Caesalpinia sappan L. (C. sappan L.), a traditional Chinese medicine, has been widely used to treat bruises and dysmenorrhea, performing pharmacological activities such as anti-inflammatory and anti-tumor. C. sappan L. has been reported to contain compounds such as protosappanins, brazilins, and homoisoflavones. In the pre-experiments, we discovered that there were many unstable components in the characteristic chromatogram of C. sappan L. Here, silica-gel column chromatography, high-speed counter-current chromatography, and preparative high-performance liquid chromatography were combined and applied to isolate the unstable components from alcohol extract of C. sappan L. The results showed that four unstable compounds were collected with the purity higher than 95.0%, characterized as episapponal, brazilin, sapponal, and 4-O-methylsapponal by hydrogen-1 and carbon-13 nuclear magnetic resonance. Based on the above results, the characteristic chromatogram of C. sappan L. was established, and the characteristic peaks were identified. These results provided a theoretical basis for the quality assessment of C. sappan L.
Caesalpinia sappan L. (C. sappan L.) is a species of the legume family and cultivated widely in Southeast Asia, especially in Guangxi Province, China, where a complete industry chain has been formed [1,2]. As a valuable traditional Chinese medicine (TCM), C. sappan L. has a wealth of pharmacological activities, including anti-bacterial [3,4], hypoglycemic , anti-inflammatory , and vasodilator . Various types of compounds have been isolated and identified from the heartwood of C. sappan L., such as protosappanins, brazilins, and homoisoflavones by previous research . In the study of Mitani et al. , six active compounds were isolated and characterized from C. sappan L. extract. And a new compound, (6aS,11bR)-7,11b-dihydro-6H-indeno[2,1-c]chromene-3,6a,10,11-tetrol, was identified for the first time. Brazilin and protosappanin B were isolated from 70% (v/v) ethanol extract of C. sappan L. and they exhibited effects on inhibiting the formation of melanin in vivo .
Separation methods for isolating polar compounds of C. sappan L. included silica gel column chromatography (SGCC), preparative column, high-speed counter-current chromatography (HSCCC), and gel column chromatography (GCC) . Notably, preparative high-performance liquid chromatography (prep-HPLC) was a powerful method developed by a high-loading and high-separation preparative column, which had been commonly employed in separating natural products and was suitable for preparative-scale separation of target compounds with high purity and recovery rate . Furthermore, HSCCC represented a liquid–liquid chromatographic separation technique whose stationary and mobile phases were liquids without irreversible adsorption during separation [13–15]. The method applying HSCCC provided significant advantages over other traditional separation methods, including non-contamination, higher separation efficiency, low sample loss, high recovery, and high separation volume [16–18]. Previously, some researchers had achieved the separation of homoisoflavonoids from C. sappan L. by liquid–liquid partition chromatography. Uddin et al.  used centrifugal partition chromatography to separate sappanol and brazilin from the ethyl acetate-soluble ingredient (350 mg) of C. sappan L. As described by Xu et al. , 3′-deoxysappanol (5 mg), 3-deoxysappanone B (8 mg), 4-O-methylsappanol (20 mg), and brazilin (18 mg) were isolated from an ethyl acetate extract (120 mg) of C. sappan L. by HSCCC.
Currently, the quality control (QC) of C. sappan L. in the Chinese Pharmacopoeia (2020) was limited to thin layer chromatography (TLC), water content determination and extractive determination , and lack of quantitative indicators and fingerprint. There has been no adequate research on its fingerprint, and identification of the characteristic peaks in the fingerprint has been restricted to protosappanin B and brazilin. In therapies of TCM, C. sappan L. was typically used in decoction. Several characteristic peaks were discovered to be substantially reduced after the adjustment of the decoction time, while information on characteristic peaks was still not clear. As a result, it was required and meaningful to isolate and identify the characteristic peaks in the fingerprint of C. sappan L. The principal aim of this study was to isolate and identify unstable components of C. sappan L. by column chromatography, counter-current chromatography, and prep-HPLC, so as to establish a characteristic chromatogram for comprehensively controlling the quality of C. sappan L.
2 Materials and methods
2.1 Reagents and materials
The heartwood of C. sappan L. was collected from Luchuan, Guangxi, China. Methanol, n-hexane, n-butanol, ethyl alcohol, petroleum ether, chloroform, and ethyl acetate were purchased from Huipu chemical store, Hangzhou, China. The above reagents were all of analytical grade. Acetonitrile (chromatographic grade) and methanol (chromatographic grade) were obtained from Tedia (Fairfield, USA). Protosappanin B (purity >98.0%) was purchased from Chengdu Pusi Biotechnology Company Inc. The silica gel (200–300 mesh) and TLC plates were purchased from Yantai Jiangyou Silica Gel Development Co., Ltd.
