Corresponding author: Ngoc-Van Thi Nguyen ( ntnvan@ctump.edu.vn ) Academic editor: Paraskev Nedialkov
© 2021 Kim-Ngan Huynh Nguyen, Ngoc-Van Thi Nguyen, Kyeong Ho Kim.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Nguyen K-NH, Nguyen N-VT, Kim KH (2021) Determination of phenolic acids and flavonoids in leaves, calyces, and fruits of Physalis angulata L. in Viet Nam. Pharmacia 68(2): 501-509. https://doi.org/10.3897/pharmacia.68.e66044
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In Vietnam, Physalis angulata L. is a wild species growing throughout the country that is often used in traditional medicine. The aim of study was to quantify seven major compounds, including phenolic acids (chlorogenic acid, caffeic acid, p-coumaric acid) and flavonoids (rutin, quercitrin, quercetin, and kaempferol) in three aerial parts of P. angulata. Chromatographic separation was carried out on a Kromasil C18 column (150 mm × 4.6 mm i.d., 5 µm) with a gradient elution of 0.1% formic acid in acetonitrile, 0.2% ammonium acetate/0.1% formic acid in water and methanol at a flow rate of 1.0 mL/min; detection was at 250 and 300 nm. The limits of detection and quantification were in the ranges of 0.1–0.3 and 0.3–1.0 µg/mL, respectively. The validated method was successfully applied to analyze active compounds in P. angulata and may be a useful tool for quality control of this plant.
rutin, HPLC-DAD, Physalis angulata, phenolic acid, ultrasound-assisted extraction
Physalis angulata L. (P. angulata) is an edible and annual plant belonging to the Solanaceae family, which is mainly distributed in tropical and subtropical regions including Asia, Central and South America, Africa, and the Pacific Islands. In Vietnam, P. angulata is called “thu lu,” a wild species growing throughout the country that is often used in traditional medicine due to its ethnopharmacological value. Moreover, this medicinal plant is commonly used in folk medicine worldwide to treat chronic rheumatism as well as kidney, bladder, liver and skin diseases (Renjifo et al. 2013). Recently, P. angulata has been reported to exhibit a broad spectrum of pharmacological activities including anti-inflammatory, antibacterial, molluscicidal, antiprotozoal, anticancer, cytotoxic, and immunomodulatory effects (
Phenolic compounds are secondary metabolites that are synthesized and accumulated in all plant parts. These compounds play important roles in plants, such as defense against ultraviolet radiation and resistance against pathogens (Ali and Neda 2011; Samar Al Jitan 2018). In addition, several previous studies reported that phenolic compounds are the major bioactive phytochemicals because of their remarkable pharmacological activities (Radek et al. 2011;
Common phenolic acid reference standards (chlorogenic – 98.2%, caffeic – 97.5%, and p-coumaric – 99.5%) and flavonoids reference standards (rutin – 98.5%, quercitrin – 99.5%, quercetin – 98.2%, and kaempferol – 99.3%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Methanol, ethanol (≥99.8%), acetonitrile, n-hexane, and HPLC-grade water were acquired from Honeywell (North Carolina, USA). Ammonium acetate and formic acid were purchased from Merck (Darmstadt, Germany).
Fresh P. angulata were collected in the Kien Giang, Ca Mau, and Dong Thap provinces, Mekong delta, Vietnam, in January 2020. The plants were identified at the Department of Biology, Can Tho University (scientific name as Physalis angulata L.). The leaf, calyx, and fruit of fresh samples were collected separately, dried at room temperature, pulverized, and analyzed individually. All samples were stored in black glass containers and kept at room temperature. Dried samples were collected from traditional drugstores located in these provinces. Following this, the leaf, calyx, and fruit of dried samples were separated, pulverized, and stored in a similar manner to those of fresh samples.
Individual standard stock solutions of chlorogenic acid and rutin (2000 μg/mL), caffeic acid, p-coumaric acid, quercitrin, quercetin, and kaempferol (1000 μg/mL) in methanol were prepared, and were stable for approximately 12 months. Working standard solutions were prepared daily by diluting the stock solutions with methanol to provide different concentrations. The standard stock and working solutions were protected from light and stored at 4 °C.
