Study on the chemical constituents and antioxidant activity of Hibiscus sabdariffa L. Calyx

Vietnam Journal of Science and Technology 58 (6A) (2020) 174-180 doi:10.15625/2525-2518/58/6A/15511 STUDY ON THE CHEMICAL CONSTITUENTS AND ANTIOXIDANT ACTIVITY OF HIBISCUS SABDARIFFA L. CALYX Tran Thu Huong 1, * , Ha Manh Tuan 2 , Le Huyen Tram 1 , Nguyen Van Thong 1 , Nguyen Hoang Minh 1 , Tran Thi Minh 1 , Dinh Thi Thu Hien 1 , Tran Thuong Quang 1 , Le Thi Thuy 1 , Nguyen Tuan Anh 1 1 School of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet, Ha Noi, Viet Nam 2 College of Pharmacy, Daegu Catholic University, Gyeongbuk 38430, Republic of Korea * Email: huong.tranthu@hust.edu.vn Received: 16 September 2020; Accepted for publication: 6 Dêcmber 2020 Abstract. Hibiscus sabdariffa L. is an herbaceous medicinal plant, believed to be native to Western Africa. The calyx is commercially important in the food industry and various parts of the sorrel plant have been used in folk medicine for the prevention of diseases such as cardiovascular disease and hypertension. However, to date, the studies on the chemical composition as well as the biological activity of H. sabdariffa are still limited. In this study, four flavonoids, including tiliroside, quercetin-3-O-(6''-O-(E)-p-coumaryl-β-D-glucopyranoside, isoquercetin, and quercetin were isolated from the calyx of H. sabdariffa collected in Bac Giang province, Viet Nam. In addition, the methanol extract of the H. sabdariffa calyx showed the ability to scavenge DPPH free radicals with SC50 values ranging from 103.61-119.16 µg/ml, in comparison with that of the positive control, ascorbic acid (SC50 = 10.75 µg/ml). Keywords: Hibiscus sabdariffa, Malvaceae, flavonoids, antioxidants, DPPH. Classification numbers: 1.1.1, 1.1.6, 1.2.1. 1. INTRODUCTION Hibiscus sabdariffa L. is an annual, herbaceous medicinal plant that belongs to the Malvaceae family. This plant is believed to be native to Western Africa, cultivated and harvested widely in tropical and subtropical countries such as India, Saudi Arabia, China, Malaysia, Indonesia, Philippines, Viet Nam, Thailand, Egypt, Sudan, and Mexico [1, 2]. The H. sabdariffa calyx is commercially important in the food industry for the production of beverages and foods such as tea, juices, jams, jellies, and syrup. In addition, various parts of the sorrel plant have been used in folk medicine for the prevention of diseases such as cardiovascular disease and hypertension [1]. However, to date, the studies on the chemical composition as well as the biological activity of H. sabdariffa are still limited. A few previous studies have reported on the main constituent of H. sabdariffa including organic acid, anthocyanins, polysacccharides, and Study on the chemical constituents and antioxidant activity of Hibiscus sabdariffa L. calyx 175 flavonoids [1]. In Viet Nam, this plant widely distributed from the northern midland and mountainous provinces such as Hoa Binh; central regions such as Thanh Hoa, Nghe An, and Lam Dong plateau to the southern provinces such as Kien Giang, Can Tho [2]. The calyx of H sabdariffa , when harvested, has an attractive red color, characteristic cool sour taste and high anthocyanin content [2]. In this paper, we describe the isolation of 4 flavonoids including tiliroside, quercetin-3-O-(6''-O-(E)-p-coumaryl-β-D-glucopyranoside, isoquercetin, and quercetin from the calyx of H. sabdariffa L. collected in Bac Giang province, Viet Nam. In addition, the methanol extract from calyx of Hibiscus sabdariffa L. showed the ability to scavenge DPPH free radicals with SC50 values ranging from 103.61 - 119.16 µg/ml, in comparison with that of the positive control, ascorbic acid (SC50 = 10.75 µg/ml). 2. MATERIALS AND METHODS 2.1. General experiment procedures The NMR spectra were recorded using a Bruker AV-400 spectrometer (Bruker Corporation, Switzerland) using TMS as the internal standard. Electron spray ionization-mass spectrometry (ESI-MS) was performed using an LC-MSD-Trap-SL Agilent 1100. RP-18 F 254s and Merck precoated silica gel F 254 plates were used to perform thin-layer chromatography (TLC). Compounds were visualized after spraying with aqueous 10 % H2SO4, and heating for 3 - 5 min. 2.2. Plant material Calyx of H. sabdariffa L. was collected in Bac Giang province, Viet Nam in May 2018 (sample 1, HB18-1) and November 2018 (sample 2, HB18-2), and authenticated by Assoc. Prof. Tran Huy Thai, Institute of Ecology and Biological Resources, Vietnam Academy of Science and Technology. The voucher specimens (HB18-1 and HB18-2) were deposited at the Department of Bioactive Products, Institute of Marine Biochemistry, Vietnam Academy of Science and Technology. 2.3. DPPH radical scavenging assay The DPPH radical scavenging assay was conducted using the previously described protocol [3]. This is the standard method carried out at the Institute of Biotechnology, Vietnam Academy of Science and Technology. Briefly, the extracts were dissolved in 100 % dimethyl sulfoxide (DMSO) to obtain a stock solution (500 mg/mL) for antioxidant assay. DPPH radical solution was prepared for 0.25 μM with 100 % ethanol. The extracts were prepared by two times dilution method in 96-well microtiter plates. An aliquot of extract (100 μL) was mixed to 100 μL of ethanolic DPPH in 96-well microtiter plates. The reaction mixtures were incubated at room temperature for 30 min in the dark. Absorbance was measured at 517 nm by Microplate Reader. The free radical scavenging activity (SA) was calculated as follows: % SA = [(ODcontrol – ODsample)/ODcontrol] X 100 where: ODcontrol was the absorbance of without samples and ODsample was the absorbance of the test sample. The values are expressed as the means of triplicate analyses. The SC50 values were determined using the software TableCurve2Dv4. 2.4. Extraction and isolation Tran Thu Huong, et al. 176 The air-dried powdered calyx (3 kg) of H. sabdariffa L. was extracted with methanol (5L x 3 times) at 40 - 50 o C and then filtered. The MeOH solution was concentrated under reduced pressure to give crude (40 g). The crude was suspended in distilled water (H2O) and successively partitioned with CH2Cl2 and EtOAc to give CH2Cl2 (2 g) and EtOAc (12 g) fraction, respectively. The EtOAc fraction (12 g) was subjected to column chromatography (CC) over silica gel and eluted with CH2Cl2/acetone gradient from 50:1 to 1:100. Ten fractions F1-F10 were successively obtained. The