Piperlongosides A–C,Three New Phenolic Constituents From Piper longum L.

Abstract

Phytochemical study on the methanolic extract of the aerial parts of this plant led to the isolation of 7 phenolic compounds (1-7), including 3 new ones (1-3). Their chemical structures were determined to be (7,8-threo)-1-O-β-D-[6′-O-syringoylglucopyranosyl]-2-methoxy-7,8,9-trihydroxyphenylpropane (1), (7,8-threo)-8-O-(2′′-methoxyacetophenone-1′′-yl)-1-O-β-D-glucopyranosyl-2-methoxy-7,9-dihydroxyphenylpropane (2), 1-O-β-D-glucopyranosyl-2-O-(2′′-methoxy-4′′-acetylphenyl)-3-hydroxypropane (3), (−)-isolariciresinol 4-O-β-D-glucopyranoside (4), (−)-isolarisiresinol 9′-O-β-D-glucopyranoside (5), (+ )-lyoniresinol 9′-O-β-D-glucopyranoside (6), (+ )-isolarisiresinol 9′-O-β-D-glucopyranoside (7) by the spectroscopic data and in comparison with those reported in the literature. Compounds 3 to 7 significantly inhibited NO production in LPS-activated RAW264.7 cells with IC₅₀ values ranging from 32.09 ± 3.34 to 38.16 ± 1.35 µM, similar to those of the positive control, N^G-monomethyl-L-arginine acetate (IC₅₀ 32.51 ± 3.70 µM).

Keywords

Piperaceae piperlongoside A piperlongoside B piperlongoside C NO inhibition activity

Introduction

Piper longum L. (family Piperaceae) is native to Asia and widely distributed in Vietnam. Its seeds have long been used as a spice and Vietnamese traditional folk medicine for treating inflammatory diseases, diabetes, coughs, bronchitis, snakebites, and colds.^1,2 Previous studies have revealed alkaloids and phenolics from this plant.^3–6 Its fruit has antiparasitic, antimicrobial, mosquito-larvicidal, anti-inflammatory, anthelminthic, analgesic, anticancer, antioxidant, neuro-pharmacological, antihyperglycemic, hepato-protective, antihyperlipidemic, immunomodulatory, antiangiogenic, antiulcer, and anti-arthritic effects.⁷ During our search for anti-inflammatory substances, we found that the methanol extract of the aerial parts of P. longum showed good nitric oxide (NO) inhibitory activity (IC₅₀ = 68.9 µg/mL). Therefore, it was selected for further study. This article reports the isolation and structure elucidation of 3 new and 4 known phenolic compounds isolated from the methanol extract of this plant and their inhibition of NO production in LPS-stimulated RAW 264.7 cells.

Results and Discussion

A phytochemical study of the water layer of the methanolic extract of the aerial parts of P. longum, which showed strong NO production inhibition activity, led to the isolation of 7 phenolic compounds (1-7). The known compounds were identified as (−)-isolariciresinol 4-O-β-D-glucopyranoside (4),⁸ (−)-isolarisiresinol 9′-O-β-D-glucopyranoside (5),⁹ (+ )-lyoniresinol 9′-O-β-D-glucopyranoside (6),¹⁰ (+ )-isolarisiresinol 9′-O-β-D-glucopyranoside (7),¹¹ by analysis their NMR, HR-ESI-MS, and CD spectra in comparison with the previously published data (Figure 1).

Figure 1.

Chemical structures of 1 to 7.

