Stereocalpin B, a New Cyclic Depsipeptide from the Antarctic Lichen Ramalina terebrata

Abstract

Stereocalpin B, a new cyclic depsipeptide (<b>1</b>), and a new dibenzofuran derivative (<b>3</b>), were isolated from the Antarctic lichen, <i>Ramalina terebrata</i> (Ramalinaceae), along with a known cyclic depsipeptide (<b>2</b>). 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 (<b>1</b>) 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 <b>1</b> 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 <b>1</b>. Compounds <b>1-3</b> exhibited moderate antimicrobial activities against <i>E. coli</i>, with the IC₅₀ values ranging from 18-30 μg/mL. Compound <b>2</b> exhibited cell growth inhibition against HCT116 cell lines, with the IC₅₀ value of 20 ± 1.20 μM, and compounds <b>1</b> and <b>2</b> also showed potent anti-inflammatory activities against lipopolysaccharide (LPS)-induced RAW264.7 macrophages with the IC₅₀ values ranging from 5-7 μM.