fraction F6 (1.6 g) was separated on silica gel CC eluted with CH2Cl2/MeOH (30/1, v/v) to give seven sub-fractions (F6.1-F6.7). Compounds HS2 (2 mg) and HS1 (3.6 mg) were obtained from the fraction F6.4 (120 mg) by using reversed-phase silica gel (RP-18) column chromatography eluting with MeOH/H2O (1/1). The fraction F7 (782 mg) was chromatographed on silica gel CC and eluted with CH2Cl2/acetone (15/1) to give 5 subfractions F7.1-F7.5. The fraction F7.3 (241 mg) was chromatographed on a reversed-phase silica gel (RP-18) column eluted with a solvent system of MeOH/H2O (1.5/1) to give HS4 (5.7 mg) and HS3 (2 mg). 2.5. Physical and spectroscopic data of isolated compounds from H. sabdariffa 2.5.1. Tiliroside (HS1): Yellow amorphous powder; ESI-MS: m/z 595.15 [M+H] + ; 1H NMR (400 MHz, CD3OD)  (ppm): δH 8.00 (2H, d, J = 8.0 Hz, H-2', 6'); 7.42 (1H, d, J =15.6 Hz, H-7'''); 7.32 (2H, d, J = 7.6 Hz, H-2''', 6'''); 6.83 (2H, d, J = 8.0 Hz, H-3', 5'); 6.80 (2H, d, J = 7.6 Hz, H- 3''', 5'''); 6.31 (1H, s, H-8); 6.13 (1H, s, H-6); 5.25 (1H, d, J = 6.8 Hz, H-1''); 6.09 (1H, d, J =15.6, H-8'''); 4.31 (1H, dd, J = 12.0; 2.0 Hz, H-6''a); 4.21 (1H, dd, J = 12.0; 6.4 Hz, H-6''b); 3.48 (1H, m, H-2''); 3.46 (1H, m, H-3''); 3.35 (1H, m, H-4''); 3.49 (1H, m, H-5''). 13 C NMR (100 MHz, CD3OD)  (ppm): (Table 1) 2.5.2. Quercetin-3-O-(6''-O-(E)-p-coumaryl-β-D-glucopyranoside (HS2): Yellow amorphous powder; ESI-MS: m/z 611.14 [M+H] + ; 1 H NMR (400 MHz, CD3OD)  (ppm): δH 7.56 (1H, d, J = 2.0 Hz, H-2'); 7.53 (1H, dd, J = 8.0; 2.0 Hz, H-6'); 7.42 (1H, d, J =15.6, H-7'''); 7.38 (2H, d, J = 8.0 Hz, H-2''', 6'''); 7.30 (1H, d, J = 8.0 Hz, H-5'); 6.78 (2H, d, J = 7.6 Hz, H-3''', 5'''); 6.28 (1H, s, H-8); 6.18 (1H, d, J =15.6, H-8'''); 6.10 (1H, s, H-6); 5.25 (1H, d, J = 6.8 Hz, H-1''); 4.29 (1H, m, H-6''a); 4.18 (1H, m, H-6''b); 3.39-3.50 (4H, m, H-2'', 3'', 4'', 5''). 13 C NMR (100 MHz, CD3OD)  (ppm): (Table 1). 2.5.3. Isoquercetin (HS3): Yellow amorphous powder; ESI-MS: m/z 465.10 [M+H] + ; 1 H NMR (400 MHz, CD3OD)  (ppm): δH 7.68 (1H, d, J = 2.0 Hz, H-2'); 7.57 (1H, dd, J = 8.4; 2.0 Hz, H- 6'); 6.85 (1H, d, J = 8.4 Hz, H-5'); 6.37 (1H, s, H-8); 6.18 (1H, s, H-6); 5.25 (1H, d, J = 7.2 Hz, H-1''); 3.70 (1H, dd, J = 12.0; 2.0 Hz, H-6''a); 3.56 (1H, dd, J = 12.0; 5.2 Hz, H-6''b); 3.45 (1H, m, H-2''); 3.43 (1H, m, H-3''); 3.32 (1H, m, H-4''); 3.37 (1H, m, H-5''). 13 C NMR (100 MHz, CD3OD)  (ppm): (Table 1). 2.5.4. Quercetin (HS4): Yellow amorphous powder; ESI-MS: m/z 303.05 [M+H] + ; 1H NMR (400 MHz, CD3OD)  (ppm): δH 7.72 (1H, d, J = 2.0 Hz, H-2'); 7.63 (1H, dd, J = 8.0; 2.0 Hz, H- 6'); 6.88 (1H, d, J = 8.4 Hz, H-5'); 6.37 (1H, s, H-8); 6.16 (1H, s, H-6). 13 C NMR (100 MHz, CD3OD)  (ppm): (Table 1). Study on the chemical constituents and antioxidant activity of Hibiscus sabdariffa L. calyx 177 Table 1. 