Compound 1 was obtained as a colorless solid. The molecular formula of 1 was determined to be C₂₅H₃₂O₁₄ by high-resolution electrospray ionization mass spectrometry (HR-ESI-MS). The ¹H nuclear magnetic resonance (¹H NMR) spectra of 1 showed 3 aromatic protons on the 1,2,4-trisubstituted benzene ring [δ_H 6.62 (dd, J = 8.0, 1.8 Hz, H-5), 7.03 (d, J = 8.0 Hz, H-6), and 7.07 (d, J = 1.8 Hz, H-3)], 2 protons of a symmetric 1,3,4,5-tetrasubstituted benzene ring [δ_H 7.36, 2H, s)], one anomeric proton at δ_H 4.90 (d, J = 7.2 Hz, H-1′), and 6 glucosyl protons (δ_H 3.46-4.71). The trihydroxypropyl moiety was identified by signals at δ_H 4.54 (1H, d, J = 6.0 Hz, H-7), 3.63 (1H, m, H-8), and 3.34/3.50 (2H, m, H₂-9), and 3 methoxy groups at δ_H 3.87 (3H, s) and 3.89 (6H, s). The ¹³C NMR, HSQC, HMBC, and COSY spectra of 1 indicated 25 carbons, including a carbonyl (δ_C 167.8), 2 aromatic rings, one glucose sugar, 1 propanyl moiety, and 3 methoxy groups. These data suggested a phenolic glycoside compound.¹² In the HSQC spectrum, H-3 (δ_H 7.07), H-5 (δ_H 6.62), H-6 (δ_H 7.03), H-7 (δ_H 4.54), H-8 (δ_H 3.63), and H-9 (δ_H 3.50/3.34) correlated with carbons at δ_C 112.4, 120.4, 117.3, 75.0, 77.3, and 64.2, respectively, while H-1′ (δ_H 4.90), H-2′ (δ_H 3.56), H-3′ (δ_H 3.54), H-4′ (δ_H 3.46), H-5′ (δ_H 3.79), and H-6′ (δ_H 4.71/4.44) correlated with carbons at δ_C 102.8, 74.9, 77.7, 72.0, 75.6, and 65.2, respectively. The COSY correlations of H-1′/H-2′/H-3′/H-4′/H-5′/ H-6′, and H-7/H-8/H-9 were observed. The key HMBC and COSY correlations of 1 are shown in Figure 2 confirming the presence of 3 methoxy groups at C-3′′, C-5′′, and C-2; the glucosyl moiety linked to C-1, trihydroxypropyl moiety linked to C-4, and 3′′,5′′-dimethoxygalloyl group linked to C-6′ of the glucosyl unit. The carbon chemical shifts of C-7 (δ_C 75.0)/C-8 (δ_C 77.3) of 1 were very similar to those of threo-2-methoxy-4-(1′,2′,3′-trihydroxyphenol (δ_C 74.9 and 77.8, respectively), and different from those of (erythro)-2-methoxy-4-(-(1′,2′,3′-trihydroxyphenol) (δ_C 76.1 and 76.5, respectively), measured in the same solvent, CD₃OD.¹² The large J value of the anomeric proton (7.2 Hz) suggested the β-form of the sugar linkage. Finally, acid hydrolysis of 1, treatment with cysteine methyl ester and O-tolyl isothiocyanate, and then followed by HPLC analysis to identify D-glucose moiety by in comparison the t_R values with those of authentic D/L-glucose.^13,14 Thus, compound 1 was determined to be (7,8-threo)-1-O-β-D-[6′-O-syringoylglucopyranosyl]-2-methoxy-7,8,9-trihydroxyphenylpropane, a new compound named piperlongoside A.

Figure 2.

Key ¹H-¹H COSY and HMBC correlations of 1 to 3.

The HR-ESI-MS of compound 2 showed ion peaks at m/z 542.2234 [M + NH₄]⁺ (calcd. for [C₂₅H₃₆NO₁₂]⁺: 542.2233, Δ = + 0.2 ppm); m/z 525.1956 [M + H]⁺ (calcd. for [C₂₅H₃₃O₁₂]⁺: 525.1967, Δ = −2.1 ppm), indicating its molecular formula was C₂₅H₃₂O₁₂. The NMR spectra of 2 revealed the presence of 2 ABX coupling proton systems [δ_H 7.03 (d, 8.4 Hz), 7.55 (dd, J = 8.4, 1.8 Hz), 7.47 (d, J = 1.8 Hz], and [6.99 (dd, J = 8.4, 1.8 Hz), 7.09 (d, J = 8.4 Hz), 7.14 (d, J = 1.8 Hz)], one acetyl moiety [δ_C 199.5 (C = O), δ_C 26.3/δ_H 2.54 (3H, s)], one glucosyl unit [δ_C/δ_H: 103.0/4.81, 74.9/3.49, 78.1/3.40, 71.3/3.40, 77.7/3.48, 62.5/3.87, and 3.70)], 2 methoxy groups [δ_C/δ_H: 56.5/3.84 (3H, s) and 56.7/3.83 (3H, s)], and a trioxygenated-propanyl moiety [δ_C/δ_H: 73.8/4.88 (d, 6.6 Hz), 84.9/4.62 (m), 62.4/3.89 (d, J = 7.2 Hz)]. These data were similar to those of lycocernuaside E, suggesting they have the same planar structure.¹⁵ This was confirmed by the HSQC, HMBC, and COSY spectra as shown in Figure 2. The HMBC correlations from H-1′ (δ_H 4.81) to C-1 (δ_C 147.5) and H-8 (δ_H 3.43) to C-1′′ (δ_C 154.1) indicated a glucose unit linked to C-1 and carbon C-1′′ linked to C-8. The 2 methoxy groups were located at C-2 and C-2′′, as evidenced by HMBC correlations from methoxy protons at δ_H 3.84 and 3.83 to C-2 (δ_C 150.4) and C-2′′ (δ_C 151.1), respectively. The carbon chemical shifts of C-7 (δ_C 73.8), C-8 (δ_C 84.9), and C-9 (δ_C 62.4) of 2 differed from those of the erythro configuration in lycocernuaside E [δ_C 73.5 (C-7), 85.4 (C-8), and 62.0 (C-9)] measured in the same solvent, CD₃OD.¹⁵ Furthermore, in the HSQC spectra, the H₂-9 protons appeared separately at δ_H 3.60 and 3.87 in lycocernuaside E, while they appeared as a doublet at δ 3.89 (2H, d, 7.2 Hz) in 2. This suggested a threo configuration of 2. The large J_{H-1′/H-2′} value (7.8 Hz) indicated the axial orientation of H-1′ and H-2′. The D-glucose of 2 was identified by hydrolysis, similar to that of 1.^13,14 Thus, compound 2 was deduced to be (7,8-threo)-8-O-(2′′-methoxyacetophenone-1′′-yl)-1-O-β-D-glucopyranosyl-2-methoxy-7,9-dihydroxyphenylpropane, a new compound named piperlongoside B.

