Structural Characterization of Withanolide Glycosides from the Roots of Withania somnifera and Their Potential Biological Activities

Withania somnifera (Solanaceae), commonly known as “ashwagandha”, is an ayurvedic medicinal plant that has been used for promoting good health and longevity. As part of our ongoing natural product research for the discovery of bioactive phytochemicals with novel structures, we conducted a phytochemical analysis of W. somnifera root, commonly used as an herbal medicine part. The phytochemical investigation aided by liquid chromatography-mass spectrometry (LC/MS)-based analysis led to the isolation of four withanolide glycosides (1–4), including one new compound, withanoside XII (1), from the methanol (MeOH) extract of W. somnifera root. The structure of the new compound was determined by nuclear magnetic resonance (NMR) spectroscopic data, high-resolution (HR) electrospray ionization (ESI) mass spectroscopy (MS), and electronic circular dichroism (ECD) data as well as enzymatic hydrolysis followed by LC/MS analysis. In addition, enzymatic hydrolysis of 1 afforded an aglycone (1a) of 1, which was identified as a new compound, withanoside XIIa (1a), by the interpretation of NMR spectroscopic data, HR-ESIMS, and ECD data. To the best of our knowledge, the structure of compound 2 (withagenin A diglucoside) was previously proposed by HRMS and MS/MS spectral data, without NMR experiment, and the physical and spectroscopic data of withagenin A diglucoside (2) are reported in this study for the first time. All the isolated compounds were evaluated for their anti-Helicobacter pylori, anti-oxidant, and anti-inflammatory activities. In the anti-Helicobacter pylori activity assay, compound 2 showed weak anti-H. pylori activity with 7.8% inhibition. All the isolated compounds showed significant ABTS radical scavenging activity. However, all isolates failed to show inhibitory activity against nitric oxide (NO) production in lipopolysaccharide-stimulated RAW 264.7 macrophage cells. This study demonstrated the experimental support that the W. somnifera root is rich in withanolides, and it can be a valuable natural resource for bioactive withanolides.


Introduction
Withania somnifera (L.) Dunal, also well-known as "Indian ginseng" or "ashwagandha" belongs to the plant family Solanaceae [1,2]. Its roots and leaves have been largely used for purposes to promote longevity and good health in the ayurvedic system of traditional Indian medicine for over 3000 years [2]. Until present, the roots of W. somnifera have been widely consumed as a functional food due to the fact of therapeutic properties, consisting of invigoration, enhancement of cognitive ability, and stress release activities. The root

Structural Elucidation of the Isolated Compounds 1-4
Compound 1, obtained as a white amorphous powder, possessed the molecular formula of C 40 H 62 O 16 deduced from positive high-resolution electrospray ionization mass spectroscopy (HR-ESIMS), which revealed a [M + Na] + ion peak at m/z 821.3920 (calcd. for C 40 H 62 NaO 16 , 821.3936). The infrared (IR) spectrum of 1 showed distinctive absorptions for the hydroxy (3439 cm −1 ) and α,β-unsaturated ketone (1705 cm −1 ) functional groups. The 1 H NMR data (Table 1) (Table 1) of 1, combined with heteronuclear multiple bond correlation (HMBC) experiment, revealed 40 carbon resonances, including 28 carbons for aglycone and 12 carbons for sugar units, where the carbon resonances assignable to the sugar units were typical of glucose [21]. Comprehensive inspection of the NMR data revealed that the NMR data of 1 was similar to those of withanoside VII, previously isolated from W. somnifera roots [22], but the apparent differences between the structures of 1 and withanoside VII were detected in the side chain due to the discrepancy of NMR data corresponding to C-20 and C-21.

