Identification and Characterization of a Library of Microheterogeneous Cyclohexadepsipeptides from the Fungus <i>Isaria</i>

Abstract

Ten new cyclic hexadepsipeptides, six isariins and four isaridins, from the fungus Isaria have been identified and characterized by high-performance liquid chromatography, coupled to tandem electrospray ionization mass spectrometry (LC-ESIMS/MS). The isariins possess a β-hydroxy acid residue and five α-amino acids, while isaridins contain a β-amino acid, an α-hydroxy acid, and four α-amino acids. One- and two-dimensional NMR spectroscopy confirmed the chemical identity of some of the isariin fractions. Mass spectral fragmentation patterns of [M + H]+ ions reveal clear diagnostic fragment ions for the isariins and isaridins. Previously described cyclic depsipeptides, isarfelins from Isaria felina (Guo, Y. X.; Liu, Q. H.; Ng, T. B.; Wang H. X. Peptides 2005, 26, 2384), are now reassigned as members of the isaridin family. Examination of isaridin sequences revealed significant similarities with cyclic hexadepsipeptides such as destruxins and roseotoxins. The structure of an isariin (isariin A) investigated by NMR spectroscopy indicated the presence of a hybrid αβ C11 turn, formed by the β-hydroxy acid and glycine residues and a dLeu-lAla type II‘ β-turn. Additionally, the inhibitory effect of isariins and an isaridin on the intra-erythrocytic growth of Plasmodium falciparum is presented.