Unguisin C, a GABA-containing cyclic peptide from the fungus Emericella unguis

Besides the known unguisins A and B, a new cyclic heptapeptide, unguisin C, containing a GABA-derived moiety in the ring, was isolated from the fungus Emericella unguis. The structure was determined by 1D and 2D NMR techniques. Marfey's method was used to determine the absolute stereochemistry. Precursor-directed biosynthesis of the unguisins was performed by supplementation of the culture medium with amino acids (-Ala, -Ser, -Phe and -Leu). A related cyclic heptapeptide, unguisin D, was detected by HPLC and characterized by sequence analysis using LC-QITMS.