A TBE-200V HSCCC separation instrument (Shanghai Tauto Biotechnology, Shanghai, China) was employed, outfitted with a constant flow pump model MP-0106 (Sanwei Science and Technology Co., Ltd, Shanghai, China), a thermostatic controller model SDC-6 (Nanjing Xinchen Biotechnology Co., Ltd, Nanjing, China), a constant flow pump Model TBP 5002 (Shanghai Tauto Biotechnology Co., Ltd, Shanghai, China), and a UV detector Model HD-2 (Husi Analytical Instruments, Shanghai, China). The separation column was constructed of 1.6 mm polytetrafluoroethylene tubes with a total volume of 190 mL. The coils were simultaneously moved in a clockwise or counterclockwise planetary motion around the central axis. The maximum coil rotation was 1,000 rpm, and a reasonable value of 800 rpm was adopted in this study. HPLC was carried out using an Agilent-1290 system (Agilent, USA) outfitted with an Agilent Eclipse C18 column (4.6 mm × 250 mm, 5 μm). Separation was carried out using instrument of Shimadzu LC-20AP (Shimadzu, Japan) with preparative column model of Shimadzu shim-pack C18 column (20 mm × 250 mm, 15 μm). The structures of the compounds were identified on a Bruker AVANCE III 400 MHz nuclear magnetic resonance (NMR) spectrometer with tetramethylsilicane as an internal standard (Bruker Company, Switzerland).
2.3 Preparation of ethyl acetate extract sample
The heartwood of C. sappan L. (1 kg, dry weight) was extracted with 5 L of 75% (v/v) ethanol for 2 h, then cooled and filtered. The herbal residues were extracted with 5 L of 75% (v/v) ethanol for 1 h and successively extracted twice. Then, the crude extracts were combined and concentrated to dry in a vacuum rotary evaporator at a water bath temperature of 50°C to obtain the alcoholic extract (97 g). After being redissolved with water (2 L), the aqueous phase was originally extracted with an equal volume of petroleum ether three times to remove small polar parts. Moreover, the lower phase was extracted with 2 L of ethyl acetate twice and concentrated under reduced pressure using a vacuum rotary evaporator at a water bath temperature of 50°C. The combined crude sample (21 g) was stored in a desiccator and was used for the next step of column chromatography and HSCCC enrichment.
2.4 SGCC combined with prep-HPLC separation procedure
Silica gel of 140 g was packed into a glass column (4 cm × 40 cm), and the crude sample (12 g) was taken and thoroughly mixed with an equal quantity of silica gel. Afterward, the crude sample (Section 2.3) was dried and loaded onto the silica gel column with the elution system of petroleum ether and ethyl acetate (5:1–1:2, v/v). Each fraction was analyzed via TLC on silica gel GF254 plate with chloroform/acetone/formic acid (8:4:1, v/v/v) as the unfolding solvent, and the ultraviolet light at 254 nm was observed. Eventually, a total of fractions (1–4) were obtained, and fraction 4 was collected from the eluting solvent of petroleum ether and ethyl acetate (1:2, v/v).
Fraction 4 was further separated by prep-HPLC to obtain compounds 1, 2, and 4 (Figure 1). Prep-HPLC separations were performed on a Shimadzu shim-pack C18 column (20 mm × 250 mm, 15 μm) using an isocratic elution program at a flow rate of 10 mL/min. The mobile phase, a solution of acetonitrile (A)/water (B) (12:88, v/v), was monitored at 285 nm. In addition, the sample concentration and the injection volume were 200 mg/mL and 300 μL, respectively. The peak fractions were collected according to the chromatogram, and the purity of compounds was analyzed by HPLC.
2.5 HSCCC combined with prep-HPLC separation procedure
The crude sample (1 g) from ethyl acetate extraction was dissolved in the mixture of mobile phase and stationary phase (ethyl acetate/n-butanol/water = 2:1:3, v/v, 10 mL) for the HSCCC separation. The entire column was filled with the stationary phase at a flow rate of 10 mL/min, and the mobile phase was pumped at a flow rate of 2.0 mL/min. The temperature of the thermostat was set to 25°C. When the upper and lower phase systems reached equilibrium, the eluate was monitored at 214 nm. At last, the mixtures of the stationary phase were pushed out of the column by compressed air, and the upper layer of the mixed solution was collected and evaporated under reduced pressure to afford the enriched sample (0.45 g).
