Penimethavone A, a flavone from a gorgonian-derived fungus Penicillium chrysogenum

Abstract A novel flavone, penimethavone A (1), possessing a rare unique methyl group at ring-B, was isolated from the fungus Penicillium chrysogenum cultured from a gorgonian Carijoa sp. collected from the South China Sea. The structure was elucidated by extensive spectroscopic analysis and by comparison with related known compound. Compound 1 showed selective and moderate cytotoxicity against cervical cancer (HeLa) and rhabdomyosarcoma cell lines with IC50 values of 8.41 and 8.18 μM, respectively.

In our investigation on the natural products from marine organisms especially microorganisms collected from the South China Sea, many biologically active compounds have been isolated (Shao et al. 2010;Chen et al. 2014;Liu et al. 2014;Jia et al. 2015). Continuously, a novel flavone, penimethavone A (1), with a methylation in the B-ring, was obtained from a gorgonian-derived fungus Penicillium chrysogenum. Herein, we report the isolation, structure elucidation and biological activity of penimethavone A.

Results and discussion
The fungus P. chrysogenum was cultured for 45 days on sea water-added rice solid medium. The etoAc extract of the mycelia was separated and purified by a combination of silica gel column chromatography, Sephadex LH-20 and semi-preparative HPLC to yield penimethavone A (1) (Figure 1).
Penimethavone A (1) was obtained as a yellowish powder. Its molecular formula was established as C 16 H 12 o 6 on the basis of the HreSIMS ion at m/z 301.0709 [M + H] + (Calcd for C 16 H 13 o 6 , 301.0707), indicating 11° of unsaturation. The Ir absorptions at 3169, 2925 and 1650 cm −1 indicated the presence of hydroxyl, olefinic and carbonyl group. The 1 H NMr (500 MHz, dMSo-d 6 ) spectrum (Table S1) exhibited four exchangeable protons at δ H 12.91, 10.84, 9.83 and 9.74. The chemical shift at δ H 12.91 indicated that a hydrogen-bonded hydroxyl group was presented in 1 (Lin et al. 1995). The deshielded area of 1 H NMr spectrum revealed the presence of two pairs of meta-coupled aromatic signals (δ H 6.35 (d, J = 1.8 Hz), 6.20 (d, J = 1.8 Hz); 6.28 (d, J = 1.5 Hz), 6.21 (d, J = 1.5 Hz)). Additionally, one olefinic singlet at δ H 6.21 and one methyl singlet at δ H 2.13 were .observed. The 13 C NMr (125 MHz, dMSo-d 6 ) spectrum revealed the presence of 1 methyl, 5 sp 2 methine groups and 10 quaternary carbon atoms. These spectroscopic features suggest that 1 belongs to the family of flavones. Careful analysis on the 1 H and 13 C NMr data suggested that 1 was similar to norartocarpetin (2) (Figure 1) which was isolated from the leaves of Morus alba L (Yang et al. 2012). The difference was the presence of a methyl group, δ H 2.13 (3H, s), δ C 19.9 (CH 3 ) in 1. The HMBC ( Figure S2) correlations from H-7′ (δ H 2.13) to C-1′ (111.2), C-5′ (108.8) and C-6′ (139.0) confirmed that the CH 3 group was attached to C-6′. on the basis of other key HMBC correlations, the structure of compound 1 was determined as shown in Figure 1. Penimethavone A (1) was assayed in vitro for its cytotoxic activity against cervical cancer (HeLa), human laryngeal epithelial (Hep-2), rhabdomyosarcoma and non-small cell lung cancer (A549) cell lines, with adriamycin as a positive control (Table S2). Compound 1 showed selective and moderate cytotoxicity against HeLa and rd cells with IC 50 values of 8.41and 8.18 μM, respectively (adriamycin, IC 50 = 0.43 and 0.09 μM). Compound 1 was also evaluated for the acetyl cholinesterase (AChe)-inhibiting activity in vitro. However, it was inactive at the concentration of 100 μg/mL.

Conclusion
Chemical investigation of etoAc extract of P. chrysogenum led to the isolation of penimethavone A (1), a novel C-methylated flavone at ring B. The structure of 1 was characterised by spectroscopic analysis. The cytotoxicity and enzyme inhibiting of penimethavone A were tested in vitro with which it showed selective and moderate cytotoxicity against HeLa and rd cell lines with IC 50 values of 8.41 and 8.18 μM, respectively.

Supplementary material
experimental details related to this article along with Figures S1-S12 and Tables S1 and S2 are available online.

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