Abstract
Stereocalpin B, a new cyclic depsipeptide (1), and a new dibenzofuran derivative (3), were isolated from the Antarctic lichen, Ramalina terebrata (Ramalinaceae), along with a known cyclic depsipeptide (2). The structures of new compounds were characterized by comprehensive spectrometric analyses; high-resolution fast atom bombardment mass spectrometry (HR-FABMS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Stereocalpin B (1) existed in a rotameric equilibrium, which was confirmed using nuclear Overhauser effect spectroscopy (NOESY)/exchange spectroscopy (EXSY) spectrum. Absolute configurations of the amino acid units in 1 were assigned using the advanced Marfey’s method and subsequent NOESY analysis of the 5-hydroxy-2,4-dimethyl-3-oxo-decanoic acid residue confirmed the complete stereochemistry of 1. Compounds 1-3 exhibited moderate antimicrobial activities against E. coli, with the IC50 values ranging from 18–30 μg/mL. Compound 2 exhibited cell growth inhibition against HCT116 cell lines, with the IC50 value of 20 ± 1.20 μM, and compounds 1 and 2 also showed potent anti-inflammatory activities against lipopolysaccharide (LPS)-induced RAW264.7 macrophages with the IC50 values ranging from 5–7 μM.
Highlights
They are known to possess a broad variety of secondary metabolites with diverse biological activities, which are reported to be useful for the treatment of heart diseases, bronchitis, bleeding pile, vomiting, asthma, inflammation, and stomach disorders [1,2]
The extract was subjected to fractionation and isolation, tion, where column chromatography and highperformance liquid chromatography (HPLC) purification techniques were used, rewhere column chromatography and HPLC purification techniques were used, resulting sulting in the isolation of acyclic new depsipeptide, cyclic depsipeptide, stereocalpin
Compound was identified to be with a known cyclic depsipeptide (2) (Figure 1)
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Lichens, which are referred to as lichenized fungi, are mutualistic symbionts involving microalga and/or cyanobacterium and fungus [1] They are known to possess a broad variety of secondary metabolites with diverse biological activities, which are reported to be useful for the treatment of heart diseases, bronchitis, bleeding pile, vomiting, asthma, inflammation, and stomach disorders [1,2]. The Antarctic lichen, Ramalina terebrata, is known to possess various secondary metabolites such as usnic acid derivatives, ramalin, and anthraquinones [1,8,9,10]. Metabolites 2022, 12, 141 usimines A–C were reported to have antibacterial activity against B. subtilis [11], ramalin to have anti-inflammatory, anticancer, and antioxidant activity [9,12], and parietin was anti-inflammatory, anticancer, and antioxidant activity [9,12], and was[10,13]. The biological activities of all isolated compounds (1–3) are discussed below
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