Abstract
Chemical investigation of a halophilic actinomycete strain belonging to the genus Nocardiopsis inhabiting a hypersaline saltern led to the discovery of new 18-membered macrolides with nitrile functionality, borrelidins C–E (1–3), along with a previously reported borrelidin (4). The planar structures of borrelidins C–E, which are new members of the rare borrelidin class of antibiotics, were elucidated by NMR, mass, IR, and UV spectroscopic analyses. The configurations of borrelidines C–E were determined by the interpretation of ROESY NMR spectra, J-based configuration analysis, a modified Mosher’s method, and CD spectroscopic analysis. Borrelidins C and D displayed inhibitory activity, particularly against the Gram-negative pathogen Salmonella enterica, and moderate cytotoxicity against the SNU638 and K562 carcinoma cell lines.
Highlights
Chemical studies of marine-derived actinomycetes in the past 20 years have resulted in the discovery of structurally and biologically diverse secondary metabolites [1,2]
NMR spectrum of 3, a relatively deshielded oxygen-bearing carbon was detected at 81.5 ppm, which was not observed in borrelidins C and D
A comprehensive analysis of the 1D and 2D NMR spectra revealed that this oxygenated carbon was at C-7, not C-20, which is different from borrelidins C and D
Summary
Chemical studies of marine-derived actinomycetes in the past 20 years have resulted in the discovery of structurally and biologically diverse secondary metabolites [1,2]. The chemistry of actinobacteria in these hypersaline environments had been mostly neglected until our report of the salternamides as the first secondary metabolites from a saltern-derived actinomycete (Streptomyces sp.) in 2015 [5,6]. Our continuing study of saltern-derived actinomycetes led to the discovery of indolosesquiterpenoids, called xiamycins C–E, from a halophilic Streptomyces sp. Xiamycin D displayed potent antiviral activity against porcine epidemic diarrhea virus (PEDV) [8] These promising results indicated that saltern-derived actinobacteria can be utilized as prolific sources of bioactive secondary metabolites with pharmaceutical potential, leading us to expand our research program from the common genus. The structures structures of borrelidins C–E (1–3) and borrelidin (4)
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