Identification and Characterization of a Library of Microheterogeneous Cyclohexadepsipeptides from the Fungus Isaria

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 beta-hydroxy acid residue and five alpha-amino acids, while isaridins contain a beta-amino acid, an alpha-hydroxy acid, and four alpha-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 alphabeta C11 turn, formed by the beta-hydroxy acid and glycine residues and a D Leu-L Ala type II' beta-turn. Additionally, the inhibitory effect of isariins and an isaridin on the intra-erythrocytic growth of Plasmodium falciparum is presented.

Identification of α- and β-hydroxy acid containing cyclodepsipeptides in natural peptide mixtures using negative ion mass spectrometry

“…Conditions for growth of fungal cultures and extracts of hyphae have been described earlier [13,14]. The crude hyphal extract was dissolved in MeOH, centrifuged to obtain a clear solution, and subsequently used for mass spectrometry and also for reverse-phase (RP) chromatographic purification.…”

“…During the course of investigations on isarial peptides, produced by the fungus Isaria, we obtained two groups of cyclohexadepsipeptides [13,14]. The isariins contain a single ␤-hydroxy residue, while isaridins contain an ␣-hydroxy residue and a single ␤-amino acid residue.…”

“…The peptides from Isaria occur as microheterogenous mixtures in fungal hyphae. In an earlier study, we have used LC-ESI-MS procedures to characterize as many as 14 components in a complex peptide mixture [14]. The isariins and isaridins are undoubtedly produced by distinct biosynthetic pathways.…”

“…In these cases, the abstraction of an acidic proton can lead to novel cleavages which provide valuable structural insights [17]. In the case of cyclic depsipeptides protonation of the ester bond has been shown to result in linearization, permitting mass spectrometric sequencing [14]. Under the conditions of negative ion mass spectrometry we anticipated that cleavage patterns arising from fragmentation at the hydroxyacid residue may yield insights into the chemical nature of the substituent.…”

“…The inset to Figure 1 illustrates fungal hyphae obtained over a solid medium, which can be directly extracted with organic solvents to yield predominantly cyclodepsipeptide libraries. The assignments of the isaridins (␣ hydroxy) and isariins (␤ hydroxy) are based on previous mass spectrometric, NMR and X-ray crystallographic investigations [13,14]. While there is a distinct difference in the molecular weight ranges of isariins and isaridins, it is evident that the higher mass components of the former and lower mass components of the latter may overlap.…”