Profiling of components of rhizoma et radix polygoni cuspidati by high-performance liquid chromatography with ultraviolet diode-array detector and ion trap/time-of-flight mass spectrometric detection.

INTRODUCTION: Rhizoma et Radix Polygoni Cuspidati (Huzhang in Chinese, HZ) is a traditional medicinal plant in China. Many of the components of HZ have been proved to be bioactive while it is difficult to conduct a comprehensive chemical profiling of HZ as a consequence of the absence of efficient separation system and sensitive detective means. We developed a simple and effective method for comprehensive characterization of constituents in HZ.
OBJECTIVE: To develop a simple and effective method to characterize the components in HZ and provide useful information for subsequent metabolic studies of HZ.
MATERIALS AND METHODS: The components in HZ aqueous extract were characterized by using high performance liquid chromatography with UV diode-array detector (HPLC-DAD) and ion trap/time-of-flight mass spectrometric detection (HPLC-IT/TOF). Stilbenes, anthraquinones, gallates and tannins, naphthalenes and some other compounds were identified and confirmed by diagnostic fragment ions with accurate mass measurements, characteristic fragmentation pathways and relevant published literatures.
RESULTS: Among the 238 constituents detected in HZ, a total number of 74 constituents were identified unambiguously or tentatively, including 29 compounds reported for the first time in HZ.
CONCLUSION: The identification and structure elucidation of these chemicals provided essential data for quality control and further in vivo metabolic studies of HZ. Key words: Polygonum cuspidatum, HPLC-DAD, HPLC-IT/TOF, qualitative analysis.

INTRODUCTION

Rhizoma et Radix Polygoni Cuspidati, well-known as Huzhang (HZ) in China and Japanese knotweed in Japan, comes from the stem and root of Polygonum cuspidatum Sieb. Et Zucc. This herb has been widely used in Chinese and Japanese folk medicine for the treatment of atherosclerosis, hypertension, cough, suppurative dermatitis, and gonorrhea. HZ contains a variety of chemical groups such as stilbenes, anthraquinones, flavones, and tannins. Previous pharmacological and clinical studies have indicated that several chemical components in HZ are bioactive.[123456]

Various methods have been used for qualitative analysis of major chemical constituents in HZ. Yi et al. developed a high-performance liquid chromatography-electrospray ionization/mass spectrometry (HPLC-ESI/MS method for simultaneously qualitative and quantitative analysis of 9 compounds.[7] Xia et al. separated HZ methanol extract by HPLC-diode array detector (HPLC-DAD) and characterized 22 major constituents using HPLC-time-of-flight (TOF/MS).[8] Sun et al. detected 20 volatile oil in HZ by gas chromatography-MS.[9] Electrophoresis and NMR have also been adopted for qualitative assessment of HZ.[1011]

Despite much effort has been dedicated to chemical profiling of HZ, limited and repeated information about HZ constituents was acquired as a consequence of the absence of efficient separation system and sensitive detector. To comprehensively identify the complex chemicals of HZ, a fast and high sensitive method need to be developed. Recently, HPLC coupled with ion trap and TOF-MS (HPLC-IT/TOF-MS) has been widely used in characterization and chemical profiling of plants.[12] The hybrid ion trap and TOF MS can integrate the advantages of ion trap in producing multistage tandem fragmentations and that of TOF in high resolution and accurate mass measurement, thus providing higher sensitivity and accuracy (error within 5 ppm) than those of TOF and IT-MS. This tandem mass technique has raised the qualitative analysis of herb medicines (HMs) to a new height both in analytical speed and accuracy and has been confirmed to be a very powerful tool on the comprehensive identification of both target and nontarget components. Moreover, our previous investigation had clearly indicated the relevance and usefulness of the combination of chromatographic, spectrophotometric, and mass-spectrometric analysis to detect and identify components in complicated samples.[13] Therefore, in the present study, HPLC with diode-array and IT/TOF MS detection was used for separation and identification of the components in HZ extract. Compared with previous methods, more components (74) were characterized or tentatively identified, and 29 of these compounds were reported in HZ for the first time.

