One triterpenoid and five phenolic compounds from the liverwort Bazzania sp.

Abstract

Introduction: The Bazzania genus, a type of liverwort known as a low-level herbaceous organism, comprises approximately 250 species. Only 8 species have been chemically and biologically studied worldwide, and none have been studied in Vietnam. This paper described the isolation and structure elucidation of six compounds isolated from the liverwort Bazzania sp. collected in Lac Duong district, Lam Dong Province, Vietnam, in July 2020.

Methods: Phytochemical investigations of the n-hexane and chloroform extracts of Bazzania sp. led to the isolation of six pure compounds. Their chemical structures were elucidated by HR-ESI-MS, 1D and 2D-NMR spectroscopic analysis and comparison with previously published data.

Results: From the studied liverwort Bazzania sp., six compounds, namely, cycloartenol (1) and five phenolic derivatives, p-coumaric acid (2), p-hydroxybenzoic acid (3), methyl 2,4-dihydroxy-3,6-dimethylbenzoate (4), barbatic acid (5), and methyl 2-hydroxy-4-methoxy-6-tetradecylbenzoate (6), were isolated and structurally elucidated.

Conclusions: Although these compounds are known in other species, this is the first time they have been reported in Bazzania sp.

INTRODUCTION

Liverworts, the species of the division Marchantiophita, belong to Bryophytes, which are known as nonvascular plants. They are mostly spread in high moisture areas, humid tropical forests, and the banks of rivers and streams. 1 The Bazzania genus, liverwort, comprises approximately 250 species. 2 , 3 In the world, only a few of them, 8 species, have been chemically and biologically studied, and none have been studied in Vietnam. 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 Previous chemical studies on the eight mentioned species showed that they contained sesquiterpenes, diterpenes, and lignans. 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 Some possess cytotoxic 14 and nitric oxide inhibitory activity. 17 Herein, we report the investigation of the n-hexane and chloroform extracts of Bazzania sp . collected in Lac Duong district, Lam Dong province, Vietnam. We isolated six compounds, including a triterpenoid compound, cycloartenol (1), and five phenolic derivatives, p-coumaric acid (2), p-hydroxybenzoic acid (3), methyl 2,4-dihydroxy-3,6-dimethylbenzoate (4), barbatic acid (5), and methyl 2-hydroxy-4-methoxy-6-tetradecylbenzoate (6). Six compounds were isolated via silica gel chromatographic processes, and their chemical structures were elucidated by HR-ESI-MS, 1D, and 2D NMR spectroscopic methods.

MATERIALS AND METHODS

General experimental procedures

The HR–ESI–MS was recorded on an MicrOTOF–Q mass spectrometer. ¹ H-NMR (600 MHz) and ¹³ C-NMR (150 MHz) spectra were measured on a Bruker Avance 600 spectrometer using CDCl ₃ ( d 7.26 and 77.16); (CD ₃ ) ₂ CO ( d 2.05 and 206.26, 29.84) and CD ₃ OD ( d 4.87 and 49.0) were used both as solvents and internal references.

Thin layer chromatography (TLC) was carried out on precoated silica gel 60 F ₂₅₄ (Merck), and the isolated compounds were visualized by spraying with vanillin + H ₂ SO ₄ in ethanol or methanol followed by heating. Gravity column chromatography was performed on silica gel 60 (0.040–0.063 mm, Himedia). Organic solvents were distilled prior to use.

Plant material

The liverwort Bazzania sp . was collected on Lang Biang Mountain, Lac Duong District, Da Lat city, Lam Dong Province, at an altitude of 150 meters in July 2020. The sample of Bazzania sp . was authenticated by MSc. Luong Thien Tam, Department of Ecology and Evolution Biology. University of Science, National University–Ho Chi Minh City. A voucher specimen, coded BazlB, was deposited in the herbarium of the Laboratory of Plant Biotechnology, Department of Plant Biotechnology and Biotransformation, Faculty of Biology and Biotechnology.

Extraction and isolation

The ground powder sample (0.37 kg) of Bazzania sp. was extracted with methanol by maceration at room temperature to obtain a crude methanolic residue (29.59 g), which was subjected to silica gel solid-phase extraction and eluted consecutively with n -hexane, chloroform, ethyl acetate, and methanol at ambient temperature. Then, each type of eluent was evaporated at low pressure to afford n -hexane (3.97 g), chloroform (6.43 g), ethyl acetate (6.82 g), and methanol (10.24 g) residues.