Structural Elucidation of the Isolated Compounds 1-4
Compound 1, obtained as a white amorphous powder, possessed the molecular formula of C40H62O16 deduced from positive high-resolution electrospray ionization mass spectroscopy (HR-ESIMS), which revealed a [M + Na] + ion peak at m/z 821.3920 (calcd. for C40H62 NaO16, 821.3936). The infrared (IR) spectrum of 1 showed distinctive absorptions for the hydroxy (3439 cm −1 ) and α,β-unsaturated ketone (1705 cm −1 ) functional groups. The 1 H NMR data (Table 1) [21]. Comprehensive inspection of the NMR data revealed that the NMR data of 1 was similar to those of withanoside VII, previously isolated from W. somnifera roots [22], but the apparent differences between the structures of 1 and withanoside VII were detected in the side chain due to the discrepancy of NMR data corresponding to C-20 and C-21.  The different partial structure of 1 was characterized by the analysis of the key HMBC correlations from H 3 -21 (δ H 1.29)/C-17 (δ C 54.3), C-20 (δ C 74.9), and C-22 (δ C 81.2) ( Figure 3). Importantly, the linkage positions of the two glucoses were confirmed by the key COSY correlations from H-1 to H 2 -4, and key HMBC correlations of H-1 /C-3, and H-1 /C-6 ( Figure 3). Finally, the complete planar structure of 1 was elucidated by analysis of COSY and HMBC experiments ( Figure 3).  The stereochemistry of 1 was established by the correlations obtained from the rotating frame Overhauser effect spectroscopy (ROESY) experiment, vicinal proton coupling constants observed in the 1 H NMR specrtum, and electronic circular dichroism (ECD)  The stereochemistry of 1 was established by the correlations obtained from the rotating frame Overhauser effect spectroscopy (ROESY) experiment, vicinal proton coupling constants observed in the 1 H NMR specrtum, and electronic circular dichroism (ECD) data. The α-position of the hydroxy groups at C-1 and C-7 was determined by the ROESY  The stereochemistry of 1 was established by the correlations obtained from the rotating frame Overhauser effect spectroscopy (ROESY) experiment, vicinal proton coupling constants observed in the 1 H NMR specrtum, and electronic circular dichroism (ECD) data. The α-position of the hydroxy groups at C-1 and C-7 was determined by the ROESY ) correlations for 1. The stereochemistry of 1 was established by the correlations obtained from the rotating frame Overhauser effect spectroscopy (ROESY) experiment, vicinal proton coupling constants observed in the 1 H NMR specrtum, and electronic circular dichroism (ECD) data. The α-position of the hydroxy groups at C-1 and C-7 was determined by the ROESY correlations of H 3 -19/H-1 and H-4β, and H-8/H-7, H 3 -18, and H 3 -19 ( Figure 4). The ECD spectrum of 1 showed a positive Cotton effect at 255 nm oriented from the n → π* transition of the α,β-unsaturated δ-lactone [18,23], which unambiguously confirmed a 22R-configuration. The configuration of C-22 was also supported by the typical coupling constants (J = 13.5 and 3.5 Hz) of H-22 showing a doublet of doublets [18,23]. In addition, almost the same 13 C NMR chemical shifts observed for C-20 and the carbons of D ring and lactone ring of 1 to the related known withanolides, including withasilolide A [18], 20β-hydroxy-1-oxo-(22R)-witha-2,5,24-trienolide [24], withacoagin, [25], dunawithanine B [26], withacoagulin E [27], and 1α,3β,20α F -trihydroxy-20R,22R-witha-5,24-dienolide [28]. Suggested that the configuration of C-20 of 1 is the same as that of the related compounds. Finally, to identify the absolute configuration of sugar units, enzymatic hydrolysis of 1 using glucosidase was carried out, which afforded aglycone 1a and the sugar moieties from 1. The absolute configuration of two glucoses of 1 was concluded as D-configuration by LC/MS-based analysis after their thiocarbamoyl-thiazolidine derivatization [29]. The typical coupling constant (J = 8.0 Hz) of the anomeric protons was characteristic of β-form in glucopyranose [30], which demonstrated that both the sugar units of 1 were β-D-glucopyranoses. Therefore, the chemical structure of 1 was elucidated as shown in Figure 1, and compound 1 was named withanoside XII.  Compound 1a, obtained as an aglycone of 1 by enzymatic hydrolysis, possessed the molecular formula of C28H42O6 confirmed by HRESIMS, which showed molecular ion peak at m/z 473.2904 [M − H] − (calcd. for C28H41O6, 473.2903) in the negative-ion mode. The NMR data of 1a showed clearly similar signals to those of compound 1, without signals responsible for the sugar moieties. The up-field shifted signals (δH 3.91/δC 65.1) for C-3 of 1a indicated the absence of the sugar units at C-3. Likewise, the planar structure of 1a was further confirmed by the interpretation of COSY and HMBC experiments ( Figure 5). The observed ROESY correlations and a positive Cotton effect at 251 nm in the ECD spectrum of 1a confirmed the same stereochemistry of 1a to that of 1. Accordingly, the structure of 1a was also assigned as a new compound, as illustrated in Figure 5 and compound 1a, was named withanoside XIIa.  The NMR data of 1a showed clearly similar signals to those of compound 1, without signals responsible for the sugar moieties. The up-field shifted signals (δ H 3.91/δ C 65.1) for C-3 of 1a indicated the absence of the sugar units at C-3. Likewise, the planar structure of 1a was further confirmed by the interpretation of COSY and HMBC experiments ( Figure 5). The observed ROESY correlations and a positive Cotton effect at 251 nm in the ECD spectrum of 1a confirmed the same stereochemistry of 1a to that of 1. Accordingly, the structure of 1a was also assigned as a new compound, as illustrated in Figure 5 and compound 1a, was named withanoside XIIa.