The powdered leaves, fruits, and calyces (0.50 g) were accurately weighed, and extracted with 10 mL of methanol:water (70:30, v/v) in an ultrasonic bath three times for 15 min each. The extracted solutions were combined in a 50 mL volumetric flask and diluted with methanol. For the cleanup procedure, 4 mL of the extract was transferred to a 15 mL centrifuge tube and dried under a nitrogen stream at 40 °C. The residue was reconstituted with 0.4 mL of acetonitrile:water (50:50, v/v) and 0.4 mL of methanol:water (50:50, v/v), and partitioned with n-hexane to eliminate non-polar matrix interferences. The lower layer was filtered through a 0.22 mm PTFE filter and transferred to a vial.
Several methods have been previously used for the extraction of phenolic acids and flavonoids from P. angulata, including microwave-assisted extraction (MAE), maceration, subcritical fluid extraction (SFE), and UAE (
The UAE procedure was optimized with regard to the solvent (ethanol, methanol, methanol – water), solid–liquid ratio (1:5, 1:10, 1:20, 1:30, and 1: 40 g/mL), extraction time (5, 10, 15, 20, and 30 min), and sonication time (once, twice, three, four, and five times) under investigation. Phenolic acid and flavonoid peaks were identified based on the comparison of their retention time (RT) and UV spectrum obtained by using a DAD with those of the reference compounds. i.e., chlorogenic acid, caffeic acid, p-coumaric acid, rutin, quercitrin, quercetin, and kaempferol, as well as the UV spectroscopy theory for phenolic acids and flavonoids developed by
Method development, quantification, and validation studies were performed using an Agilent LC 1100 series (Agilent Technologies, Mississauga, ON, Canada) equipped with a G-1310A pump, G-1316A column thermostat, G-1313A autosampler, and G1315B diode array detector.
Chromatographic separation was performed on a Kromasil C18 column (150 mm × 4.6 mm I.D, 5 µm). The mobile phase consisted of 0.1% formic acid in acetonitrile (A), methanol (B), and 0.2% ammonium acetate/0.1% formic acid in water (C). The gradient elution program is shown in Table
The proposed method was validated for selectivity, linearity, limit of detection (LOD), limit of quantification (LOQ), precision, and accuracy according to the Association of Official Analytical Chemists (AOAC) and ICH guidelines.
The results of the optimization of the sample preparation procedure are shown in Fig.
A higher volume of extraction solvent can dissolve analytes more effectively. Thus, the liquid-to-solid ratio is also an important factor during extraction. To evaluate the effect of this factor on the extraction yields, we examined different ratios ranging from 1:5 to 1:40 g/mL. It was found that the phenolic acid and flavonoid contents increased with increase in the solid–liquid ratio from 1:5 to 1:20 g/mL before stabilizing at ratios of 1:30 and 1:40 g/mL (Fig.
As shown in Figure
To analyze phenolic acids and flavonoids, acidic modifiers are usually added to the mobile phase to minimize the ionization of polyphenolic compounds and silanols. Moreover, the addition reduces the interaction between analytes and ionized silanols on the stationary phase, which might result in peak tailing (
HPLC chromatogram of the extraction solvent (a), mixed standards solution (b), P. angulata leaves (c), calyces (d) and fruits (e). All peaks were identified based on the UV spectra theory of phenolic acids and flavonoids developed by
The system stability was tested by carrying out six replicate injections of a mixed standard solution (10 µg/mL) and determining the theoretical plate number (N), resolution (Rs), symmetry factor (As), and repeatability [relative standard deviation (RSD) of RT and area] of the analytes. The %RSD values of the peak area and RT of all analytes were less than 2.0%. Therefore, the proposed method met this requirement.