Subsequently, the enriched sample was also further separated by prep-HPLC to obtain compound 3. The elution gradient was as follows: 15–15% A (v/v) in 0–30 min and 15–30% A (v/v) in 30–60 min. The graphical flow chart is shown in Figure 1.
2.6 HPLC fingerprint analysis
Chromatograms were captured on an Agilent 1290 series HPLC system. The mobile phase consisted of phase A (acetonitrile) and phase B (water). A gradient program was used according to the following profile: 0–15 min, 15–15% A (v/v), 15–30 min, 15–30% A (v/v), and 30–40 min, 30–100% A (v/v) with the flow rate of 1.0 mL/min, and the detection wavelength was set at 285 nm. In addition, the injection volume was 10 μL.
3 Results and discussion
3.1 Discovery of the unstable components from C. sappan L.
The traditional usage of C. sappan L. was commonly using water extraction. During the preparation of the aqueous extract of C. sappan L., a noticeable difference in the relative content of characteristic peaks was observed in the fingerprint after decocting for different times. Five characteristic peaks were selected from the fingerprint, and four characteristic peaks were found to be unstable, as illustrated in Figure 2.
The relative content of characteristic peaks at various decoction times (20, 40, 60, 80, 120, 140, 160, and 180 min) is shown in Figure 3. The temperature of the decoction process was 120–150°C. It should be noted that the relative content was calculated as a percentage of the total chromatographic peaks from the target peaks. The results indicated that the relative content of peak 1 and peak 3 decreased from 11.05 to 4.91% (0–180 min) and 12.00 to 4.03%, respectively, while the relative content of peak 2 increased from 19.45 to 24.89%. The pH of the extracting solution was also measured at various decoction times, and its value was 5 before heating, while its value decreased to 4 after heating. These findings might indicate a potential transformation relationship between peaks 1–3 with extended heating time in acidic solutions. The obtained findings should nevertheless be helpful when C. sappan L. was used as herbal medicine, and we would like to investigate these unstable components further. As a result, isolation and identification of unstable components from C. sappan L. were essential for controlling the preparation process and ensuring the integrity of the QC assessment system of C. sappan L.
3.2 Separation of the characteristic compounds by SGCC combined with prep-HPLC
In order to separate unstable components of C. sappan L., the SGCC combined with prep-HPLC was used. The binary solvent system of petroleum ether and ethyl acetate was finally chosen by optimizing the elution condition. One column volume was eluted with petroleum ether and ethyl acetate (5:1, v/v) and a gradient of petroleum ether and ethyl acetate (3:1, v/v). The column was successively eluted by increasing the polarity of mixtures of petroleum ether/ethyl acetate, and each fraction was checked using TLC. Fractions 1–4 were obtained by SGCC. Nevertheless, spots on the TLC plate were observed when the gradient was changed to ether and ethyl acetate (1:2, v/v) and washed at 250 mL. Fractions with similar TLC patterns were combined to give fraction 4 (2.67 g) and identified by HPLC.
3.3 Separation of the unstable compounds by HSCCC
Peak 3 could not be obtained by column chromatography due to significant loss during the silica gel column separation. Therefore, we used HSCCC as a sample pretreatment methodology to improve peak 3 enrichment. In addition, some experiments were performed to optimize the biphasic solvent system for HSCCC. In the chosen solvent system, the solubility of the crude sample in the stationary and mobile phases differed significantly. The number of peaks in the two phases differed considerably, thus favoring the enrichment of peak 3. K-values that were not in the range of 0.5–2.0 were chosen, and the enrichment of peak 3 was achieved. Finally, the biphasic solvent system of ethyl acetate/n-butanol/water (2:1:3, v/v) was chosen as the elution system.
In addition, a single injection volume up to the gram level was used in this experiment, eliminating the need to repeat collection of fractions. At first, 200 mg of crude sample were dissolved in a 10 mL two-phase solvent system by HSCCC analysis for 300 min. It was found that the upper phase of the nitrogen-blown fraction under this system contained peak 3 of 16%, which was suitable for further separation by prep-HPLC. So we considered the nitrogen-blown fraction as the next step of separation sample, and 1 g was finally chosen as the sample size. The upper solution of the nitrogen-blown push-out fraction (57 mL) was collected after the crude sample was analyzed for 350 min by HSCCC, and the mixture of peak 2 and peak 3 was obtained with a relatively large proportion of both components after enrichment. From the HPLC chromatogram shown in Figure 4a and b, we knew this method effectively increased the percentage content of peak 2 from 18 to 34% and peak 3 from 7 to 15% (area normalization method). An enriched sample (0.45 g) was also obtained in a single pass. Eventually, ethyl acetate/n-butanol/water (2:1:3, v/v) was chosen as a solvent system for enriching peak 3. The stationary phase retention of ethyl acetate/n-butanol/water (2:1:3, v/v) was 30%.