MATERIALS AND METHODS

Reagents and chemicals

Reference standards of gallic acid (HPLC >98%), (+)-catechin (HPLC >98%), (-)-epicatechin (HPLC >98%), emodin (HPLC >98%), rhein (HPLC >98%), aloe-emodin (HPLC >98%), physcion (HPLC >98%) were purchased from the National Institutes for Food and Drug Control (Beijing, China). Emodin-1-O-β-D-glucopyranoside (HPLC >98%), emodin-8-O-β-D-glucopyranoside (HPLC >98%), resveratrol (HPLC >98%), and polydatin (HPLC >98%) were purchased from Shanghai Yilin Biotech Co. Ltd (Shanghai, China). Methanol of HPLC grade was purchased from Merck (Darmstadt, Germany). Formic acid (analytical reagent) was purchased from the First Chemical Company of Nanjing (Jiangsu, China). Deionized water was prepared with a Milli-Q system (Millipore, MA, USA).

Plant materials

The rhizome et radix of P. cuspidatum was collected in Hubei Province, China and authenticated by Associate Professor Rui Song (State Key Laboratory of Natural Medicine, China Pharmaceutical University, Nanjing, China). The voucher specimens (No. zzj-P-20130813) were deposited at the Herbarium of China Pharmaceutical University, Nanjing, China.

Sample preparation

The preparation of HZ aqueous extraction took published literature as reference with some modifications.[14] 25 g of HZ was immersed in 500 mL distilled water for 1 h and heated with a heating mantle. After boiling, gentle heating was continued until the volume reduced below 250 mL. The decoction was filtered while hot and condensed to 50 mL and stored at −80°C for later use.

High-performance liquid chromatography-diode array detector analysis

The analysis was performed using an Agilent Series 1100 liquid chromatograph (Agilent Technologies, Palo Alto, CA, USA). Chromatography was carried out on a Shimadzu Inertsil C8-3 reversed phase column (150 mm × 2.1 mm, 3 μm). The mobile phase was composed of A (methanol) and B (0.1% formic acid aqueous solution) under gradient elution conditions: 5% A at 0–5 min, 5–30% A at 5–25 min, 30–50% A at 25–45 min, 50–90% A at 45–60 min, and then maintained at 90% A for 2 min. The flow rate was 0.3 mL/min. The column oven temperature was set at 35°C. The DAD detector scanned from 190 nm to 600 nm.

High performance liquid chromatography-ion trap/time-of-flight analysis

The experiments were performed using a Shimadzu (Kyoto, Japan) LC system and a hybrid IT/TOF MS. The LC condition was the same as described above. IT/TOF-MS was operated with ESI source in negative mode. The optimized analytical conditions were as follows: Interface voltage, −3.5 kV; detector voltage, 1.7 kV; nebulizing gas (N2) flow, 1.5 L/min; drying gas (N2) flow, 100 kPa; curved desorption line temperature and heat block temperature, 200°C; ion accumulated time, 30 ms; precursor ion selected width, 3.0 amu; collision energy, 30–80%. Mass spectra and chromatograms were acquired and processed with LC-MS solution version 3.6 (Shimadzu, Japan). Shimadzu's Composition Formula Predictor software was adopted to predict chemical formulas for both precursor and productions.

RESULTS AND DISCUSSION

Identification of major constituents in Huzhang

Figure 1 shows the typical total ion chromatogram (TIC) of HZ extract acquired in negative mode. In the present study, 238 components were detected in the HZ extract and 74 of them had been confirmed or tentatively identified. Compounds were first classified into several categories based on the characteristic fragment ions obtained from the HPLC-IT/TOF MS analysis. The formulae of detected components were then predicted by the formula predictor software according to the accurate mass provided by their deprotonated ions [M − H]− or [M + HCOO]−. Following that, the fragmentation pathways of these compounds were proposed on account of their MSn behaviors. By comparison with the available standard samples, previously published data and the path of biosynthesis, the components were ultimately identified or putatively characterized. The obtained ultraviolet (UV) spectra of some compounds were hired for auxiliary confirmation.