The known compounds were identified as withagenin A diglucoside (2) [31], withanoside II (3) [22], and withanoside IV (4) [22] by comparing their physical features and NMR spectroscopic data with those previously reported, and the data from LC/MS analysis. To the best of our knowledge, the compound 2 (withagenin A diglucoside) has not been isolated as a natural product, and the structure of withagenin A diglucoside (2) was previously proposed by HRMS and MS/MS spectral data without NMR experiment [31]. Compound 2 was isolated as a white amorphous powder, and the molecular formula of 2 was deduced as C 40 17 ) determined by (−)-HRESIMS data, the compound 2 was unambiguously confirmed as withagenin A diglucoside, which was also supported by the comparable UV and ECD data of 2 to withanoside II (3) [22]. Here, we reported the physical and spectroscopic data of withagenin A diglucoside (2) for the first time. Compound 1a, obtained as an aglycone of 1 by enzymatic hydrolysis, possessed the molecular formula of C28H42O6 confirmed by HRESIMS, which showed molecular ion peak at m/z 473.2904 [M − H] − (calcd. for C28H41O6, 473.2903) in the negative-ion mode. The NMR data of 1a showed clearly similar signals to those of compound 1, without signals responsible for the sugar moieties. The up-field shifted signals (δH 3.91/δC 65.1) for C-3 of 1a indicated the absence of the sugar units at C-3. Likewise, the planar structure of 1a was further confirmed by the interpretation of COSY and HMBC experiments ( Figure 5). The observed ROESY correlations and a positive Cotton effect at 251 nm in the ECD spectrum of 1a confirmed the same stereochemistry of 1a to that of 1. Accordingly, the structure of 1a was also assigned as a new compound, as illustrated in Figure 5 and compound 1a, was named withanoside XIIa. The known compounds were identified as withagenin A diglucoside (2) [31], withanoside II (3) [22], and withanoside IV (4) [22] by comparing their physical features and NMR spectroscopic data with those previously reported, and the data from LC/MS analysis. To the best of our knowledge, the compound 2 (withagenin A diglucoside) has not been isolated as a natural product, and the structure of withagenin A diglucoside (2) was previously proposed by HRMS and MS/MS spectral data without NMR experiment [31]. Based on the NMR data, and the molecular formula (C40H62O17) determined by (-)-HRESIMS data, the compound 2 was unambiguously confirmed as withagenin A diglucoside, which was also supported by the comparable UV and ECD data of 2 to withanoside II (3) [22]. Here, we reported the physical and spectroscopic data of withagenin A diglucoside (2) for the first time. The different partial structure of 1 was characterized by the analysis of the key HMBC correlations from H3-21 (δH 1.29)/C-17 (δC 54.3), C-20 (δC 74.9), and C-22 (δC 81.2) ( Figure  3). Importantly, the linkage positions of the two glucoses were confirmed by the key COSY correlations from H-1 to H2-4, and key HMBC correlations of H-1′/C-3, and H-1″/C-6′ (Figure 3). Finally, the complete planar structure of 1 was elucidated by analysis of COSY and HMBC experiments (Figure 3). The stereochemistry of 1 was established by the correlations obtained from the rotating frame Overhauser effect spectroscopy (ROESY) experiment, vicinal proton coupling constants observed in the 1 H NMR specrtum, and electronic circular dichroism (ECD) data. The α-position of the hydroxy groups at C-1 and C-7 was determined by the ROESY correlations of H3-19/H-1 and H-4β, and H-8/H-7, H3-18, and H3-19 ( Figure 4). The ECD spectrum of 1 showed a positive Cotton effect at 255 nm oriented from the n → π* transition of the α,β-unsaturated δ-lactone [18,23], which unambiguously confirmed a 22R-configuration. The configuration of C-22 was also supported by the typical coupling constants (J = 13.5 and 3.5 Hz) of H-22 showing a doublet of doublets [18,23]. In addition, almost the same 13 C NMR chemical shifts observed for C-20 and the carbons of D ring and lactone ring of 1 to the related known withanolides, including withasilolide A [18], 20β-hydroxy-1-oxo-(22R)-witha-2,5,24-trienolide [24], withacoagin, [25], dunawithanine B [26], withacoagulin E [27], and 1α,3β,20αF-trihydroxy-20R,22R-witha-5,24-dienolide [28]. Suggested that the configuration of C-20 of 1 is the same as that of the related compounds. Finally, to identify the absolute configuration of sugar units, enzymatic hydrolysis of 1 using glucosidase was carried out, which afforded aglycone 1a and the sugar moieties from 1. The absolute configuration of two glucoses of 1 was concluded as D-configuration by LC/MS-based analysis after their thiocarbamoyl-thiazolidine derivatization [29]. The typical coupling constant (J = 8.0 Hz) of the anomeric protons was characteristic of β-form in glucopyranose [30], which demonstrated that both the sugar units of 1 were β-D-glucopyranoses. Therefore, the chemical structure of 1 was elucidated as shown in Figure 1, and compound 1 was named withanoside XII.

