Phytochemical Constituents Isolated From Distichochlamys citrea M.F. Newman Rhizomes and Their Antioxidant and α -glucosidase Inhibitory Activities

Abstract

Objective/Background: Distichochlamys citrea M.F. Newman (Zingiberaceae) is an endemic plant of Vietnam and has been used as a folk medication for treatments of infection-related diseases, diabetes, and used as spice. This study aimed to characterize the antioxidant and α-glucosidase inhibitory activities of isolated compounds from D. citrea rhizomes. Methods: Compounds have been isolated from D. citrea rhizomes by different chromatographic techniques. Their chemical structures were determined based on mass spectrometry and nuclear magnetic resonance spectroscopy in comparison with those reported in the literature. The antioxidant effect was evaluated by 2,2-diphenyl-1-picrylhydrazyl-free radical neutralization whereas α-glucosidase inhibitory activity was determined by using the method of optical density measurement. Results: Seven compounds including 5-hydroxy-3,7,4'-trimethoxyflavone (1), kaempferol (2), platyphyllone (3), 5-O-caffeoylquinic acid (4), a mixture of phytosterols (β-sitosterol (5) and stigmasterol (6)), and glycerol monostearate (7) were isolated and identified from D. citrea rhizomes. Compounds 1 and 2 simultaneously exhibited antioxidant activities (IC₅₀ = 794.71 ± 2.68 µM, 14.08 ± 5.60 µM, respectively) and α-glucosidase inhibitory effects (IC₅₀ = 189.40 ± 1.55 µM, and 31.86 ± 5.38 µM, respectively). Meanwhile, compound 3 and the mixture of 5 and 6 singly inhibited the enzyme α-glucosidase (IC₅₀ = 69.41 ± 6.11 µM, 1204.82 µM, respectively), and showed weak antioxidant effects. On the contrary, compound 4 only had antioxidant activity (IC₅₀ = 30.20 ± 1.86 µM). Compound 7 did not exhibit either antioxidant or enzymatic inhibitory effects. Conclusion: This is the first time that all of the seven compounds (1-7) and their biological activities were reported from D. citrea. Moreover, the α-glucosidase inhibitory effect of platyphyllone (3) is reported for the first time in the literature. This study revealed that flavonoids, phenolics, sterols, and monoglycerides in D. citrea rhizomes could be responsible for the antioxidant property and α-glucosidase inhibitory effect of this Vietnamese endemic medicinal plant.

Keywords

Zingiberaceae DPPH endemic plant phenolics flavonoids

Introduction

Oxidative stress is included in the most potential triggers of metabolic diseases and diabetes mellitus (DM) progression.^1,2 Increased and irregular synthesis of reactive oxygen species (ROS) and reactive nitrogen species explicitly results in oxidative stress, overloading the instinct antioxidant systems of the body and possibly exacerbating the conditions of DM.^1,2 Furthermore, hyperglycemia is a pathogenically indicating condition in patients with DM, which originates from insufficient secretion of insulin or insulin resistance of target tissues.³ Long-term excessive level of glucose in the bloodstream leads to multimetabolomic disorders of carbohydrates, protides, and lipids, which directly damage essential organs.³ Taking part in the glucose intake process, the enzyme α-glucosidase cleaves subjected di- and oligosaccharides to produce glucose, initiating the absorption of glucose through the gut lumen.⁴ Hence, the search for natural α-glucosidase inhibitors and ROS neutralizers became already a hotspot in DM drug research, as suppression of α-glucosidase activity and neutralization of ROS could potentially relieve severe exacerbation of DM. As part of our ongoing research for candidates of DM drug from native medicinal plants in Central Vietnam, a phytochemical study accompanied with screening for biological activities on Distichochlamys citrea M.F. Newman rhizomes was carried out.⁵ The plant D. citrea is native to the central region of Vietnam, where it was discovered in Thua Thien Hue, Quang Binh, Quang Tri, Quang Nam, and Nghe An Provinces.^6–8 Up to now, the investigations of chemical profiles and the biological activities of D. citrea have been limited. In addition, recent studies on this plant are scarcely available despite raising scientific interest in the genus Distichochlamys, except for reports on compositions and biological activities of the essential oils such as anti-inflammation and anticancer,^9,10 and our previous study on the corresponding subject which reported that the ethyl acetate (EtOAc) fraction apparently exhibited both α-glucosidase inhibitory effect (IC₅₀ = 115.75 µg/mL) and antioxidant potential via 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging capacity (IC₅₀ = 90.27 µg/mL).⁵ This study was carried out to isolate and determine the corresponding bioactivities of phyto-constituents from potential fractions of D. citrea. The present study reports the isolation, α-glucosidase inhibitory activities, and DPPH scavenging capacities of 7 compounds from D. citrea rhizomes.