The selectivity was tested by employing the HPLC method to analyze the extracts of the leaf, calyx, and fruit parts of P. angulata. It was evaluated by comparing the RT and UV absorption spectrum of each component in standard solutions with those of the peaks obtained by analyzing the extracts. As shown in Figure
The stock solutions were diluted and mixed to seven different concentrations ranging from 5.0 to 500 µg/mL of chlorogenic acid and rutin, and from 2.0 to 80 µg/mL of caffeic acid, p-coumaric acid, quercitrin, quercetin, and kaempferol. To evaluate the linearity, each mixed standard sample was injected in triplicates into the HPLC system, and calibration curves were obtained by plotting the average of the peak area responses versus concentration for each sample. The results of the regression equations and square correlation coefficients (r2) are summarized in Table
Recovery (%) | Precision (n = 6) | Calibration curve (n = 3) | ||||||
---|---|---|---|---|---|---|---|---|
Low-level | Mid-level | High-level | Intra-day RSD (%) | Inter-day RSD (%) | Slope (±SD) | y-intercept (±SD) | r2 (±SD) | |
Chlorogenic acid | 105.3 | 100.5 | 101.6 | 1.16 | 0.88 | 10.82 ± 0.14 | 19.76 ± 9.25 | 0.9990 ± 0.0002 |
Caffeic acid | 101.5 | 101.2 | 103.5 | 1.59 | 1.44 | 13.62 ± 0.36 | -9.880 ± 2.04 | 0.9997 ± 0.0001 |
p-Coumaric acid | 101.8 | 99.8 | 102.5 | 1.38 | 1.97 | 57.36 ± 0.74 | -27.15 ± 8.49 | 0.9997 ± 0.0002 |
Rutin | 105.6 | 100.6 | 101.7 | 0.80 | 0.77 | 12.32 ± 0.17 | 23.10 ± 12.33 | 0.9990 ± 0.0002 |
Quercitrin | 103.3 | 101.5 | 103.9 | 2.20 | 2.76 | 20.55 ± 0.69 | -14.14 ± 5.36 | 0.9997 ± 0.0002 |
Quercetin | 102.0 | 101.4 | 102.8 | 1.22 | 3.09 | 33.27 ± 0.73 | -39.85 ± 4.10 | 0.9995 ± 0.0001 |
Kaempferol | 102.2 | 100.9 | 103.6 | 2.58 | 4.03 | 24.72 ± 0.81 | 24.37 ± 17.40 | 0.9991 ± 0.0010 |
The precision of the method was verified by evaluating the intra-day and inter-day precisions. The relative standard deviation (%RSD) was selected as a measure of precision. The intra-day precision was examined by analyzing six samples from each matrix (leaf, calyx, and fruit) in a single day, while the inter-day precision was determined by analyzing six samples each day for three days. The precision results for the leaf matrix, as shown in Table
The accuracy of the method was investigated by performing recovery studies. Three different concentrations, including low, medium, and high amounts of reference compounds, were added to the blank leaf samples. Then, the spiked samples were extracted and quantified according to the methods mentioned above. The results indicated that the developed method exhibited good accuracy, with an overall recovery ranging from 101.35 to 102.87%, while the RSD% was less than 2.38% for all the analytes (Table
The proposed method was applied to the evaluation of the content of phenolic acids and flavonoids in P. angulata leaves, calyces, and fruits. Data in Table
Content of phenolic acids and flavonoids in leaf, calyx and fruit of fresh P. angulata samples.
