Xanthones from a lignicolous freshwater fungus (BCC 28210)

Abstract Four xanthones (1‒4) and a known compound, mansonone D (5), were isolated from the lignicolous freshwater fungus BCC 28210 (family, Chaetosphaeriaceae). The structures of these compounds were elucidated by extensive spectroscopic analysis. Among the isolated metabolites, compound 2 and the known mansonone D (5) displayed antimalarial activity against Plasmodium falciparum K1 with IC50 values of 7.75 and 0.55 μg/mL, respectively. Compound 4 displayed antibacterial activity against Bacillus cereus with an MIC value of 6.25 μg/mL. Graphical Abstract


Introduction
Xanthones are a large group of metabolites found from various species within plant, fungi, lichens, and bacteria (Peres et al. 2000;Vieira and Kijjoa 2005;Masters and Br€ ase 2012). Based on different oxidation states, types and positions of side chains and oxygenated substituents, a wide variety of naturally occurring xanthones have been described. Xanthones are known to exhibit an array of biological activities including antibacterial, antifungal, antiviral, antimalarial, and anticancer activities (Fotie and Bohle 2006;Masters and Br€ ase 2012;Ruan et al. 2017;Fouotsa et al. 2014). As part of our ongoing research on novel bioactive compounds from fungi, a crude extract of BCC 28210 showed weak antifungal activity against Curvularia lunata with an IC 50 value of 100 lg/mL. This fungus was isolated from a submerged wood, collected from Tai Rom Yen National Park (Surat Thani province, Thailand), and identified as a member of the family Chaetosphaeriaceae (order Chaetosphaeriales, class Sordariomycetes) based on phylogenetic analysis. Chemical investigation of the BCC 28210 led to the isolation and structure elucidation of four new trihydroxyxanthone derivatives (1-4) and a known compound, mansonone D (5) (Bett olo et al. 1965;Puckhaber and Stipanovic 2004) (Figure 1).
Compound 4 was isolated as yellow powder with a molecular formula of C 19 H 18 O 7 , indicating a lack of two protons comparing to that of 1. The NMR data of 4 showed signals corresponding to an E-double bond (d C 126.7/d H 5.90 (dd, J ¼ 15.5, 5.5 Hz), CH-2'; d C 139.1/d H 6.01 (dd, J ¼ 15.5, 1.2 Hz), CH-3') in the side chain at the expense of the two methylene (d C 32.4/d H 1.98 and 1.81, CH 2 -2'; d C 39.6/d H 1.72 and 1.60, CH 2 -3') of 1.
Antimalarial, antibacterial, antifungal, and cytotoxic activities were evaluated for all isolated compounds (Table 1). Compound 2 and the known mansonone D (5) exhibited antimalarial activity against Plasmodium falciparum K1 with respective IC 50 values of 7.75 and 0.55 lg/mL. The inhibitory activity against Candida albicans and cytotoxicity against Vero (African green monkey kidney fibroblast) cells of 2 were observed with close IC 50 values (18.4 and 19.1 lg/mL, respectively), as also found for mansonone D (1.95 and 1.97 lg/mL, respectively). Compound 4 displayed antibacterial activity against Bacillus cereus with an MIC value of 6.25 lg/mL, and showed cytotoxicity against Vero cell lines with an IC 50 value of 47.9 lg/mL. At the maximum concentration of 50 lg/mL, compounds 1, 2, and 5 were inactive against Curvularia lunata.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the Thailand Research Fund (Grant No. DBG5980002).