Figure 1

Total ion chromatogram of Huzhang aqueous extraction

Identification of stilbenes derivatives

A total of 16 compounds were assigned to stilbenes derivatives [Tables S1 and S2], among which three components (S1, S13, S16) were first reported in HZ. By comparison with the reference substance, S4 and S10 were identified as polydatin and resveratrol, respectively. A resveratrol-O-diglucoside (S1), which showed [M + HCOO]− ion at m/z 597.18 and [M − H]− ion at m/z 551.17, was detected in HZ extract. It formed fragment ions at m/z 389.12 ([M-H-Glu]−) and 227.07 ([M-H-2Glu]−). S3 showed the same [M + HCOO]− and [M − H]− ions with those of polydatin. Meanwhile, the MS2 spectra of S3 displayed characteristic fragment ions at m/z 227.07 and 185.06, indicating that it was resveratrol-O-glucoside as well. By comparing the UV spectra of S3 with resveratrol [Figure 2], it was tentatively identified as resveratroloside.[11516] Both S8 and S9 displayed deprotonated ion at m/z 541.13 and shared fragment ions at m/z 227.07 and 185.06. The neutral loss of 314 Da and the dominant fragment ions at m/z 313.05 and 169.01 were the symbols of the galloyl glucose. Hence, they were tentatively characterized as resveratrol 3-O-D-(6’ or 2’-galloyl)-glucopyranoside.[17]

Table S1

Identification of stilbenes derivatives

Table S2

Structures of identified stilbenes derivatives

Figure 2

Extracted ion chromatograms of ions at m/z 435.12 (a), 469.08 (b) and 541.13 (c) and ultraviolet spectra of some stilbenes

Besides the glucosides discussed above, several sulfates were detected in HZ extract. S13 and S16 all gave [M − H]− ion at m/z 307.03. The following neutral loss of 80 Da forming ions at m/z 227.07 ([M-H-SO3]−) and other MS2 fragmentation ions consistent with those of resveratrol led to the tentative characterization of S13 and S16 as resveratrol-O-sulfates. S5 and S7 both showed deprotonated ion at m/z 469.08 and characteristic MS2 fragmentation ions at m/z 307.02 ([M-H-Glu]−), 227.07([M-H-Glu-SO3]−) and 185.06 ([M-H-Glu-SO3 C2H2O]−), revealing that they might be sulfonylresveratrol-3-O-glucosides.[17] S11, S12, S14, and S15 shared the same precursor ion but almost distinct fragment ions with those of S5 and S7. In their MS2 spectra, the dominant ions were all at m/z 241.00, which was 228 Da less than the precursor ion. With the extra MS3 spectra of the ion at m/z 241.00, we predicted it as sulfonyl glucose [Figure 3]. Therefore, these four constituents were believed to be resveratrol-3-O-(sulfonyl)-glucosides.[1819]

Figure 3

The proposed fragmentation pathways of stilbenes

IDENTIFICATION OF ANTHRAQUINONES DERIVATIVES

A total of 24 anthraquinone derivatives were characterized or plausibly identified [Tables S3 and S4]. A3, A8, and A23 were separately characterized as emodin 1-O-β-D-glucopyranoside, emodin-8-O-β-D-glucopyranoside and emodin by direct injection of authentic standards.