13 C NMR spectral data of compounds HS1-HS4. No. HS1 HS2 HS3 HS4 No. HS1 HS2 HS3 δC δC δC δC δC δC δC 2 159.4 158.3 158.6 146.1 1'' 104.0 103.7 104.3 3 135.3 135.1 135.7 137.3 2'' 75.8 75.8 75.8 4 179.5 179.4 179.6 177.3 3'' 78.1 78.0 78.1 4a 105.7 105.6 105.8 104.5 4'' 71.8 71.7 71.3 5 163.1 163.2 163.2 162.4 5'' 75.9 75.6 78.5 6 100.1 99.9 99.9 99.2 6'' 64.3 64.2 62.6 7 166.0 165.8 166.1 165.6 1''' 127.1 127.1 8 94.9 94.6 94.8 94.4 2''' 131.3 131.2 8a 158.5 159.1 159.1 158.2 3''' 116.9 116.7 1' 122.8 123.0 123.3 124.1 4''' 161.1 160.1 2' 132.3 115.8 116.1 116.0 5''' 116.9 116.7 3' 116.1 146.5 146.0 147.9 6''' 131.3 131.2 4' 161.6 149.7 149.9 148.7 7''' 146.6 145.9 5' 116.1 117.2 117.6 116.2 8''' 114.8 114.7 6' 132.3 123.4 123.2 121.6 9''' 168.9 168.8 3. RESULT AND DISCUSSION 3.1. Structural elucidation of compounds HS1-HS4 Compound HS1 was obtained as yellow amorphous powder. The molecular formula of C30H26O13 was determined by the electrospray ionization mass spectrometry (ESI-MS) with a protonated molecular ion peak at m/z 595.15 [M+H]+ (calcd for C30H27O13, 595.15). The 1 H and 13 C NMR of HS1 exhibited characteristic signals of kaempferol glucoside including: a para- substituted benzene ring at δH 8.00 (2H, d, J = 8.0 Hz, H-2', 6') and 6.83 (2H, d, J = 8.0 Hz, H-3', 5'); a meta-substituted benzene ring at δH 6.31 (1H, s, H-8) and 6.13 (1H, s, H-6); one anomeric proton at δH 5.25 (1H, d, J = 6.8 Hz, H-1'') and one carbonyl carbon at δC 179.5 (C-4). The β- configuration of the D-glucose moiety was deduced from the large coupling constant (J = 6.8 Hz) of anomeric proton (H-1''). In addition, the 1 H and 13 C NMR spectra of HS1 displayed characteristic signals of (E)-p-coumaroyl group at δH 7.42 (1H, d, J =15.6 Hz, H-7'''), 6.09 (1H, d, J =15.6 Hz, H-8'''), 7.32 (2H, d, J = 7.6 Hz, H-2''', 6'''), and 6.80 (2H, d, J = 7.6 Hz, H-3''', 5''') and δC 127.1 (C-1'''), 131.3 (C-2''', 6'''), 116.9 (C-3''', 5'''), 161.1 (C-4'''), 146.6 (C-7'''), 114.8 (C- 8'''), and 168.9 (C-9''') [4]. The 13 C NMR and HMQC spectra of HS1 exhibited 30 carbon signals including two carbonyl carbons, one methylene carbon, 12 sp 2 methines, 5 sp 3 methines, and 10 quaternary carbons. The spectroscopic data suggested that HS1 is a flavonoid (kaempferol glucoside derivative) attached by an (E)-p-coumaroyl moiety. This suggestion was confirmed by Tran Thu Huong, et al. 178 the HMBC spectrum. Specifically, the HMBCcorrelation from anomeric proton H-1'' (δH 5.25) to C-3 (δC 135.3) (Figure 2) indicated that the β- D-glucose moiety was attached at position 3 of the kaempferol skeleton. The HMBC interactions of oxygenated methylene protons at δH 4.31 (H-6''a) and 4.21 (H-6''b) and δH 7.42 (H-7''') with carbonyl carbon at δC 168.9 (C-9''') indicated that (E)-p-coumaroyl group linked to hydroxyl group at position 6'' of the sugar moiety. The above spectroscopic data of HS1 completely resembled those of kaempferol-3-O-(6''-O-(E)- p-coumaryl-β-D-glucopyranoside) (tiliroside) reported in the literature [4]. Thus, the chemical structure of HS1 (tiliroside) was determined as shown in Figure 1. Figure 1. Chemical structures of HS1-HS4 and key HMBC correlations of HS1. Compound HS2 was obtained as a yellow amorphous powder. The molecular formula of C30H26O14 was determined by the ESI-MS with a protonated molecular ion peak at m/z 611.14 [M+H] + (calcd for C30H27O14, 611.14). The 1 H and 13 C NMR spectra of HS2 were similar to those of HS1 by the signals a meta-substituted benzene ring at δH 6.28 (1H, s, H-8) and 6.10 (1H, s, H-6); one anomeric proton at δH 5.25 (1H, d, J = 6.8 Hz, H-1''); one carbonyl carbon at δC 179.4 (C-4); and typical signals of (E)-p-coumaroyl {δH 7.42 (1H, d, J =15.6 Hz, H-7'''), 6.18 (1H, d, J =15.6 Hz, H-8'''), 7.38 (2H, d, J = 8.0 Hz, H-2''', 6'''), and 6.78 (2H, d, J = 8.0 Hz, H-3''', 5''') and δC 127.1 (C-1'''), 131.2 (C-2''', 6'''), 116,7 (C-3''', 5'''), 160.1 (C-4'''), 145.9 (C-7'''), 114.7 (C-8'''), and 168.8 (C-9'''). By the same analytical method as applied to HS1, the sugar moiety in HS2 was also identified as β-D-glucose by the large coupling constant (J = 6.8 Hz) of the anomeric proton (H-1''). The only difference between the 1 H spectra of HS1 and HS2 is that the AABB coupling system in B-ring of HS1 is replaced by the ABX coupling system [δH 7.56 (1H, d, J = 2.0 Hz, H-2'), 7.53 (1H, dd, J = 8.0, 2.0 Hz, H-6'), and 7.30 (1H, d, J = 8.0 Hz, H-5')] in HS2. This observation suggested that the aglycone part in HS2 is quercetin instead of kaempferol as in HS1. The 13 C NMR spectrum of HS2 displayed 30 carbon signals including nine (E)-p-coumaroyl carbons, six glucose carbons, and fifteen carbons belonging to aglycone moiety (quercetin) (Table 1). The above spectroscopic data completely resembled those of quercetin-3-O-(6''-O-(E)-p-coumaryl-β-D-glucopyranoside published in the literature [5]. Thus, the chemical structure of HS2 was determined as shown in Figure 1. Compound HS3 was isolated as a yellow, amorphous powder. The molecular formula of Study on the chemical constituents and antioxidant activity of Hibiscus sabdariffa L. calyx 179 C21H20O12 was determined by the ESI-MS with a protonated molecular ion peak at m/z 465.10 [M+H] + (calcd for C21H21O12, 465.10). Similar to HS2, the 1 H NMR spectrum of HS3 exhibited signals of an AX coupling system at δH 6.37 (1H, s, H-8) and 6.18 (1H, s, H-6); an ABX coupling system at δH 7.68 (1H, d, J = 2.0 Hz, H-2'), 7.57 (1H, dd, J = 8.4, 2.0 Hz, H-6'), and 6.85 (1H, d, J = 8.4 Hz, H-5'); and one anomeric proton at δH 5.25 (1H, d, J = 7.2 Hz, H-1''). However, the 1 H and 13 C NMR of HS3 did not show any signals of the coumaroyl group. The 13 C NMR spectrum of HS3 displayed the signals of 21 carbons including six glucose carbons, fifteen carbons belonging to aglycone moiety (quercetin), one typical signal of carbonyl carbon at δC 179.6 (C-4), six oxygenated carbons at δC 166.1, 163.2, 159.1, 158.6, 149.9, and 146.0 (Table 1). Based on the above analysis, and compared to the literature [6], HS3 is confirmed to be quercetin-3-O-β-D-glucopyranoside (isoquercetin). Compound HS4 was obtained as a yellow, amorphous powder. The molecular formula of C15H10O7 was determined by the ESI-MS with a protonated molecular ion peak at m/z 303.05 [M+H] + (calcd for C15H11O7, 303.05). Similar to HS3, the 1 H NMR spectrum of HS4 exhibited signals of an AX coupling system at δH 6.37 (1H, s, H-8) and 6.16 (1H, s, H-6); an ABX coupling system at 7.72 (1H, d, J =2.0 Hz, H-2'), 7.63 (1H, dd, J =8.0, 2.0 Hz, H-6'), and 6.88 (1H, d, J = 8.0 Hz, H-5'); and one carbonyl carbon at δC 177.3 (C-4). However, the 1 H and 13 C NMR of HS4 did not show any signals of the glucose moiety. The 13 C NMR of HS4 showed 15 carbon signals including one carbonyl carbon at δC 177.3 (C-4), five sp 2 methine carbons, and nine quarternary carbons (Table 1). The above spectral data completely resembled those of quercetin published in the literature [7]. Thus, the structure of HS4 (quercetin) was identified as shown in Figure 1. 