Evaluation of Biological Activity of the Isolated Compounds
Helicobacter pylori is a major health problem worldwide, affecting approximately 50% of the global population [32,33]. In our anti-H. pylori activity assay, we found that the crude MeOH extract of W. somnifera roots showed weak antibacterial activity with 22.4% inhibition against H. pylori strain 51. Several previous studies have reported anti-bacterial activity of W. somnifera, and withaferin A, a major withanolide from this plant is known to inhibit H. pylori-induced inflammation [34][35][36][37]. Thus, the isolated withanolide glycosides (1-4) were evaluated for anti-H. pylori activity. Among the isolates, compound 2 exhibited weak anti-H. pylori activity with 7.8% growth inhibition against H. pylori strain 51, and the other compounds failed to show inhibitory activity. An epoxide group at C-6/C-7 and the hydroxyl groups at C-5 and C-27 may play a role in anti-H. pylori activity of compound 2, compared with the structures of inactive compounds.
Anti-oxidant activity has been known to be related to various human diseases including cancer and inflammation, and the methanolic extract of W. somnifera has been reported to show anti-oxidant activity [36]. In this study, the crude MeOH extract of W. somnifera roots showed significant ABTS radical scavenging activity with 24.3 ± 4.9 µmol Trolox equivalent (TE)/g·DW (dry weight). Thus, the isolated compounds 1-4 were evaluated for anti-oxidant activity. Although the activity of 1-4 was lower than the extract, all the compounds also displayed significant anti-oxidant activity, and the TE values of 1-4 were 10.5 ± 3.2, 13.1 ± 2.1, 9.4 ± 1.8, and 11.6 ± 3.9 µmol TE/g·DW, respectively. Significant differences in the activity were not found among the isolated compounds.
In addition, the anti-inflammatory activity of the isolates was evaluated because ethanolic extract of W. somnifera root, withaferin A, and phenolic compounds from this plant have been reported to have anti-inflammatory activity [19,38]. However, all the isolates (1-4) failed to show inhibitory activity against lipopolysaccharide-induced nitric oxide (NO) production in RAW 264.7 murine macrophage cells.