3.4 Identification of the unstable compounds
Ethyl acetate extract of C. sappan L. was subjected to column chromatography and HSCCC for enrichment. And then, fraction 4 and enriched sample were further separated by prep-HPLC to obtain characteristic peaks 1–4. The results showed that four unstable compounds were collected with the purity higher than 95.0% (Figure 4c–f). The structures of the unstable compounds were identified as episapponal, brazilin, sapponal, and 4-O-methylsapponal by comparing the 1H NMR and 13C NMR data with literature values, as reflected in Figure S1.
Episappanol (1): 1 H NMR (400 MHz, DMSO-d 6): δ 6.97 (1H, d, J = 8.3 Hz, H-5′), 6.72 (1H, s, H-2′), 6.61 (1H, d, J = 8.3 Hz, H-5′), 2 6.53 (1H, d, J = 8.3 Hz, H-6′), 6.28 (1H, d, J = 8.3 Hz, H-6), 6.13 (1H, s, H-8), 3.98 (1H, s, H-4), 3.88 (1H, d, J = 10.4 Hz, Hb-2), 3.60 (1H, d, J = 10.4 Hz, Ha-2), 2.69 (1H, d, J = 13.7 Hz, Hb-9), and 2.49 (1H, d, J = 13.7 Hz, Ha-9). 13 C NMR (100 MHz, DMSO-d 6): δ 29.5 (C-9), 68.3 (C-2), 68.8 (C-3), 70.4 (C-4), 102.2 (C-8), 108.2 (C-6), 115.3 (C-5′), 116.6 (C-2′), 118.9 (C-4a), 122.1 (C-6′), 128.1 (C-5), 130.1 (C-1′), 143.8 (C-4′), 144.8 (C-3′), 154.7 (C-7), and 158.0 (C-8a). Peak 1 was identified as episappanol  based on the reported data.
Brazilin (2): 1 H NMR (400 MHz, DMSO-d 6): δ 7.14 (1H, d, J = 8.3 Hz, H-5), 6.62 (1H, s, H-5′), 6.53 (1H, s, H-2′), 6.40 (1H, d, J = 8.3 Hz, H-6), 6.20 (1H, br s, H-8), 3.84 (1H, s, H-4), 3.84 (1H, d, J = 11.3 Hz, Hb-2), 3.56 (1H, d, J = 11.3 Hz, Ha-2), 2.87 (1H, d, J = 15.6 Hz, Hb-9), and 2.69 (1H, d, J = 15.6 Hz, Ha-9). 13 C NMR (101 MHz, DMSO-d 6): δ 156.96 (C-7), 154.51 (C-8a), 144.75 (C-3′), 144.41 (C-4′), 136.01 (C-6′), 131.38 (C-5), 130.21 (C-1′), 114.79 (C-4a), 112.51 (C-2′), 112.12 (C-5′), 109.17 (C-6), 103.23 (C-8), 76.78 (C-3), 70.06 (C-2), 49.8 (C-4), and 42.40 (C-9). Peak 2 was identified as brazilin  based on the reported data.
Sappanol (3): 1 H NMR (400 MHz, DMSO-d 6): δ 10.87 (s, 1H, H-7), 10.18 (s, 1H, H-4′), 10.12 (s, 1H, OH), 8.48 (d, J = 8.3 Hz, 1H, H-5), 8.11-8.02 (m, 2H, H-6, H-8), 8.03 (s, 1H, H-2′), 7.80 (dd, J = 14.8, 2.1 Hz, 1H, H-5′), 7.80 (s, 1H, H-6′), 7.62 (d, J = 2.3 Hz, 1H, OH), 5.77 (s, 1H, H-2), 5.53 (d, J = 5.2 Hz, 1H, H-4), 5.21 (d, J = 10.5 Hz, 1H, OH), and 3.91 (s, 2H, H-9). 13 C NMR (101 MHz, DMSO-d 6): δ 158.1 (C-7), 154.2 (C-8a), 154.2 (C-8a), 143.6 (C-4′), 131.5 (C-5), 127.6 (C-1′), 121.4 (C-6′), 118.1 (C-5′), 115.7 (C-2′), 114.3 (C-4a), 108.4 (C-6), 102.0 (C-8), 69.1 (C-4), 67.8 (C-3), and 67.1 (C-2) (C-9 was hidden by solvent ). Peak 3 was identified as sappanol  based on the reported data.