No. | Location | Part | Phenolic acids (mg/kg dry weight) | Flavonoid (mg/kg dry weight) | |||||
---|---|---|---|---|---|---|---|---|---|
Chlorogenic | Caffeic | p-Coumaric | Total | Rutin | Quercetin | Total | |||
1 | Rach Gia city, Kien Giang provine | Leaf | 4545 ± 131.3 | 832.6 ± 0.876 | 32.96 ± 1.403 | 5855 | 5038 ± 87.68 | 66.10 ± 0.747 | 6920 |
Calyx | 1464 ± 11.27 | 182.2 ± 4.226 | 38.74 ± 0.218 | 1775 | 87.74 ± 4.041 | ND | 87.74 | ||
Fruit | 1462 ± 40.03 | 134.0 ± 3.436 | ND* | 1908 | ND | ND | ND | ||
2 | Chau Thanh district, Kien Giang province | Leaf | 3201 ± 16.08 | 458.2 ± 1.383 | 25.34 ± 0.480 | 4222 | 1812 ± 12.81 | 40.12 ± 0.597 | 2724 |
Calyx | 4971 ± 71.53 | 577.6 ± 0.211 | ND | 5706 | 165.8 ± 6.337 | ND | 165.8 | ||
Fruit | 808.3 ± 4.018 | 286.9 ± 1.360 | 45.98 ± 0.064 | 1455 | ND | ND | ND | ||
3 | Ca Mau province | Leaf | 188.1 ± 9.115 | ND | ND | 546.3 | 636.6 ± 10.12 | ND | 923.1 |
Calyx | 249.1 ± 10.25 | ND | 39.22 ± 2.102 | 824.1 | 43.23 ± 1.579 | ND | 43.23 | ||
Fruit | 80.52 ± 2.342 | ND | 18.80 ± 0.926 | 485.3 | ND | ND | ND | ||
4 | Dong Thap province | Leaf | 647.1 ± 4.256 | ND | 39.33 ± 0.045 | 727.0 | 622.6 ± 2.341 | ND | 701.9 |
The data illustrated in Table
Flavonoids varied between plant parts, similar to phenolic acids. In fresh P. angulata samples, leaves showed the highest content of total flavonoids (701.9–6920 mg/kg dry weight), while the samples collected in Rach Gia city, Kien Giang province, exhibited the highest total flavonoid content (6920 mg/kg dry weight). This value was two times higher than that reported for the leaves of P. peruviana (3238.5 mg/kg dry weight) by
The dried P. angulata samples showed a significant decrease (3–10 fold) of total phenolic acid and flavonoid contents when compared to the fresh samples collected in the same province (Table
Content of phenolic acids and flavonoids in leaf, calyx and fruit of dried P. angulata samples.
No. | Location | Part | Phenolic acids (mg/kg dry weight) | Flavonoid (mg/kg dry weight) | |||||
---|---|---|---|---|---|---|---|---|---|
Chlorogenic | Caffeic | p-Coumaric | Total | Rutin | Quercetin | Total | |||
1 | Rach Gia city, Kien Giang provine | Leaf | 1390 ± 53.32 | 407.2 ± 7.829 | 21.06 ± 0.457 | 2739 | 1697 ± 5.339 | 43.53 ± 0.082 | 2452 |
Calyx | 386.8 ± 24.74 | 194.9 ± 9.085 | ND | 581.7 | ND | ND | ND | ||
Fruit | ND | 68.76 ± 2.120 | 26.21 ± 0.420 | 246.2 | ND | ND | ND | ||
2 | Ca Mau province | Leaf | ND | ND | ND | 82.81 | 327.2 ± 11.31 | ND | 542.4 |
Calyx | ND | ND | ND | ND | ND | ND | ND | ||
Fruit | ND | ND | ND | ND | ND | ND | ND | ||
3 | Dong Thap province | Leaf | 19.32 ± 0.501 | ND | ND | 19.32 | 136.4 ± 10.78 | ND | 136.4 |
Calyx | 19.01 ± 0.383 | ND | ND | 19.01 | ND | ND | ND | ||
Fruit | ND | ND | 26.26 ± 0.106 | 26.26 | ND | ND | ND |
In this study, an ultrasound-assisted extraction and RP-HPLC method combined with diode array detection was developed for determining antioxidant active compounds in the leaves, calyces, and fruits of P. angulata. The optimum extraction procedure allowed for good efficiency and extraction yields (> 99%). In addition, the HPLC protocol permitted a qualitative separation of the compounds and proved to be efficient, precise, and accurate. Therefore, it could be used for the simultaneous determination of phenolic acids and flavonoids present in the three aerial parts of P. angulata. The experimental results of fresh samples collected from three provinces of the Mekong delta indicated that both leaves and calyces were important sources of phenolic acids, whereas leaves were the best source of flavonoids. According to these results, chlorogenic acid and rutin were the major active compounds in P. angulata. In addition, the data suggested that the composition was significantly influenced by different plant parts and collection locations. In addition, a considerable decrease in phenolic compounds was observed in the dried samples, which may be ascribed to the uncontrolled drying and storage processes.
The authors would like to express their hearty gratitude to Can Tho University of Medicine and Pharmacy. We also thank all of our colleagues for their excellent assistance.