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MS/MS-based molecular networking strain prioritization led to the discovery of a group of cyclic depsipeptides from an endolichenic Xylaria sp. The main component, xylaroamide A (1), was obtained by LC-MS-guided isolation. The planar structure of compound 1 was elucidated via 1D and 2D NMR, as well as MS/MS data. The configurations were fully determined by the combination of advanced Marfey's analysis, partial hydrolysis, Mosher's reaction, and GIAO NMR calculation based on a restricted conformational search. A plausible biosynthetic pathway for xylaroamide A (1) involving a rare trans-acting N-methyltransferase is proposed based on bioinformatics analysis. Xylaroamide A (1) exhibited inhibitory activity against cancer cell lines BT-549 and RKO with IC 50 values of 2.5 and 9.5 μM, respectively.

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Four new lipid siderophores bearing both L-threo-and L-erythro-β-hydroxyaspartic acids, potashchelins A-D (1-4), were isolated from the potash-salt-ore-derived extremophile Halomonas sp. MG34. The planar structures of 1-4 were elucidated on the basis of extensive 1D and 2D NMR studies and MS/MS data. Potashchelins 1-4 contain a hydrophilic nonapeptide headgroup sequentially consisting of β-hydroxyaspartic acid, serine, glycine, serine, serine, β-hydroxyaspartic acid, threonine, serine, and cyclic N(δ)-hydroxy-ornithine, which is appended by one of a series of fatty acids ranging from dodecanoic acid to tetradecanoic acid. The absolute configurations of the amino acids of potashchelins 1-4 were determined by C 3 and advanced Marfey's reaction, partial hydrolysis, and bioinformatics analysis, which revealed that potashchelins 1-4 bear both L-threo-and L-erythro-β-hydroxyaspartic acid. Phylogenetic analysis showed that the stand-alone β-hydroxylase, PtcA, and the fused domain with β-hydroxylase activity in PtcB are expected to be responsible for the formation of L-erythro and L-threo diastereomers, respectively. Additionally, utilizing a comparative genomics approach, we revealed an evolutionary mechanism for lipid siderophores in Halomonas involving horizontal transfer. Bioassays showed that potashchelin A and D had weak antibacterial activity against B. subtilis CPCC 100029 with an MIC value of 64 µg/mL.

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