The n -hexane residue was chromatographed over silica gel with n -hexane: CHCl ₃ (10:0) to (0:10) to give 10 fractions, coded HE1 (283 mg), HE2 (560 mg), HE3 (495 mg), HE4 (233 mg), HE5 (170 mg), HE6 (400 mg), HE7 (590 mg), HE8 (201 mg), HE9 (487 mg), and HE10 (228 mg). The residue of fraction HE7 was further chromatographed (CC) over silica gel with n -hexane:chloroform (100:0, 90:10, 0:100) to give five subfractions (HE7.1-HE7.5). The subfraction HE7.2 (192 mg) was silica gel chromatographed with a gradient system of n -hexane:toluene (90:10, 80:20, 70:30, 10:90) to afford compound 6 (6 mg). Subfraction HE7.5 (152 mg) was subjected to CC using the mobile phase n -hexane:chloroform (90:10, 85:15, 50:50, 10:90) to afford compound 1 (5 mg).

The chloroform residue was silica gel chromatographed and eluted by n -hexane:chloroform (80:20, 50:50, 0:100) and chloroform:ethyl acetate (90:10, 80:20, 0:100) to give seven fractions (CL1‒CL7). The fraction CL4 (840 mg) was further fractionated by silica gel chromatography using n -hexane:ethyl acetate (stepwise, 98:2, 95:5, 90:10, 0:100) to obtain six subfractions (CL4.1‒CL4.6). Further fractionation of CL4.2 (187 mg) and elution with n -hexane:ethyl acetate (90:10 to 0:100) afforded compound 4 (7 mg). The same procedure was applied to subfraction CL4.6 (200 mg), eluted by n -hexane:ethyl acetate (90:10 to 0:100) to obtain compound 5 (4 mg). Fraction CL6 (540 mg) was divided into four subfractions (CL6.1‒CL6.4) by CC using n -hexane:acetone (99:1, 95:5, 90:10, 80:20, 0:100). Subfraction CL6.4 (213 mg) was repeatedly chromatographed followed by preparative TLC to afford compound 2 (3 mg) and compound 3 (4 mg).

RESULTS

Using silica gel column chromatography, the chemical investigation of the -hexane and chloroform extracts of the liverwort Bazzania sp. led to the isolation of six compounds. Their structures are shown in Figure 1 by using 1D and 2D NMR and HR-ESI-MS.

Figure 1 . Chemical structures of six compounds isolated from the liverwort Bazzania sp.

DISCUSSION

The chemical structures of six isolated compounds were elucidated based on modern spectroscopic methods such as HR-ESI-MS and 1D and 2D NMR, and their data were compared with those in the literature. Compounds possessing a similar framework were structurally discussed.

Compound 1 (5 mg) was obtained as a white solid that was highly soluble in chloroform. The HR-ESI-MS spectrum (positive mode) showed a pseudomolecular ion peak at m/z 427.4030 [M+H] ⁺ (calcd. for C ₃₀ H ₅₀ O+H, 427.3934). The ¹ H-NMR spectrum showed signals of seven methyl groups [ δ _H 0.81 (3H, s , H-29), 0.96 (6H, s , H-18, H-28), 0.91 (3H, d, 6.5 Hz, H-21), 0.93 (3H, s , H-30), 0.96 (3H, s , H-18), 1.60 (3H, s , H-27), 1.66 (3H, s , H-26)], two nonequivalent methylene protons [ δ _H 0.37 (1H, d , 3.5 Hz, H-19a) and 0.58 (1H, d , 3.5 Hz, H-19b)], one oxymethine [ δ _H 3.22 (1H, dd , 10.0, 5.5 Hz, H-3)] and one olefinic proton [ δ _H 5.12 (1H, m , H-24)]. The ¹³ C-NMR spectrum showed thirty signals with two olefinic carbons [ δ _C 125.98 (C-24) and 131.25 (C-25)], one oxygenated methine carbon [ δ _C 78.45 (C-3)], and many signals in the high-field zone (14‒53 ppm). The good compatibility of its NMR and MS data with those in the literature 19 , 20 proposed that 1 was cycloartenol.