4-O-Methysapponal (4): 1 H NMR (400 MHz, DMSO-d 6): δ 8.66 (s, 2H, OH, OH), 7.04 (d, J = 8.3 Hz, 1H, H-5), 6.60 (d, J = 7.5 Hz, 3H, H-6, H-8, H-2′), 6.36 (t, J = 7.7 Hz, 2H, H-5, H-6′), 6.17 (s, 1H, OH), 5.20 (s, 1H, OH), 4.29-4.08 (s, 2H, H-2), 3.76 (d, J = 10.5 Hz, 1H, H-4), 3.54-3.17 (s, 2H, H-9), and 2.53-2.47 (m, 3H, OMe). 13 C NMR (100 MHz, DMSO-d 6): δ 24.1 (C-9), 49.1 (OMe), 67.4 (C-2), 68.2 (C-3), 69.5 (C-4), 102.4 (C-8), 108.8 (C-6), 115.4 (C-5′), 116.0 (C-2′), 118.5 (C-4a), 121.7 (C-6′), 127.9 (C-5), 131.9 (C-1′), 144.1 (C-4′), 144.9 (C-3′), 154.3 (C-7), and 158.4 (C-8a). Peak 4 was identified as 4-O-methylsapponal  based on the reported data.
3.5 Interconversion between unstable components
According to the structural properties of the isolated compounds, it was determined that all four characteristic peaks were homoisoflavones. Peaks 1 and 3 belonged to the homoisoflavones(iv), peak 2 was homoisoflavones(viii), and peak 4 was homoisoflavones .
Based on the similarity of their structures, there were transformation relationships between peaks 1–4. The possible conversion pathways are shown in Figure S2, which demonstrated that episapponal (peak 1), sapponal (peak 3), and 4-O-methylsapponal (peak 4) would convert to brazilin (peak 2) . We discovered that episapponal (peak 1) and sapponal (peak 3) were unstable in acidic circumstances, according to Mueller et al. . As a result, we hypothesized that the reduction in pH after prolonged heating would enhance the conversion of these two compounds to brazilin. Our findings perhaps explained the phenomenon observed in Section 3.1. Furthermore, due to the structural properties of the chemical components involved, the heating time in the preparation of C. sappan L. should be controlled within 1 h. The analysis of unstable components by HPLC fingerprint demonstrated that content was not the only indicator for assessing the preparation process. Furthermore, the pharmacological actions of TCM were thought to result from the synergistic effects of multiple ingredients rather than a single component. And the characteristic compounds obtained in isolation were the material basis of the pharmacological effects of C. sappan L., among which episappanol and sappanol have been shown to have anti-inflammatory effects . 4-O-Methylsappanol has been reported to possess multiple biological activities, such as antifungal activity , intense melanin inhibitory activity , and neuroprotective effects . Brazilin also performed abundant pharmacological activities, including anti-inflammatory  and anti-bacterial . In summary, these characteristic peaks made different contributions to the pharmacological activities of C. sappan L. Based on the above discussions, an appropriate analytical method was developed. The HPLC fingerprint of C. sappan L. is shown in Figure S3, and five characteristic peaks were identified for HPLC fingerprint of C. sappan L.
During the optimization of the decoction time of C. sappan L., four peaks were discovered to be unstable. Therefore, four characteristic peaks were separated and identified as episapponal, brazilin, sapponal, and 4-O-methylsapponal by NMR. Episapponal, brazilin, and 4-O-methylsapponal were obtained from column chromatography combined with prep-HPLC, and sappanol was obtained from HSCCC combined with prep-HPLC. Moreover, episapponal, sappanol, and 4-O-methylsapponal might transform to brazilin after prolonged heating. These results were consistent with the phenomenon mentioned above. These findings provided a useful theoretical foundation for improving the quality of C. sappan L., and specifically for targeting control of production processes of C. sappan L.
Funding information: This research was supported by the Basic Public Welfare Research Project of Zhejiang Province (Grant No. LBY22H280003) and Zhejiang Provincial Department of Science and Technology Project (Grant No. 2022C03062).
Author contributions: Yameng Wu – conceptualization, methodology, writing – original draft; Jianhui Xie – investigation, methodology; Jielin Zeng – data curation, validation; Rui Bai – data curation, supervision; Hui Zhang – writing – review and editing; and Jizhong Yan – writing – review and editing.
Conflict of interest: The authors declare no conflicts of interest.
Ethical approval: This conducted research is not related to either human or animal use.
Data availability statement: This work contains all of the data that were collected or analyzed throughout the research.
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