Xanthones from the herb of Swertia elata and their anti-TMV activity

Abstract Two new xanthones (1–2), together with four known ones (3–6), were isolated from whole herb of Swertia elata. Their structures were elucidated by spectroscopic methods including extensive 1D- and 2D-NMR techniques. Their anti-tobacco mosaic virus (anti-TMV) activity test revealed that 1–6 showed weak anti-TMV activities with inhibition rate in the range of 15.2–28.8% at the concentration of 20 μM.


Introduction
Swertia elata, belonging to the genus Swertia, is a plant of the family Gentianaceae. It is one of the Tibetan medicinal herbs in China, used for the treatment of hepatitis ), pneumonia, influenza (Zhou et al. 2011), osteomyelitis and other diseases (Zhang et al. 2011). New pharmacological studies also found the extract of the Swertia genus has significant activity against the sexually transmitted virus (including HIV), skin cancer (Zhu et al. 2012), and has anti-HBV (Geng et al. 2009), neural protection activities (Du et al. 2010), antioxidant activities (Kshirsagar P et al. 2015), etc. Previous studies have shown that xanthones (Kumar et al. 2015;Uvarani et al. 2015), triterpenoids (Yue et al. 2014;Kshirsagar PR et al. 2015), iridoids (Wan et al. 2013) and steroids are main constituents from the genus Swertia (Wang et al. 2013 Motivated by a search for more new bioactive metabolites from this plant, we now investigated the chemical constituents of the whole plants of S. elata in shangri-la Prefecture, Yunnan Province. This led to the isolation of six xanthones (1-6), including two new compounds (1 and 2). In this paper, we report the isolation, structure elucidation and anti-tobacco mosaic virus (anti-TMV) activity of the isolated compounds.
Compound 2 was also isolated as a pale yellow gum, and its molecular formula was determined as C 18 H 16 o 6 through HReSI-MS analysis (pseudomolecular ion [M + Na] + at m/z 351.0840). The 1 H and 13 C spectra data of 2 were very similar to those of 1 (see Table S1), except for the appearance of an acetyl group signal at (δ C 198.6 s, 30.7 q; δ H 2.51 s) and the disappearance of a 2-hydroxyethyl group signal on ring B. This indicated the 2-hydroxyethyl group in 1 was replaced by an acetyl group in 2. an acetyl group located at C-6 was supported by the HMBC correlations of H-2′ (δ H 2.51) with C-6 (δ C 134.0). Three methoxy groups at C-1, C-3 and C-7 were supported by HMBC correlations of three methoxy protons (δ H 3.86 s, 3.81 s and 3.88 s) with the C-1 (δ C 154.8), C-3 (δ C 158.4) and C-7 (δ C 162.9), respectively. The structure of 6-acetyl-1,3,7-trimethoxy-xanthone (2) is therefore determined.
Since some xanthones exhibit potential anti-TMV activity (Wu et al. 2013a(Wu et al. , 2013bZhao et al. 2013), the inhibitory activities of compounds 1-6 against TMV replication were tested using the half-leaf method (Hu, Zhou, et al. 2013). Ningnanmycin, a commercial product for plant disease in China, was used as a positive control. The antiviral inhibition rates of compounds 1-6 at the concentration of 20 μM are listed in Table 1. The results indicated that compounds 1-6 showed weak anti-TMV activity with inhibition rate in the range of 15.2-28.8%.

General experimental procedures
UV spectra were obtained using a Shimadzu UV-2401a spectrophotometer. 1D and 2D NMR spectra were recorded on a DRX-500 NMR spectrometer with TMS as internal standard. Unless otherwise specified, chemical shifts (δ) are expressed in ppm with reference to the solvent signals. HR-eSI-MS was performed on a VG autospec-3000 spectrometer. Semipreparative HPLC was performed on a Shimadzu LC-8a preparative liquid chromatograph with Zorbax  Note: all results are expressed as mean ± sd; n = 3 for all groups.

Plant material
Whole herb of S. elata was collected from Xianggelila, Yunnan Province, People's Republic of China, in July 2013. The plant was identified by Physician Zhaxiduoze (Diqing Tibetan Medicine Hospital). a voucher specimen (No. 1301005) was deposited at the Herbarium of Key laboratory of ethnic Medicine Resource Chemistry, Yunnan University of Nationalities.

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

Funding
This work was financially supported by the National Natural Science Foundation of China [grant number 21262048] and the Program for Innovative Research Team (in Science and Technology) in University of Yunnan Province [IRTSTYN 2014-11].