Compound 2 (3 mg) was obtained as colorless needles and was highly soluble in methanol. The NMR data ( Table 2 , Table 3 ) showed two aromatic signals [ δ _H 6.84 (2H, d , 8.6 Hz; δ _C 130.3, C-3, C-5); 7.43 (2H, d , 8.6 Hz; δ _C 116.1, C-2, C-6)] of a 1,4-disubstituted benzene ring and two olefinic protons [ δ _H 6.25 (1H, d, 15.6 Hz, H-8) and 7.63 (1H, d , 15.6 Hz; H-7)]. The large coupling constants suggested the E -configuration for this double bond. The other chemical shifts for the remaining carbons are presented in Table 3 . The molecular formula of 2 was confirmed by the pseudomolecular ion peak (negative mode) at m/z 163.0241 [M–H] ^– (calcd. for C ₉ H ₈ O ₃ ‒H, 163.0401), as well as the good compatibility in the comparison of its NMR data with those in the literature 21 , suggested that 2 was -coumaric acid.

Compound 3 (4 mg) was isolated as a white solid that was highly soluble in MeOH. Its HR-MS spectrum showed a deprotonated molecular ion peak at m/z 137.0103 [M–H] ^– (calcd. for C ₇ H ₆ O ₃ ‒H, 137.0244), as well as good compatibility in the comparison of its NMR data with those in the literature 22 , suggested that 3 was p -hydroxybenzoic acid.

Compound 4 (7 mg) was isolated as a white amorphous powder. Its molecular formula was determined to be C ₁₀ H ₁₂ O ₄ through its pseudomolecular ion peak at m/z 195.0673 [M–H] ^– (calcd. for C ₁₀ H ₁₂ O ₄ ‒H, 195.0662) in the HR-ESI-MS spectrum. The ¹ H-NMR spectrum showed one aromatic methine proton at 𝛿 _H 6.21 (3H, s ), two methyl groups at 𝛿 _H 2.10 (3H, s ) and 2.46 (3H, s ), and one methoxy group at 𝛿 _H 3.92 (3H, s ). The ¹³ C-NMR spectrum showed 10 resonances of one benzene ring, two methyl groups, and one methoxycarbonyl group ( Table 2 , Table 3 ) . HMBC cross-peaks of the methyl protons at 𝛿 _H 2.10 (3H, s , H-8) with ¹³ C carbon signals at 𝛿 _C 108.7 (C-3), 158.2 (C-4), 163.3 (C-2), and 𝛿 _H 2.46 (3H, s , H-9) with 𝛿 _C 110.7 (C-5), 140.3 (C-6) and 172.7 (C-7) ( Figure 2 ) evidenced their positions. The presence of the methoxycarbonyl group in 3 was proven by cross-peaks of 𝛿 _H 3.92 (H-10) with 𝛿 _C 172.7 (C-7). The spectrum also confirmed the position of the chelated hydroxyl group through cross-peaks of 𝛿 _H 12.00 (1H, s , 2-OH) with 𝛿 _C 105.5 (C-1), 108.7 (C-3), and 163.3 (C-2). Therefore, 4 was identified as methyl 2,4-dihydroxy-3,6-dimethylbenzoate, as shown in the literature. 23

Figure 2 . Selected HMBC correlations of compounds 4 , 5 , and 6

Compound 5 (4 mg) was isolated as a white solid that was highly soluble in acetone. The ¹ H-NMR spectrum showed four methyl signals [ δ _H 1.99 (3H, s , H-8′), 2.03 (3H, s , H-8), 2.61 (3H, s , H-9′), 2.69 (3H, s , H-9)], one methoxy group [ δ _H 3.93 (1H, s , 4-OCH ₃ )] and two aromatic protons of two benzene rings at δ _H 6.43 (1H, s , H-5′) and 6.59 (1H, s , H-5). The spectrum also showed one chelated hydroxyl group at δ _H 11.59 (1H, s , 2-OH). The ¹³ C-NMR spectrum ( Table 3 ) showed 18 signals of 19 carbons of four methyl groups ( δ _C 7.8, 9.4, 23.5, 24.8), one methoxy group ( δ _C 55.1), nine signals of 12 aromatic carbons, two carboxyl carbons including one ester ( δ _C 171.1), and one carboxylic acid ( δ _C 177.2). The HMBC experiment ( Figure 2 ) revealed that 5 was composed of two benzene rings. This was well supported by the HR-ESI-MS spectrum (negative mode) with a pseudomolecular ion peak at m/z 359.1129 [M–H] ^– (calcd. for C ₁₉ H ₂₀ O ₇ ‒H, 359.1125). The good compatibility of its NMR data with those in the literature 24 suggested that 5 was a barbatic acid.

