Biological Activities of Lichen-Derived Monoaromatic Compounds

Lichen-derived monoaromatic compounds are bioactive compounds, associated with various pharmacological properties: antioxidant, antifungal, antiviral, cytotoxicity, and enzyme inhibition. However, little is known about data regarding alpha-glucosidase inhibition and antimicrobial activity. Very few compounds were reported to have these activities. In this paper, a series of monoaromatic compounds from a lichen source were isolated and structurally elucidated. They are 3,5-dihydroxybenzoic acid (1), 3,5-dihydroxybenzoate methyl (2), 3,5-dihydroxy-4-methylbenzoic acid (3), 3,5-dihydroxy-4-methoxylbenzoic acid (4), 3-hydroxyorcinol (5), atranol (6), and methyl hematommate (7). To obtain more derivatives, available compounds from the previous reports such as methyl β-orsellinate (8), methyl orsellinate (9), and D-montagnetol (10) were selected for bromination. Electrophilic bromination was applied to 8–10 using NaBr/H2O2 reagents to yield products methyl 5-bromo-β-orsellinate (8a), methyl 3,5-dibromo-orsellinate (9a), 3-bromo-D-montagnetol (10a), and 3,5-dibromo-D-montagnetol (10b). Compounds were evaluated for alpha-glucosidase inhibition and antimicrobial activity against antibiotic-resistant, pathogenic bacteria Enterococcus faecium, Staphylococcus aureus, and Acinetobacter baumannii. Compound 4 showed stronger alpha-glucosidase inhibition than others with an IC50 value of 24.0 µg/mL. Synthetic compound 9a exhibited remarkable activity against Staphylococcus aureus with a MIC value of 4 µg/mL. Molecular docking studies were performed to confirm the consistency between in vitro and in silico studies.