General Experimental Procedure and Plant Material
Detailed information on the general experimental procedure and identification of plant material are provided in the Supplementary Materials.

Extraction and Separation of the Compounds
Dried roots of W. somnifera (1.28 kg) were extracted using 80% aqueous MeOH (each 3.0 L for 3 days) under reflux three times and filtered at room temperature. After the filtrate was concentrated through a rotavapor, the methanolic extract (189.6 g) was suspended in water (700 mL) and then partitioned with each 700 mL of hexane, dichloromethane (CH 2 Cl 2 ), ethyl acetate (EtOAc), and n-butanol (n-BuOH). Four fractions were provided in corresponding order: hexane (3.4 g), CH 2 Cl 2 (4.5 g), EtOAc (2.0 g), and n-BuOH soluble (18.6 g) fractions. Four fractions from the solvent partitioning were subjected to LC/MS analysis combined with the reference of an in-house-built UV spectra library, which indicated the presence of withanolide glycosides in the n-BuOH fraction since some peaks in the n-BuOH fraction exhibited the UV pattern (λ max 200-230 nm) related to that reported for withanolides [18] and the molecular ion peaks ranging m/z 780-820.

Enzymatic Hydrolysis and Absolute Configuration Determination of the Sugar Moieties of Compound 1
The absolute configurations of the sugar moieties were determined using the method previously described [24] with minor modifications. Briefly, compound 1 (1.0 mg) was hydrolyzed with crude glucosidase (10 mg, from almonds, Sigma-Aldrich, St. Louis, MO, USA) for 72 h at 37 • C, and CH 2 Cl 2 was used for the extraction of aglycone. The CH 2 Cl 2 fraction was confirmed to contain the aglycone 1a by LC/MS analysis. Detailed description for absolute configuration determination of the sugar moieties in the aqueous layer is provided in the Supplementary Materials.

Anti-Helicobacter pylori Activity
The anti-H. pylori activity was evaluated using a previously described method [39]. The detailed information on the experimental procedure is included in the Supplementary Materials.

Antioxidant Activity Test with ABTS Radical
The ABTS radical was generated using a previously reported method [40]. Each sample (20 µL) was reacted with 180 µL of the ABTS •+ solution at room temperature, and the absorbance was measured at 734 nm after 10 min. The antioxidant activity of each sample was expressed as Trolox (Sigma, St. Louis, MO, USA) equivalents per gram (µmol TE/g).

Anti-Inflammatory Activity
The anti-inflammatory activity was evaluated using our previously reported method [19]. The detailed information on the experimental procedure is included in the Supplementary Materials.

Statistical Analysis
All of the contents and the antioxidant activities are expressed as the means ± standard deviations (SDs) of triplicate determinations. The differences among samples were statistically evaluated via one-way analysis of variance (ANOVA). The values were evaluated at the 5% significance level using two-sided tests. Pearson's correlation coefficients were obtained using IBM SPSS Statistics 24.0 software (Armonk, NY, USA).

Conclusions
In conclusion, we isolated and characterized four withanolide glycosides (1-4), including one new compound, withanoside XII (1), in the n-BuOH fraction of the MeOH extracts of W. somnifera root via the LC/MS-based analysis [41]. The structure of withanoside XII was established by 1D and 2D NMR spectroscopic methods, HR-ESIMS, and ECD data as well as enzymatic hydrolysis followed by LC/MS analysis. We also characterized the new compound, withanoside XIIa (1a), as an aglycone of 1, obtained by enzymatic hydrolysis of 1. The physical and spectroscopic data of withagenin A diglucoside (2) were reported in this study for the first time. In the anti-H. pylori activity assay, compound 2 showed weak anti-H. pylori activity with 7.8% inhibition. In addition, all the isolated withanolide glycosides showed significant anti-oxidant activity in the ABTS radical scavenging assay. The present study provided the experimental support that W. somnifera root is rich in withanolides, and it can be a valuable natural resource for bioactive withanolides.