Table S3

Identification of anthraquinones

Table S4

Structures of identified anthraquinones derivatives

In the present study, 11 hydroxyl aloe-emodin derivatives were first reported in HZ. A7, A9, A11, A16, A24 shared the [M − H]− ion at m/z 285.04, 16 Da more than that of aloe-emodin. The sequential loss of CO and CHO radical further indicated that they were aloe-emodin derivatives [Figure 4]. Therefore, they were tentatively identified as hydroxyl aloe-emodin. Compared with aloe-emodin, the UV spectrum of A9 displayed some changes in the shape and bathochromic shift in the band III [Figure 5], revealing that there was a β-OH on the other side of methylol group.[12] Consequently, A9 was tentatively identified as 6-hydroxylaloe-emodin. Five isomers (A1, A2, A4, A5, A13), which could form the aglycone ion at m/z 285.04 after having lost 162 Da and displayed characteristic fragment ions of hydroxyl aloe-emodin, were consequently identified as hydroxyl aloe-emodin-O-glucosides. The precursor ion of A20 produced fragment ion at m/z 285.04 after losing 80 Da, which yield a series of characteristic ions of hydroxyl aloe-emodin at m/z 257.04, 241.05, and 211.04 in MS3 spectrum, providing evidence for the assumption that A20 was hydroxyl aloe-emodin-O-sulfate.

Figure 4

The proposed fragmentation pathways of anthraquinone derivatives

Figure 5

Extracted ion chromatograms of ions at m/z 285.04 (a), 283.06 (b), 431.10 (c) and 299.02 (d); and ultraviolet spectra of some anthraquinone derivatives

Both A6 and A14 showed the [M − H]− ion at m/z 473.11, which was identical with that of emodin-O-(acteyl)-glucoside. Nevertheless, the MS2 fragment ion at m/z 311.05 formed by direct loss of 162 Da from the precursor ion denied the deduction. Based on this evidence, those two compounds were plausibly identified as acetylemodin-O-glucosides. In addition, an acetylemodin (A21), which showed a deprotonated ion at m/z 311.06 and shared most of the fragment ions with emodin, was detected at 55.707 min.

The [M − H]− ion and a series of characteristic fragment ions of compound A10 and A17 led to the initial assumption that they were physcion and its isomer. The injection of standard sample of physcion overthrow the inference then as both of them had weaker retention than physcion. Besides, the UV spectrum of A10 was quite different from that of physcion [Figure 5]. Those comprehensive hints indicated that they were possibly two more polar isomers of physcion. Taking the retention times of emodin-1-O-β-D-glucopyranoside and emodin-8-O-β-D-glucopyranoside as reference, A10 was tentatively identified as emodin-1-questin which was first reported in HZ, while A17 was characterized as emodin-8-questin reported in literatures.[7]

A11 showed the [M − H]− ion at m/z 431.10, and shared most of the fragment ions with those of emodin-1 or 8-O-β-D-glucopyranoside. In the UV spectra of A11, the characteristic band III and band V of anthraquinone displayed hypochromatic shift compared with emodin [Figure 5], indicating that the β-OH of emodin was substituted. Consequently, A11 was tentatively identified as emodin-6-O-glucoside.

The formula of A18, which showed [M − H]− ion at m/z 299.02, was predicted as C15H7O7. In the MS2 spectrum, the ion at m/z 255.03 indicated that a carboxyl was attached to the anthraquinone moiety. Thus, A18 was tentatively characterized as hydroxyl rhein. By comparing the UV spectra of A18 with that of rhein [Figure 5], the substitution position was found to be more likely on the other side of the carboxyl. Since the C-6 position was easier to be substituted, A18 was consequently qualified as 6-hydroxylrhein.

A19 and A22 exhibited the same deprotonated ion at m/z 511.05 and roughly identical fragment ions despite little difference in intensity. A consecutive loss of SO3 and glucoside led to the formation of aglycone ion at m/z 269.04, which was believed to be emodin proved by the diagnostic ions of emodin at m/z 241.05 and 225.05. In the meantime, the fragment ion at m/z 241.00 (sulfonyl glucose) provided further clue that these two compounds were likely to be emodin-O-(sulfonyl)-glucosides.

