α-Furanones, secondary metabolites from the fungus Cephalotrichum microsporum and their antibacterial activities
Graphical abstract
Introduction
Nature produces a variety of species, prokaryotes and eukaryotes (Bräse et al., 2009). Among them, fungi play a very important role in the ecosystem for plants, humans, and animals (Bräse et al., 2009). Fungi produce metabolites belonging to highly diverse structural classes, including aromatic compounds, anthracenones, amino acids, butanolides, cytochalasans, butenolides, macrolides, pyrones, naphthalenones, and terpenes, which exhibit significant biological activities, such as antifungal, antitumor, and antibacterial activities (Evidente et al., 2014). Fungi-derived natural products have been an excellent source of pharmaceuticals, such as the antibacterial agent penicillins, immunosuppressive drug cyclosporin A, antifungal drug echinocandin B, and cholesterol-lowering agent lovastatin (Evidente et al., 2014). Therefore, it has been more and more attractive to explore the bioactive substances from the metabolites of fungi.
Cephalotrichum microsporum was previously known as Doratomyces microsporum (Woudenberg et al., 2017). The genus Cephalotrichum is characterized by the formation of dry-spored, indeterminate synnemata and enteroblastic percurrent conidiogenesis (Woudenberg et al., 2017). It is well known to produce an extracellular keratinase, which is closely related to proteinase K (Gradisar et al., 2000; Orabi et al., 1999; Peterson et al., 1992; Vignardet et al., 1999). Additionally, various antibacterial aromatic and indole compounds are obtained from C. microsporum, the production of which can be induced by valproic acid (Zutz et al., 2016).
In our previous research, a C. microsporum strain was obtained from the rhizosphere soil of the traditional Chinese medicine Panax notoginseng. In the present study, three new secondary metabolites (1 ‒ 3) and one known analogue (4) were isolated from a culture of C. microsporum using various chromatographic techniques (Figs. 1 and S1). Their structures were determined using spectroscopic analyses. The antibacterial activities of 1 ‒ 4 were evaluated.
Section snippets
Results and discussion
Compound 1 was obtained as a primrose yellow powder. The molecular formula was determined as C13H16O4 using HRESIMS at m/z 259.0950 [M + Na]+ (calcd. for C13H16NaO4, 259.0941), combined with the NMR data (Table 1). The 1H NMR spectrum of 1 exhibited resonances attributable to four olefinic protons at δH 6.35 (d, J = 15.6 Hz, H-4”), 7.23 (dd, J = 15.6, 10.8 Hz, H-3”), 6.55 (dd, J = 16.2, 10.8 Hz, H-2”), and 7.00 (d, J = 16.2 Hz, H-1”); one oxygenated methine group at δH 4.13 (m, H-2'); one
General experimental procedure
ECD spectra were recorded on a Bio-Logic Science MOS-450 spectrometer. Optical rotations were recorded using a Perkin-Elmer 241 polarimeter (Perkin-Elmer, Waltham, USA). HRESIMS data were acquired on an Agilent 6210 TOF mass spectrometer. 1D- and 2D-NMR spectra were obtained at 600 MHz for 1H and 150 MHz for 13C, respectively, on a Burker 600 MHz spectrometer with solvent peaks as references. Analytical HPLC experiments were conducted on a Dionex UltiMate 3000 instrument (Thermo Scientific)
Conclusions
An investigation of the bioactive metabolites of the fungus C. microsporum was performed. Four metabolites (1 ‒ 4) were isolated from its culture using various chromatographic techniques. On the basis of widely available spectroscopic data, including 1D-NMR, 2D-NMR, ECD, and HRESIMS, the structures of the isolated compounds were determined. Compounds 1 ‒ 3 were undescribed compounds. In the antibacterial bioassay, compounds 2 and 3 displayed the moderate inhibitory effects on S. aureus and B.
Conflicts of interest
The authors declare that there are no conflicts of interest.
Acknowledgments
This research program is financially supported by the National Natural Science Foundation of China (Nos. 81803683, 81872970, 81503201, 81622047, 81703397), the Science and Technique Programs of Yunnan Province (2016ZF001-001, 2017IB038, 2015IC017), the National Science and Technology Major Project (2018ZX09735001-002-002) and the Program for the Innovative Research Team of the Ministry of Education and the Program for the Liaoning Innovative University Research Team.
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These authors contributed equally to this work.