Structural Characterization and Assessment of Anti-Inflammatory Activities of Polyphenols and Depsidone Derivatives from Melastoma malabathricum subsp. normale

The roots of Melastoma malabathricum subsp. normale (D. Don) Karst. Mey have been used in traditional ethnic medicine systems in China to treat inflammation-triggered ailments, such as trauma, toothache, and fever. Therefore, the aim of this study is to screen for compounds with anti-inflammatory activity in the title plant. The extract of M. malabathricum subsp. normale roots was separated using various chromatographic methods, such as silica gel, ODS C18, MCI gel, and Sephadex LH-20 column chromatography, as well as semi-preparative HPLC. One new complex tannin, named whiskey tannin D (1), and an undescribed tetracyclic depsidone derivative, named guanxidone B (2), along with nine known polyphenols (2–10) and three known depsidone derivatives (12–14) were obtained from this plant. The structures of all compounds were elucidated by extensive NMR and CD experiments in conjunction with HR-ESI-MS data. All these compounds were isolated from this plant for the first time. Moreover, compounds 1–4, 8, and 10–14 were obtained for the first time from the genus Melastoma, and compounds 1, 2, and 11–14 have not been reported from the family Melastomataceae. This is the first report of complex tannin and depsidone derivatives from M. malabathricum subsp. normale, indicating their chemotaxonomic significance to this plant. Compounds 1–12 were investigated for their anti-inflammatory activities on the production of the nitric oxide (NO) in lipopolysaccharide (LPS)-stimulated RAW264.7 cells, and compounds 1, 11, and 12 showed anti-inflammatory activities with IC50 values of 6.46 ± 0.23 µM, 8.02 ± 0.35 µM, and 9.82 ± 0.43 µM, respectively. The structure–activity relationship showed that the catechin at glucose C-1 in ellagitannin was the key to its anti-inflammatory activity, while CH3O- at C-16 of aromatic ring A in depsidone derivatives had little effect on its anti-inflammatory activity. The study of structure–activity relationships is helpful to quickly discover new anti-inflammatory drugs. The successful isolation and structure identification of these compounds, especially complex tannin 1, not only provide materials for the screening of anti-inflammatory compounds, but also provide a basis for the study of chemical taxonomy of the genus Melastoma.


Introduction
The genus Melastoma (Melastomataceae), with approximately 100 species, is widespread in southern Asia, northern Oceania, and the Pacific islands, and a total of 9 species and 1 variety are found in China [1]. Some species of this genus are used for the treatment Molecules 2022, 27, 1521 2 of 10 of diarrhea, dysentery, leucorrhoea, ulcers, and wounds [2]. Flavonoids, tannins, phenylpropanoids, organic acids (esters), terpenoids, and other components were previously characterized from this genus [3]. Some of them exhibited anti-inflammatory [4], hemostatic activity [5], anticoagulant activity [6], antibacterial activity [7], antioxidant activity [8,9], hepatoprotective activity [10], gastroprotective activity [11], hypoglycemic activity [12], and cytotoxic activities [13]. Melastoma malabathricum subsp. normale (D. Don) Karst.Mey, a shrub of the family Melastomataceae, grows mainly in Xizang, Sichuan, Guangxi, and Fujian provinces of China [1]. Its roots have been used in Zhuang and Yao medicines for the treatment of inflammation-triggered ailments, such as trauma, toothache, and fever [14,15]. With the aim to find compounds with anti-inflammatory activity in the title plant, the roots of M. malabathricum subsp. normale were extracted by 80% aqueous acetone, and subsequently separated using silica gel, MCI, ODS C18, and Sephadex LH-20 column chromatography, as well as semi-preparative HPLC to yield ten polyphenols and four depsidone derivatives. The structures of these compounds were characterized by experimental and published spectroscopic data analyses. As we all know, complex tannin is a kind of flavono-ellagitannin, which has a unique C-C condensation structure of C-glycoside tannin (vescalagin-type or stachyurin-type) and flavane-3-alcohol. To date, these compounds have only been found in a few plant families, including Combretaceae, Myrtaceae, Melastomataceae, Fagaceae, and Theaceae [16]. Compound 1 is the only complex tannin isolated from M. malabathricum subsp. normale, and its analogs have also been reported from M. malabathricum L. in the family Melastomataceae [17], suggesting their closely chemotaxonomic relationships between M. malabathricum subsp. normale. and M. malabathricum. L. Depsidone derivatives have never been reported from the family Melastomataceae [3,18]. These compounds enrich the chemical diversity of M. malabathricum subsp. normale and provided a basis for the chemotaxonomic studies of the species of the genus Melastoma. Moreover, the anti-inflammatory activities of compounds 1-12 were investigated to develop polyphenols or depsidone derivatives as a novel anti-inflammatory drug. In the present study, the isolation and structural elucidation of compounds 1 and 11, as well as the anti-inflammatory activities of 1-12, are reported in detail.

