Targeted Discovery of Tetrapeptides and Cyclic Polyketide‐Peptide Hybrids from a Fungal Antagonist of Farming Termites

Abstract

Dedicated to Prof. Rolf Huisgen. Herein, we report the targeted isolation and characterization of four linear nonribosomally synthesized tetrapeptides (pseudoxylaramide AD) and two cyclic nonribosomal peptide synthetase-polyketide synthase-derived natural products (xylacremolide A and B) from the termite-associated stowaway fungus Pseudoxylaria sp. X187. The fungal strain was prioritized for further metabolic analysis based on its taxonomical position and morphological and bioassay data. Metabolic data were dereplicated based on high-resolution tandem mass spectrometry data and global molecular networking analysis. The structure of all six new natural products was elucidated based on a combination of 1D and 2D NMR analysis, Marfey's analysis and X-ray crystallography.

GNPS-guided discovery of xylacremolide C and D, evaluation of their putative biosynthetic origin and bioactivity studies of xylacremolide A and B

“…Based on the assumption that xylacremolides share the same biosynthetic origin and the similar NMR chemical shift pattern, we deduced the identical stereochemical assignment for compound 1 and 2 as shown for xylacremolide B ( 4 ) by X-ray crystallography, 11 while the configuration at position C-21 remained unsolved.…”

“…1b and S1 † ). 11 GNPS-networks were screened for molecular ion peaks belonging to xylacremolide A and B and putative structural congeners that varied in chain length and oxidation of the PKS moiety. The identified m / z subnetwork related to xylacremolide A and B ( m / z 403.223 ( 3 ), m / z 417.238 ( 4 )) contained a total of 50 connected nodes with molecular ion peaks that were partially assigned to congeners carrying additional methylene units (Δ m / z 14.016, Δ m / z 28.032) and derivatives with varying C 2 H 4 OH moieties (Δ m / z 45.057).…”

“…Subsequent dereplication of HRMS values using Antibase and Scifinder indicated that X187 might produce yet unreported xylacremolide congeners. 11 …”

“…Similar to their closest structural counterparts (acremolides), xylacremolides revealed so far no cytotoxic or antibacterial activities. 11 We then questioned which other cellular and/or ecological role xylacremolides might impose. A literature survey revealed that structurally related fungal metabolites trapoxin and FR235222 were previously investigated and found to be potent inhibitors of histone deacetylase (HDACi), 23,24 and are thus being investigated in cancer therapy, as well as in parasitic and inflammatory diseases.…”

“… 9 Processing of the MS 2 -data using Global Natural Products Social Molecular Networking Analysis (GNPS) 10 revealed the production of four linear peptides called pseudoxylaramides A–D and two PKS-NRPS based hybrids named xylacremolides A and B. 11 The identified xylacremolides share similar structural features with acremolides 12 and FR235222, 13 a known histone deacetylase inhibitor, isolated from Acremonium spp. and saroclides from the mangrove-derived fungus Sarocladium kiliense HDN11-112.…”