Results and Discussion

Determination of Isolated Compounds

Bioactivity-guided isolation of the extract of D. citrea rhizomes led to the identification of 7 known compounds comprising 2 flavonoids, 2 phenolics, 2 phytosterols, and 1 monoglyceride. Their chemical structures were elucidated based on a comparison of their spectroscopic data (ESI-mass spectrometry [MS], HR-ESI-MS, 1D- and 2D-nuclear magnetic resonance [NMR]) with those of the literature. These compounds were 5-hydroxy-37,4′-trimethoxyflavone (1),¹¹ kaempferol (2),¹² platyphyllone (3),¹³ 5-O-caffeoylquinic acid (4),¹⁴ a mixture of phytosterols (β-sitosterol (5) and stigmasterol (6)),^15,16 and glycerol monostearate (7)¹⁷ (Figure 1). To the best of our knowledge, all of these compounds were found in D. citrea rhizomes for the first time.

Figure 1.

Chemical structures of seven compounds (1-7) isolated from Distichochlamys citrea rhizomes.

Antioxidant Activity Assay

The evaluation of the antioxidant activity of seven compounds showed that compound 2 (IC₅₀ = 14.08 ± 5.60 μM, R² = 0.9808) had the strongest antioxidant effect and was stronger than ascorbic acid (ca. 2 times, IC₅₀ = 30.33 ± 8.63 μM, R² = 0.9938). Meanwhile, compound 4 (IC₅₀ = 30.20 ± 1.86 μM, R² = 0.9952) had an antioxidant effect comparable to ascorbic acid. In addition, compounds 1 (IC₅₀ = 794.71 ± 2.68 μM, R² = 0.9931), 3, and 5/6-7 exhibited weaker activity than ascorbic acid. At a concentration of 500 μg/mL, compounds 3, 5/6, and 7 had antioxidant activities of 41.89 ± 0.47%, 28.96 ± 0.74%, and 25.69 ± 1.47%, respectively (Table S1). The above results suggested that D. citrea is a potential candidate to look for antioxidant agents.

α-Glucosidase Inhibitory Assay

As shown in Table S2, the isolated compounds all showed α-glucosidase inhibitory effects. Compound 2 (IC₅₀ = 31.86 ± 5.38 μM, R² = 0.9970) exhibited the strongest α-glucosidase inhibitory activity in all compounds, two-fold stronger than that of compounds 3 (IC₅₀ = 69.41 ± 6.11 μM, R² = 0.9962) and 6-fold stronger than that of compound 1 (IC₅₀ = 189.40 ± 1.55 μM, R² = 0.9894). To the best of our knowledge, compound 3 is the first time to be reported for α-glucosidase inhibitory activity in the literature. Additionally, compounds 1 to 3 showed better activity than acarbose (IC₅₀ = 818.59 ± 2.91 μM, R² = 0.9962). At a concentration of 500 μg/mL (eq., 1204.82 μM), compounds 5/6 showed α-glucosidase inhibitory effect with 66.30 ± 1.89%. Meanwhile, both the compounds 4 and 7 showed very weak α-glucosidase inhibitory activity at 400 μg/mL concentration, regarding as of 3.34 ± 0.36% and 4.76 ± 0.19%, respectively. This result demonstrated the ability to inhibit α-glucosidase of D. citrea rhizomes, a precious medicinal plant endemic to Vietnam, contributing to the source of materials in the development of products with hypoglycemic effects.

In previous studies, 5-hydroxy-37,4′-trimethoxyflavone has been reported to have antioxidant, α-glucosidase inhibitor,^18,19 antibacterial,²⁰ and anti-inflammatory properties.²¹ The present results show that the antioxidant capacity of compound 1 is very weak compared to ascorbic acid, which is consistent with the report of Teffo et al²² This can be explained through the structure of compound 1 because the hydroxyl groups at positions C-3, C-7, and C-4ʹ have been methoxidized leading to a decrease in the antioxidant effect of compound 1.^23,24 Besides, the α-glucosidase inhibitory effect of compound 1 was better than that of acarbose. The present study results show that compound 1 has the ability to inhibit α-glucosidase, which is consistent with previous reports.^18,19

The antioxidant activity as well as the free radical scavenging capacity of kaempferol (2) is due to the high reactivity of the hydroxyl substituents.^23,24 Compound 2 acts as a potent antioxidant by reducing oxidative stress²² and this compound has antitumor effects,²⁵ which is currently under review as a cancer treatment.²⁵ Furthermore, compound 2 has many other biological effects such as antibacterial, antiviral, anti-inflammatory, anti-allergic, and antidiabetic, which have been reported in previous studies.^26–29 The structure and activity relationships (SARs) between essential moieties in the flavonoid skeletons have been established through experimental-based meta-analysis and computer-aided tools. The number of free, nonalkylated, and nonacylated aromatic hydroxyl groups directly affects the capacity to neutralize free radical species, in a proportional manner, which is contributable to the antioxidant effect. Meanwhile, the reduction or substitution of the olefinic group Δ²⁽³⁾ or ketone group C-4 diminishes both the antioxidant and α-glucosidase activities. When it comes to our results, compounds 1 and 2 resemble in structure, but compound 1 is a trimethyl ether of compound 2 in which three in four hydroxyl groups of compound 2 are alkylated. Both flavonoids 1 and 2 have their olefinic and ketone groups preserved, but three in four hydroxyl groups of compound 1 are methoxylated compared to compound 2^23,24; therefore, the antioxidant effect of compound 1 is 56-fold weaker than that of compound 2; also, the α-glucosidase capacity of compound 1 is 6-fold weaker than that of compound 2. This result has an agreement with previous establishment of SARs between the number of aromatic hydroxyl groups and the antioxidant property, the α-glucosidase inhibitory effect of flavonoids.