Structure Elucidation
Compound 1, a pale brown amorphous powder, shows the positive coloration characteristic of complex tannin when reacting with anisaldehyde-sulfuric acid (pink) and NaNO 2 -AcOH (brown) reagent. A deprotonated molecular ion peak at m/z 1249.1580 [M − H] − (calcd, 1249.1586) was observed in the HR-ESI-MS spectrum, indicating that the molecular formula of 1 is C 58 H 42 O 32 . The 1 H NMR data (Table 1) revealed at least two hexahydroxydiphenoyl (HHDP) groups at δ 6.63 (s), 6.80 (s), and 7.02 (s); an ethoxyl at δ 4.21 (q, 7.1, 2H) and 1.20 (t, 7.1, 3H). As shown in Figure 1, the 1 H-1 H COSY correlations among methylene (δ 3.86 and 4.81) and five methine protons (δ 4.36-5.53) revealed a polyalchohol unit, which exhibited similar NMR data to the open-chain glucose core of stenophyllanine B [31]. The 13 C NMR data ( Five downfield signals at δ C 63.6-83.2 suggested that the hydroxyl at C-2-C-6 was esterified. The large difference in chemical shifts between H-6a (δ 4.81) and 6b (δ 3.86) suggested one of the HHDP moieties was located at C-4 and C-6, which can be explained by the anisotropic effect of a C-6 ester carbonyl group. It is restrained to be rigidly coplanar with one of the C-6 methylene protons in the eleven-membered diester ring, so the proton was placed in a strongly deshielding environment [32]. This was also supported by the correlations of H-4 and H-6 with carbonyl carbons C HHDP-7 (δ 168.2) of the HHDP group and C HHDP-7 (δ 167.4) of the HHDP group in the HMBC spectrum (  -6 ) suggested that this carbonyl was connected by a double bond and formed a δ-lactone ring with glucose O-2. This was confirmed by the correlation of H-2 with carbonyl carbon (δ 164.2) in the HMBC spectrum. The correlations of H-3 with C HHDP-7" (δ 168.0) of the HHDP group and H-5 with C HHDP-7 (δ 167.8) of the HHDP group in the HMBC spectrum indicated that these two carbonyl carbons (C HHDP-7" and C HHDP-7 ) were connected to glucose O-3 and glucose O-5, respectively. The correlations of H Cp -1 with C HHDP-2" (δ 124.7) and C HHDP-3" (δ 112.8) of the HHDP group in the HMBC spectrum indicated that Cp-1 was linked to C HHDP-3 of the HHDP group. Comparison between the NMR data of whiskey tannin B [19] and 1 (Table 1) revealed that -OH at C-1 in whiskey tannin B was replaced by a 5,7,3 ,4 -tetrahydroxy flavan-3-ol moiety in 1. This moiety could be constructed by analysis of the 1 H NMR data of a 1,2,4-trisubstituted aromatic ring at δ 6.94 (d, 0.9), 6.86 (dd, 8.2, 0.9), and 6.78 (d, 8.2) and a phloroglucinol aromatic ring at δ 6.00 (s), as well as the C-ring characteristic protons of a 2,3-trans flavan-3-ol at 4.72 (br s), 4.11 (m), 2.57 (dd, 16.1, 7.7), and 2.83 (d, 16.1) [33]. This is further supported by the 1 H-1 H COSY correlations of catechin H-3 with catechin H-2 and catechin H-4 ( Figure 1), and by the correlations of catechin H-2 with aromatic carbons (114.2 and 120.6) and catechin H-3 with aromatic carbon (δ 131.3) in the HMBC spectrum. This moiety has also been found in stenophyllanine B [31]. To determine the C-6 or C-8 linkage between catechin and C-glycosylated ellagitannin