3.2. DPPH radical scavenging activity of the methanol extract of H. sabdariffa L. Table 2. DPPH free radical scavenging activity the H. sabdariffa methanol extract. DPPH free radical scavenging percent (%) Concentration (µg/ml) MeOH extract (sample 1) MeOH extract (sample 2) Concentration (µg/ml) Ascorbic acid (vit C) 200 79.73 ± 5.98 83.07 ± 3.59 100 87.24 ± 3.69 100 43.10 ± 2.86 50.61 ± 4.02 50 86.47 ± 2.55 50 21.10 ± 1.77 24.89 ± 1.14 25 82.75 ± 1.06 25 7.63 ± 0.65 10.01 ± 1.03 12.5 56.45 ± 2.71 12.5 3.21 ± 0.11 3.85 ± 0.27 6.25 29.06 ± 1.48 SC50 (µg/ml) 119.16 ± 6.27 103.61 ± 5.97 SC50 (µg/ml) 10.75 ± 1.75 The DPPH free radical scavenging activity of the methanol extracts (samples 1 and 2) of H. sabdariffa is presented in Table 2. The results showed that both the MeOH extract samples showed DPPH free radical scavenging activity with SC50 (Scavenging Concentration at 50 %) values ranging from 103.61 - 119.16 µg/ml. Ascorbic acid (SC50 = 10.75µg/ml) was used as a positive control. The values are exact with r 2 > 0.99. Tran Thu Huong, et al. 180 4. CONCLUSION A phytochemical investigation of the methanol extract of calyx of H. sabdariffa led to the isolation of 4 flavonoids, including tiliroside, quercetin-3-O-(6''-O-(E)-p-coumaryl-β-D- glucopyranoside, isoquercetin, and quercetin. The chemical structures were identified based on the comprehensive analysis of the spectroscopic data and in the comparisons with the literature. In addition, the methanol extract of the H. sabdariffa calyx showed the ability to scavenge DPPH free radicals with SC50 values ranging from 103.61-119.16 µg/ml, in comparison with that of the positive control, ascorbic acid (SC50 = 10.75 µg/ml). Acknowledgements. This work was supported by a grant from National Foundation for Science and Technology Development (No. 07/2018/TN). Author contributions: Tran Thu Huong: funding acquisition, supervision, and review of the manuscript. Ha Manh Tuan: structural determination of compounds and writing the original manuscript. Tran Thu Huong, Le Huyen Tram, Nguyen Van Thong, Nguyen Hoang Minh, Tran Thi Minh, Dinh Thi Thu Hien, Tran Thuong Quang, and Le Thi Thuy: contributed to collect the sample, isolate compounds, and process NMR spectral data. Nguyen Tuan Anh: contributed to the review of the manuscript. All authors have read and agreed to the published version of the manuscript. Conflict statements: The authors declare that they have no conflict of interest. REFERENCES 1. Da-Costa-Rocha I., Bonnlaender B., Sievers H., Pischel I., Heinrich M. - Hibiscus sabdariffa L. - a phytochemical and pharmacological review, Food Chemistry 165 (2014) 424-443. https://doi.org/10.1016/j.foodchem.2014.05.002. 2. Hien N. T., Thuy N. T.T., Loan N.T. - Study on Extraction and Separation of Anthocyanin from Hibiscus Sabdariffa Calyx. Application to Produce the Rapid Indicator Paper of Detecting the Borax in Food, Vietnam Journal of Science and Development 10 (5) (2012) 738-746. 3. Cuc N. T., Tram N. C. T., Phuong D. T., Phuong V. T. T., Nga N. T., Truan G., Thao D. T. - Study on t