Abstract
Four compounds (1–4) were isolated from the extracts of Streptomyces sp. CT37 using bioassay in conjunction with mass spectrometric molecular networking (MN) driven isolation. Their complete structures were established by high-resolution electrospray ionization mass spectrometry (HR-ESIMS), and 1D and 2D nuclear magnetic resonance (NMR) data. Legonimide 1 was identified as a new alkaloid containing a rare linear imide motif in its structure, while compounds 2–4 were already known and their structures were elucidated as 1H-indole-3-carbaldehyde, actinopolymorphol B, (2R,3R)-1-phenylbutane-2,3-diol, respectively. The biosynthetic pathways of 1–4 were proposed based on the reported biogenesis of indole alkaloids in literature. Bioactivity tests for 1 and 2 revealed moderate growth inhibition activity against Candida albicans ATCC 10231 with MIC95 values of 21.54 µg/mL and 11.47 µg/mL, respectively.
Highlights
Microbial natural products (NPs) encompass several chemical structures that constitute a treasure trove of high-value molecules, such as antibiotics, anticancer and antioxidant drugs [1]
Were selected based on the recommended medium for Streptomyces species, which differ with respect to carbon source and salt concentration [22]
Based on the reported biogenesis of indole alkaloids from bacteria [11,13,35,36,37,38], we proposed the biosynthesis pathways of compounds 1–4 (Figure 2)
Summary
Microbial natural products (NPs) encompass several chemical structures that constitute a treasure trove of high-value molecules, such as antibiotics, anticancer and antioxidant drugs [1]. Metabolomics guided microbial NPs discovery processes have been widely used for the prediction of bioactive metabolite structures and prioritization of extracts for isolations [15,16]. A useful analytical technique that has been integrated into the metabolomics-guided microbial NPs discovery is HR-ESIMS and associated MS/MS fragmentation data. The MN database is a web-based server that is extremely useful for dereplicating natural product structures by comparing both experimental HR-ESIMS (MS/MS) spectra with the Molecules 2020, 25, 1108; doi:10.3390/molecules25051108 www.mdpi.com/journal/molecules. The targeted isolation afforded four compounds: legonimide 1, 1H-indole-3-carbaldehyde 2, actinopolymorphol B 3, and (2R,3R)-1-phenylbutane-2,3-diol 4
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