Compound 3 had molecular formula of C₁₈H₂₆O₁₀ determined by HR-ESI-MS [found m/z 420.1872 [M + NH₄]⁺ (calcd. for [C₁₈H₃₀NO₁₀]⁺: 420.1864, Δ = + 2.0 ppm); m/z 403.1596 [M + H]⁺ (calcd. for [C₁₈H₂₇O₁₀]⁺: 403.1599, Δ = −0.6 ppm). The ¹H NMR, ¹³C NMR, HSQC, COSY, and HMBC spectra of 3 confirmed the presence of glucose and glyceryl units, together with a 2′-methoxy-4′-acetylphenoxyl moiety (Table 1). The HMBC correlations from H-1′ (δ_H 4.36) to C-1 (δ_C 68.9) and H-2 (δ_H 4.69) to C-1′′ (δ_C 153.6) indicated that the glucosyl moiety was linked to C-1 and the 2′-methoxy-4′-acetylphenoxyl moiety to C-2. The large J value (7.8 Hz) of the anomeric proton revealed β-linkage of the glucose moiety. The D-glucose of 3 was identified by hydrolysis, similar to that of 1 and 2.^13,14 Therefore, compound 3 was determined to be 1-O-β-D-glucopyranosyl-2-O-(2′′-methoxy-4′′-acetylphenyl)-3-hydroxypropane, a new compound named piperlongoside C.

Table 1.

¹H NMR and ¹³C NMR Spectroscopic Data for 1 to 3.

Pos.	1		2		Pos.	3
Pos.	^a,b δ _C	^a,cδ_H (mult, J in Hz)	^a,b δ _C	^a,cδ_H (mult, J in Hz)	Pos.	^a,b δ _C	^a,cδ_H (mult, J in Hz)
1	147.2	–	147.5	–	1	68.9	4.09 (dd, 11.4, 4.8) 3.90*
2	150.5	–	150.4	–	2	80.3	4.69 (m)
3	112.4	7.07 (d, 1.8)	112.9	7.14 (d, 1.8)	3	62.1	3.86/3.90
4	121.0	–	137.4	–	1′	104.9	4.36 (d, 7.8)
5	120.4	6.62 (dd, 8.0, 1.8)	121.2	6.99 (dd, 8.4, 1.8)	2′	75.1	3.22 (dd, 9.0, 7.8)
6	117.3	7.03 (d, 8.0)	117.5	7.09 (d, 8.4)	3′	78.0	3.37 (t, 9.0)
7	75.0	4.54 (d, 6.0)	73.8	4.88 (d, 6.6)	4′	71.6	3.21 (t, 9.0)
8	77.3	3.63 (m)	84.9	4.62 (m)	5′	78.0	3.29 (m)
9	64.2	3.50* 3.34*	62.4	3.89 (d, 7.2)	6′	62.7	3.67 (dd, 12.0, 5.4) 3.84*
1′	102.8	4.90 (d, 7.2)	103.0	4.81 (d, 7.8)	1′′	153.6	–
2′	74.9	3.56 (dd, 9.0, 7.8)	74.9	3.49 (dd, 9.0, 7.8)	2′′	151.3	–
3′	77.7	3.54 (t, 9.0)	78.1	3.40 (t, 9.0)	3′′	112.5	7.58 (d, 1.8)
4′	72.0	3.46 (t, 9.0)	71.3	3.40 (t, 9.0)	4′′	132.2	–
5′	75.6	3.79 (m)	77.7	3.48 (m)	5′′	124.6	6.76 (dd, 8.4, 1.8)
6′	65.2	4.71 (dd, 12.0, 2.4) 4.44 (dd, 12.0, 7.8)	62.5	3.87 (dd, 12.0, 1.8) 3.70 (dd, 12.0, 4.8)	6′′	115.7	7.23 (d, 8.4)
1′′	138.3	–	154.1	–	7′′	199.4	–
2′′	108.6	7.36 (s)	151.1	–	8′′	26.3	2.58 (s)
3′′	149.0	–	112.4	7.47 (d, 1.8)	OCH₃	56.5	3.91 (s)
4′′	142.0	–	131.8	–
5′′	149.0	–	124.4	7.55 (dd, 8.4, 1.8)
6′′	108.6	7.36 (s)	115.7	7.03 (d, 8.4)
7′′	167.8	–	199.5	–
8′′			26.3	2.54 (s)
2-OCH₃	56.7	3.87 (s)	56.5	3.84 (s)
2′′-OCH₃			56.7	3.83 (s)
3′′-OCH₃	57.1	3.89 (s)
5′′-OCH₃	57.1	3.89 (s)