Identification of gallates and tannins

Gallates and tannins can be found in most of the Polygonaceae plants. In the present study, 19 gallates and tannins were detected, and 18 of them were characterized [Tables S5 and S6]. By comparing the retention times, characteristic fragment ions and UV spectra [Figure S1] with references, G3, G12, G15 were identified as gallic acid, (+)-catechin, and (-)-epicatechin, respectively. Five isomers (G1, G2, G4, G5, G6, and G8), which displayed [M − H]− ion at m/z 331.07 and shared the same characteristic ions at m/z 169.01 and 125.02 [Figure S2], were characterized as galloyl glucose.

Table S5

Identification of gallates and tannins

Table S6

Structures of identified gallates and tannins

Figure S1

Ultraviolet spectra of some gallates and tannins

Figure S2

The proposal fragmentation pathways of gallates and tannins

Protocatechuic acid was once detected in HZ.[20] In our present study, a protocatechuic acid-O-glucoside (G7) rather than the aglycone itself was found in the herb. The [M − H]− ion at m/z 315.07 first lost 162 Da to form the deprotonated ion of protocatechuic acid, which produced the ion at m/z 108.02 by a carboxyl crashed down.

G9 and G13 yielded the [M − H]− ion at m/z 451.12, which could give rise to a further fragment ion at m/z 289.07 after a loss of 162 Da. G13 was eluted between (+)-catechin and (-)-epicatechin, provided evidence that it was more likely to be (-)-epicatechin glucoside under the consideration that (+)-catechin glucoside should have weaker retention than (+)-catechin on reverse phase chromatography. Therefore, G13 was believed to be (-)-epicatechin glucoside while G9 was tentatively identified as (+)-catechin-5-O-glucoside.[21] Another pair of positional isomers, G17 and G18, gave the deprotonated ion at m/z 441.08. In the MS2 spectra, the dominant ion at m/z 289.07 and other characteristic fragment ions implied that they were (+)-catechin or (-)-epicatechin derivatives. Since the precursor ion was 152 Da more than the dominant fragment ion at m/z 289.07, thus guiding us to tentatively identify G17 and G18 as (+)-catechin or (-)-epicatechin gallates. The fragment ion at m/z 169.01 also provided evidence for this assumption.

Three enantiomers of procyanidin B (G10, G11, and G14) were found in the EICs at m/z 577.13. The deprotonated molecule of these compounds could further yield the most abundant ion at m/z 407.07, which might result from Retro Diels-Alder reaction (RDA) and the following loss of H2O. The fragment ion at m/z 289.07 can be attributed to the quinone methide (QM) fission cleavage of the type-B interflavan bond resulting from the loss of an (+)-catechin or (-)-epicatechin residue.[22] Besides, a type-B galloylated procyanidin (G16) with the precursor ion at m/z 729.14 was detected at 23.840 min. In the MSn spectrum, RDA reaction, heterocyclic ring fission, and QM fission cleavage had led to the formation of characteristic ions at m/z 407.07, 451.10, and 289.07 [Figure S2].

Identification of naphthalene derivatives

Six naphthalene derivatives were detected and characterized in HZ [Tables S7 and S8]. Five of them (N1, N2, N3, N5, N6) formed the same fragment ion at m/z 245.08, which was predicted as torachrysone supported by the characteristic fragment ions at m/z 230.05, 215.03, 187.03, and 159.05 [Figure S3]. N1 was identified as torachrysone-8-O-β-D-glucoside.[23] N2 and N5 were characterized as two positional isomers of torachrysone-8-O-(acetyl)-glucosides since they directly lost 204 Da (acetyl glucose) and formed the base peak at m/z 245.08. N3 and N6 shared the same [M − H]− ion at m/z 487.09, which was 80 Da more than that of N1, indicating that the two compounds were highly likely the sulfate ester of N1, and that the sulfate moiety was connected to glucose since there was no fragment ion which formed by directly loss of hexoside.