Compound 6 (6 mg) was isolated as a white amorphous solid that was highly soluble in chloroform. Its NMR data ( Table 1 , Table 3 ) showed a benzene ring with four substitutions via two meta -coupled aromatic proton signals ( 𝛿 _H 6.33, 1H, d , 2.7 Hz; H-3) and 6.28 (1H, d , 2.7 Hz; H-5), a chelated hydroxyl at 𝛿 _H 11.69 (1H, s , 2-OH), and two methoxy groups ( 𝛿 _H 3.92, 3H, s , 7-OCH ₃ ) and ( 𝛿 _H 3.80, 3H, s ; 4-OCH ₃ ). The data also showed that 1 possessed an alkyl side chain with some methylene groups ( δ _H 1.25‒2.84, δ _C 22.8‒37.1) and one terminal methyl group ( δ _H 0.89, 3H, t , 6.6 Hz; δ _C 14.2). These results revealed that 6 contained one benzene ring, one methoxy group, one methoxycarbonyl group, and one aliphatic side chain. The HMBC spectrum ( Figure 2 ) suggested that the chemical structure of 6 was similar to that of 4, except that it further possessed a side chain. The HR-ESI-MS spectrum of 6 showed a pseudomolecular ion peak at m/z 377.2642 [M–H] ^– (calcd. for C ₂₃ H ₃₈ O ₄ –H, 377.2682), which suggests that the alkyl chain moiety is a tetradecyl group. Therefore, 6 was elucidated as methyl 2-hydroxy-4-methoxy-6-tetradecylbenzoate. It is a new compound from the plant.

Table 1 The ¹ H-NMR data of compounds 1 and 6

Table 2 ¹ H-NMR data of compounds 2-5

Table 3 ¹³ C-NMR data of compounds 1-6

CONCLUSION

Six compounds were isolated from the n -hexane and chloroform extracts of the liverwort Bazzania sp. , collected at Lac Duong district, Lam Dong province, Vietnam, using chromatographic methods. Those compounds are cycloartenol ( 1 ) and five phenolic derivatives, -coumaric acid ( 2 ), -hydroxybenzoic acid ( 3 ), methyl 2,4-dihydroxy-3,6-dimethyl benzoate ( 4 ), barbatic acid ( 5 ), and methyl 2-hydroxy-4-methoxy-6-tetradecyl benzoate ( 6 ). Although 1-5 were known in other species, this is the first time they have been reported in Bazzania sp . After being checked in Scifinder (on September 30 ^th , 2022), compound 6 was determined to be new. Further research on the remaining extracts is ongoing.

ABBREVIATIONS

HR-ESI-MS: High resolution- Electrospray ionization-Mass spectrometry

¹ H-NMR: Proton nuclear magnetic resonance

¹³ C-NMR: Carbon-13 Nuclear Magnetic Resonance

HMBC: Heteronuclear Multiple Bond Correlation

s : singlet

d : doublet

dd : doublet of doublets

t : triplet

m : multiplet

COMPETING INTEREST

The authors declare no competing financial interest.

AUTHORS’ CONTRIBUTION

Tran Q. T. collected samples and prepared extracts. Nguyen N. N., Nguyen T. B. T., and Nguyen H. C. performed column chromatography on the n -hexane extract and isolated compounds. Nguyen X. K., Phan T. T., and Le H. K. performed column chromatography on the chloroform extract and isolated compounds. Duong T. H and Huynh N. V. interpreted the NMR and MS data. Nguyen X. K. prepared the manuscript and searched the bibliography. Nguyen K. P. P. gave the final correction for the manuscript.