moieties, methylation of 1 was carried out, giving 1a, and its 13 C-NMR data showed an unsubstituted A-ring carbon signal at δ 89.6, indicating the presence of a substituent at C-8 of the flavan-3-ol moiety [34]. The side-chain moiety linked to C-1 was confirmed by the correlations of H-1 with aromatic carbons (δ 102.9, 154.9, and 156.2) in the HMBC spectrum and by the upfield chemical shift (δ 33.9). Compound 1 was refluxed in 20% acetic acid ethanol, and then chromatographed on Sephadex LH-20 to obtain a crystalline compound [m.p. 170-171 • C; [α] 25 D + 14 • (acetone)] that was identical with (+)-catechin [34]. Thus, the planar of 1 was identified. the 1D NMR data of 11 (Table 2) revealed high structural similarity to the co-isolated excelsione (13) [29]. The only difference between 11 and 13 was the replacement of the -CH2OH (C-16) by a methyl group, which was supported by the chemical shifts of C-16 (δC 8.7). The structure of 11 was therefore established and named guanxidone B.   The coupling constant between H-1 and H-2 is 0 Hz (<2.0 Hz), indicating that the configuration at C-1 of the glucose core in 1 is the same as that of vescalagin (J = 2.0 Hz) [35] and different from that of whisky tannin B (J = 6.4 Hz) [19]. This is also evidenced by the nuclear Overhauser effect (NOE) correlations of H-1 with H-3. Assuming that 1 is derived from vescalagin, inspection of a Dreiding model of 1 showed that the proton Hc p-1 of the cyclopentenone ring must be β oriented because its fusion ring system is so rigid that it is impossible to build an alternative model [35]. No proton was correlated with Hc p-1 in the ROESY spectrum indicated that the ethoxycarbonyl in 1 is α-orientation. The 2R-and 3S-configurations of the flavan C-ring were deduced from the absence of the NOE cross peaks between catechin H-2 and catechin H-3 in the ROESY spectrum of 1, as well as acid hydrolysis of 1 gave (+)-catechin. The atropisomerism of the HHDP group in 1 was determined to be S, as indicated by a positive Cotton effect at 240 nm and a negative one at 265 nm in its CD spectrum [36]. Thus, the structure of compound 1, named whiskey tannin D, was characterized as depicted in Figure 1.
Compound 11 was obtained as a white powder and had a molecular formula of C 18 Table 2), requiring 12 degrees of unsaturation. MS analysis and evaluation of NMR data suggested that compound 11 was a tetracyclic depsidone [28]. The 1 H NMR data of 11 showed one singlet at δ H 6.63 (1H, s, H-8 (Table 2) revealed high structural similarity to the co-isolated excelsione (13) [29]. The only difference between 11 and 13 was the replacement of the -CH 2 OH (C-16) by a methyl group, which was supported by the chemical shifts of C-16 (δ C 8.7). The structure of 11 was therefore established and named guanxidone B. the 1D NMR data of 11 (Table 2) revealed high structural similarity to the co-isolated excelsione (13) [29]. The only difference between 11 and 13 was the replacement of the -CH2OH (C-16) by a methyl group, which was supported by the chemical shifts of C-16 (δC 8.7). The structure of 11 was therefore established and named guanxidone B.