Abbreviation: NMR, nuclear magnetic resonance spectroscopy.

^aMeasured in CD₃OD.

^b150 MHz.

^c600 MHz.

^*Overlapped signals.

The HR-ESI-MS and NMR spectra of compounds 1 to 7 are shown in Supplemental Figures S1 to S37.

To identify NO inhibitors, compounds 1 to 7 were evaluated in LPS-stimulated RAW 264.7 cells. These compounds were not cytotoxic in the MTT assay at a concentration of 100 µM (Supporting information). Therefore, the NO production levels in the cell medium were measured in the presence of 1 to 7 with serial dilutions (0-100 µM). Interestingly, compounds 3 to 7 potentially inhibited NO production in LPS-activated RAW264.7 cells with IC₅₀ values of 33.46 ± 2.13, 32.09 ± 3.34, 37.81 ± 1.18, 38.16 ± 1.35, and 35.32 ± 1.14 µM, respectively, similar to those of the positive control, N^G-monomethyl-L-arginine acetate (L-NMMA) (IC₅₀ 32.51 ± 3.70 µM). Compound 2 had weaker activity, with an IC₅₀ of 64.56 ± 1.18 µM, while compound 1 was inactive with an IC₅₀ >100 µM.

Materials and Methods

General Experimental Procedures

The used characterization equipment is the same as that described in our previous work.¹⁶ Refer to Supplemental Material.

Plant Materials

The aerial parts of Piper longum L. were collected from Me Linh Biodiversity Station, Vinh Phuc, Vietnam, and identified by Dr Nguyen The Cuong, Institute of Ecology and Biological Resources, VAST. The voucher specimen (NCCT-P136) has been deposited at IMBC, VAST.

Extraction and Isolation

Refer to Supplemental Material.

Piperlongoside A (1)

Colorless solid, $[α]_{D}^{25}$ : − 21.3 (c 0.03, MeOH); UV (MeOH) λ_max (log ɛ): 220 (2.20), 280 (1.79); IR ν_max (KBr) 3401, 2934, 1711, 1073 cm⁻1; HR-ESI-MS m/z 555.1702 [M-H]⁻ (calcd. for [C₂₅H₃₁O₁₄]⁻: 555.1719, Δ = −3.1 ppm); m/z 591.1472 [M + ³⁵Cl]⁻ (calcd. for [C₂₅H₃₂O₁₄³⁵Cl]⁺: 591.1486, Δ = −2.4 ppm); m/z 591.1472 [M + ³⁵Cl]⁻ (calcd. for [C₂₅H₃₂O₁₄³⁵Cl]⁺: 591.1486, Δ = −2.4 ppm); m/z 593.1469 [M + ³⁷Cl]⁻ (calcd. for [C₂₅H₃₂O₁₄³⁷Cl]⁺: 593.1456, Δ = + 2.2 ppm); m/z 592.1493 [M + ³⁵⁺³⁷Cl]⁻ (calcd. for [C₂₅H₃₂O₁₄³⁵⁺³⁷Cl]⁺: 592.1514, Δ = −3.6 ppm); ¹H NMR (CD₃OD, 600 MHz) δ (ppm), ¹³C NMR (CD₃OD, 150 MHz) δ (ppm) data shown in Table 1.