Table S7

Identification of naphthalene derivatives

Table S8

Structures of naphthalene derivatives

Figure S3

The proposed fragmentation pathways of naphthalenes

The MS2 spectrum of N4 was entirely different from those of the five components discussed above. The [M − H]− ion at m/z 339.02 lost 80 Da and yielded the dominant ion at m/z 259.06 [Figure S3], which was tentatively identified as 2-methoxy-6-acethyl-7-methyljuglone.[20] Therefore, N4 was tentatively identified as 2-methoxy-6-acethyl-7-methyljuglone-5-O-sulfate. Other fragment ions at m/z 244.04 ([M-H-CH3•]−•) and 231.06 ([M-H-CO]−) provided further evidence for the conclusion.

Identification of other compounds

Totally, 20 components which could hardly be assigned to categories above were detected, and 10 of them were tentatively identified [Tables S9 and S10]. O1 and O5 were characterized as critic acid[24] and5, 7-dihydroxy-1-(3H)-isobenzofuran-one,[25] respectively. O4 and O6 were believed to be glucosides of O5 since the precursor ion of them was 162 Da more than that of O5, and they share the other fragment ions with O5.

Table S9

Identification of other compounds

Table S10

Structures of other compounds

O6 displayed the deprotonated ion at m/z 329.09, which produced the ion at m/z 167.03 after losing 162Da, indicating that it was a glucoside. The aglycone was predicted as vanillic acid proved by the ion at m/z 123.05 ([M-H-Glu-CO2]).[26] Meanwhile, the occurrence of the two characteristic ions at m/z 239.05 ([M-H-90 Da]−) and 209.04 ([M-H-120 Da]−) proved that O6 was likely to be vanillic acid-C-glucoside though the exact substitution position remained to be further confirmed.

O7 was characterized as a sulfate since the [M − H]− ion lost 80 Da directly and formed ion at m/z 233.04, which was identical with the deprotonated ion of 5-carboxymethyl-7-hydroxy-2-methyl chromone.[27] Another sulfate, O8, shared the same fragment ion at m/z 189.05 with O7, indicating they might have the same 2, 5-dimethyl-7-hydroxylchromone nucleus. The most abundant ion at m/z 231.06 prompted that there might be an acetyl on the heterocyclic nucleus or connected to the hydroxyl group. Since sulfation usually acted on the hydroxyl group, the acetyl was more possibly substituted on the heterocyclic nucleus. Considering the electronic effect and steric hindrance, O8was tentatively characterized as 3-acetyl-2, 5-dimethylchromone-7-O-sulfate. O9 displayed the [M − H]− ion at m/z 269.02 and the fragment ion at m/z 189.05, thus indicating that the compound might be 2,5-dimethylchromone-7-O-sulfate.

The [M − H]− at m/z 285.01 of O10 directly lost 80 Da, forming an ion at m/z 205.05, which was predicted as C11H9O4−. The aglycone was tentatively characterized as 7-hydroxy-4-methoxy-5-methylcoumarin.[7] Consequently, O10 was tentatively identified as 4-methoxy-5-methylcoumarin-7-O-sulfate.

SUMMARY

A relatively comprehensive chemical profiling of HZ was achieved by combination of HPLC-DAD and HPLC-IT/TOF analysis. 74 compounds including stilbenes, anthraquinones, gallates and tannins, naphthalenes along with some other minor components were identified or presumed based on their accurate mass, fragment patterns, and characteristic UV spectra. Besides, 29 of these components were reported in HZ for the first time. Although the substitution positions of some components remained to be further confirmed by other ancillary qualitative methods, the subtle combination of HPLC-DAD and HPLC-IT/TOF method established in the present study could provide reference method for quality control of HZ and provide a basis for further metabolic studies in vivo of HZ. In addition, the high sensitive method could yet be regarded as a new perspective of componential analysis of other HMs, especially those containing components with chromophores.