DP4 probability and ECD spectrum helped identify the absolute configuration of 1. Bioactivity evaluation of the isolated compounds revealed 6, 7, 8, 11 as active tyrosinase inhibitors, and 3, 10, 13, 14 as significant antioxidants. Among them, protocetraric acid (6) (IC₅₀ 37.86  μM) and usnic acid (10) (IC₅₀ 15.92  μM) were found as the most active components. Molecular docking simulation on these two compounds further confirmed their notable activities, based on effective H-bond formation and excellent binding energy with the tyrosinase protein (2Y9X), and DPPH proteins (2CDU and 3RP8), respectively.