Piperlongoside B (2)

Colorless solid, $[α]_{D}^{25}$ : + 8.7 (c 0.03, MeOH); UV (MeOH, c 0.1 mg/mL) λ_max: (log ɛ): 230 (2.38), 274 (2.19), 305 (2.05); IR ν_max (KBr) 3440, 2935, 1712, 1638, 1073 cm⁻¹; HR-ESI-MS m/z 542.2234 [M + NH₄]⁺ (calcd. for [C₂₅H₃₆NO₁₂]⁺: 542.2233, Δ = + 0.2 ppm); m/z 525.1956 [M + H]⁺ (calcd. for [C₂₅H₃₃O₁₂]⁺: 525.1967, Δ = −2.1 ppm); ¹H NMR (CD₃OD, 600 MHz) δ (ppm), ¹³C NMR (CD₃OD, 150 MHz) δ (ppm) data shown in Table 1.

Piperlongoside C (3)

Colorless solid, $[α]_{D}^{25}$ : + 12.9 (c 0.03, MeOH); UV (MeOH, c 0.1 mg/mL) λ_max: (log ɛ): 227 (1.80), 275 (1.68), 308 (1.54); IR ν_max (KBr) 3428, 2935, 1712, 1650, 1078 cm⁻¹; HR-ESI-MS m/z 420.1872 [M + NH₄]⁺ (calcd. for [C₁₈H₃₀NO₁₀]⁺: 420.1864, Δ = + 2.0 ppm); m/z 403.1596 [M + H]⁺ (calcd. for [C₁₈H₂₇O₁₀]⁺: 403.1599, Δ = −0.6 ppm); m/z 425.1417 [M + Na]⁺ (calcd. for [C₁₈H₂₆O₁₀Na]⁺: 425.1418, Δ = −0.2 ppm); ¹H NMR (CD₃OD, 600 MHz) δ (ppm), ¹³C NMR (CD₃OD, 150 MHz) δ (ppm) data are shown in Table 1.

Acid Hydrolysis and Confirmation of Monosaccharide

Refer to Supplemental Material.

Nitric Oxide Assay

The nitric oxide assay is the same as that described in our previous work.¹⁷ Refer to Supplemental Material.

Conclusions

Bio-guided fractionation and isolation of the active component from the aerial parts of P. longum led to the isolation of 3 new phenolic compounds, piperlongosides A-C (3), and 4 known ones, (− )-isolariciresinol 4-O-β-D-glucopyranoside (4), (− )-isolarisiresinol 3α-O-β-D-glucopyranoside (5), (+ )-lyoniresinol 3α-O-β-D-glucopyranoside (6), and (+ )-isolarisiresinol 3α-O-β-D-glucopyranoside (7). Of the isolates, compounds 3 to 7 significantly inhibited NO production in LPS-activated RAW264.7 cells, with IC₅₀ values ranging from 32.09 ± 3.34 to 38.16 ± 1.35 µM, similar to those of the positive control, N^G-monomethyl-L-arginine acetate with an IC₅₀ value of 32.51 ± 3.70 µM. These results suggest further studies of the anti-inflammatory mechanisms of the active compounds were warranted.

Supplemental Material

sj-docx-1-npx-10.1177_1934578X221150367 - Supplemental material for Piperlongosides A–C, Three New Phenolic Constituents From Piper longum L.

Supplemental material, sj-docx-1-npx-10.1177_1934578X221150367 for Piperlongosides A–C, Three New Phenolic Constituents From Piper longum L. by Le Thi Huyen, Nguyen Thi Thu Hau and Phan Van Kiem in Natural Product Communications

Footnotes

Abbreviations

Acknowledgments

The authors would like to thank Dr Nguyen for The Cuong at the Institute of Ecology and Biological Resources, VAST for the plant identification.

Author Contribution

Research idea, LT Huyen, PV Kiem; Isolation, NNT Hau, LT Huyen; Structure elucidation and writing, LT Huyen, PV Kiem.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Vietnam National University, Hanoi, Vietnam (grant no. QG.22.05).

Ethical Approval

Our institution does not require ethical approval for reporting individual cases or case series.

Statement of Human and Animal Rights

This article does not contain any studies with human or animal subjects.

Statement of Informed Consent

There are no human subjects in this article and informed consent is not applicable.

ORCID iD

Phan Van Kiem

Supplemental Material

Supplemental material for this article is available online.

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Supplementary Material

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