A new pheophytin, (132S, 17S, 18S)-132-hydroxy-20-chloro-ethylpheophorbide a (3), along with two known analogues (1-2) were isolated from the lichen Usnea diffracta Vainio (Parmeliaceae). Among them, compound 3 was a rare C-20-chloro type pheophytin obtained from lichens. Their structures were elucidated by extensive spectroscopic analysis, and all the compounds were obtained for the first time from U. diffracta. Compounds (1-3) were evaluated for their xanthine oxidase (XO) inhibitory activities in vitro, and the results showed that 1-3 possessed significant enzyme inhibitory actions with IC50 values of 46.9 ± 3.8, 75.9 ± 7.4 and 42.1 ± 1.7 μg/mL, respectively.
Keywords: Usnea diffracta; pheophytin; xanthine oxidase.