Anti-Inflammatory Activity Assays
All compounds except compounds 13 and 14 were investigated for potential antiinflammatory activity by measuring the inhibition of the nitric oxide (NO) production. As shown in Table 3, compounds 1, 11, and 12 displayed significant anti-inflammatory activity with IC50 values ranging from 6.46 ± 0.23 to 9.82 ± 0.43 μM. The IC50 values for the inhibition of NO production by other compounds are all > 10 μM. The anti-inflammatory

Anti-Inflammatory Activity Assays
All compounds except compounds 13 and 14 were investigated for potential antiinflammatory activity by measuring the inhibition of the nitric oxide (NO) production. As shown in Table 3, compounds 1, 11, and 12 displayed significant anti-inflammatory activity with IC 50 values ranging from 6.46 ± 0.23 to 9.82 ± 0.43 µM. The IC 50 values for the inhibition of NO production by other compounds are all > 10 µM. The anti-inflammatory activity of compound 1 is better than that of compound 2, indicating that the effect of catechin on glucose C-1 is very important for its activity. Compound 11 has better antiinflammatory activity than compound 12, indicating that CH 3 O-at C-16 of aromatic ring A has little effect on its activity.

General Experimental Procedures
Optical rotations were measured at 25 • C with an ADP440+ polarimeter, Julabo, Seelbach, Germany (λ 589 nm, path length 1.0 cm). The UV spectra were recorded in MeOH on a TU-1901 spectrophotometer (Beijing Puxi General Instrument Co., Ltd., Beijing, China). The CD spectra were acquired in MeOH on a JASCO J-180 spectropolarimeter (Jasco, Tokyo, Japan). The NMR spectra were obtained on a Brucker Avance III HD-500 MHz spectrometer (Bruker Biospin AG, Fällanden, Switzerland), and the residual solvent peaks were used as references. Coupling constants and chemical shifts were given in Hz and on a δ (ppm) scale, respectively. ESI-MS and HR-ESI-MS were acquired on a Bruker Esquire 3000plus and Waters/Micromass Q-TOF-Ultima (Waters, Milford, MA, USA) mass spectrometers, respectively. Semi-preparative HPLC performed on a Shimadzu LC-20AT HPLC system at the rate of 2 mL/min. Sephadex LH-20 (GE Healthcare Bio-Science AB, Uppsala, Sweden), MCI gel CHP 20P (Mitsubishi Chemical Co., Tokyo, Japan), silica gel (Qingdao Marine Chemical Co., Ltd., Qingdao, China), and Chromatorex ODS (Merck, Darmstadt, Germany) were used for column chromatography (CC).

Methylation of 1
Compound 1 (10 mg) was dissolved in dimethyl sulfate (1 mL), then anhydrous potassium carbonate (0.5 g) in acetone was added and heated under reflux for 3 h. After removing inorganic salts by filtration, the filtrate was evaporated off under reduced pressure, and loaded onto a silica gel CC gradually eluting with benzene with increased proportion of acetone to yield compound 1a (2.

Anti-Inflammatory Activity
The anti-inflammatory activities of compounds 1-12 were investigated on the production of the NO in LPS-stimulated cells according to our previously described method [17]. That is, the RAW 264.7 cells were cultivated in DMEM supplemented with 10% FBS at 37 • C for 24 h. Cells in 24-well plate were treated with 200 ng/mL LPS and the test compounds. After 22 h, the media were collected, and the level of nitrite was measured using the Griess Reagent reagent System (Promega), Madison, WI, USA. The results are expressed as the mean ± SD, n = 3.

Conclusions
The study aimed to discover new anti-inflammatory drugs from the roots of M. malabathricum subsp. normale based on our previous works [18,37]. As expected, fourteen compounds were obtained from the tile plant for the first time, and compounds 1 and 11 are new compounds. In addition, this is the first report of compounds 1-4, 8, and 10-14 from the genus Melastoma, and compounds 1, 2, and 11-14 from the family Melastomataceae. Compounds 1, 11, and 12 showed anti-inflammatory activities, which make them potential anti-inflammatory drugs. The study of structure-activity relationship is helpful to quickly find out new anti-inflammatory drugs. The successful isolation and structure identification of compounds 1-14 not only provide materials for this experiment, but also contribute to the chemotaxonomic studies of the species of the genus Melastoma.