Diarylheptanoids are found mainly in several species of the genera Alpinia, Zingiber, Curcuma, Alnus, Amomum, and Myrica.^13,30–32 One of those diarylheptanoids, platyphyllone (3) exhibits antioxidant, anti-inflammatory, cytotoxic, antitumor, and anti-influenza virus activities that have been reported in previous studies.^13,33–35 Using basic search tools, platyphyllone was isolated for the first time from the genus Distichochlamys. Additionally, the ability to inhibit α-glucosidase in vitro was also reported for the first time for this compound.

In previously reported studies, 5-O-caffeoylquinic acid (4) is one of the chlorogenic acids found in plant species covering several different families, including Zingiberaceae (e.g., Etlingera elatior (Jack.) RM Smith, E. fulgens (Ridl.) CK Lim,³⁶ E. rubrostriata (Holttum) CK Lim, E. philippinensis (Ridl.) RM Smith, Zingiber officinale Rosc., Curcuma longa L., Hornstedtia conoidea Ridl., Amomum muricarpum Elmer³⁷), Asteraceae (e.g., Artemisia argentea L'Hér., Andryala glandulosa Lam., Argyranthemum pinnatifidum (L.f.) Webb, Calendula maderensis DC.),³⁸ Rubiaceae (e.g., Coffea arabica L., C. robusta L.Linden),³⁹ etc. 5-O-caffeoylquinic acid is widely recognized for its antioxidant, DNA-protective activity,⁴⁰ and a wide range of other biological effects such as antibacterial, antiviral, antifungal, antitumor, antimicrobial, anti-inflammation, antidegeneration, and enhanced neuroprotective and cardioprotective activities.^41,42 There is sufficient evidence to demonstrate that 5-O-caffeoylquinic acid acts as a potent antioxidant to inhibit the formation of ROS and plays a beneficial role in the prevention of diseases associated with related oxidation and ageing. It is surprising that the present study shows that this compound has a very weak, almost no effect on the role of α-glucosidase inhibition. Although the results of α-glucosidase inhibition of the present study are consistent with the previous report of Chen et al¹⁴ and Anh et al⁴³ In previous reports, the antidiabetic role of 5-O-caffeoylquinic acid may be attributed to other mechanisms, such as the antioxidant and anti-inflammatory activities of 5-O-caffeoylquinic can ameliorate endothelial dysfunction and reduce insulin resistance, which may be an important mechanism for the enhanced cardioprotective and antidiabetic effects of this compound.^42,44 In addition, 5-O-caffeoylquinic acid has been implicated in improved glucose and lipid metabolism by AMP-activated protein kinase.⁴⁵

β-Sitosterol (5) and stigmasterol (6) are sterols widely distributed in the plant kingdom. Previous studies have reported β-sitosterol and stigmasterol to provide significant health benefits. They have the effect of lowering cholesterol, reducing the risk of coronary heart disease, heart attack, and atherosclerosis, preventing many types of cancer, supporting the body's natural recovery process, supporting the treatment of obesity, diabetes, and other pharmacological effects (e.g., antibacterial and anti-inflammatory).^46–48

Glycerol monostearate (7), also known as Monostearin, is a primary metabolite found in a number of plants such as Clinacanthus nutans (Burm.f) Lindau,⁴⁹ Cleome chelidonii Lf.,⁵⁰ Dichapetalum filicaule Breteler,¹⁷ Saururus chinensis (Lour.) Hort. ex Loud.,⁵¹ Sargaassum sagamianum Yendo.⁵² In the report of Chama et al, it was demonstrated that glycerol monostearate isolated from Dichapetalum filicaule has the effect of killing hookworms with IC₅₀ = 306.0 μg/mL.¹⁷ In addition, glycerol monostearate isolated from Saururus chinensis and Sargaassum sagamianum also has anti-atherosclerotic and anti-inflammatory effects.^51,52 In the present study, it was shown that the antioxidant and α-glucosidase inhibitory activities of glycerol monostearate were very weak. However, Murugesu et al reported that glycerol monostearate has activity against the in silico enzyme α-glucosidase.⁴⁹ Glycerol monostearate was isolated for the first time from the rhizome of D. citrea and evidence for the in vitro antioxidant and α-glucosidase inhibitory capacity of glycerol monostearate was also reported for the first time in this study.

In conclusion, D. citrea has antioxidant and α-glucosidase inhibitory effects, providing scientific evidence for the use of D. citrea by Pako people in particular and in folk in general to treat inflammation and diabetes. These effects may be due to the presence of compounds such as kaempferol, 5-hydroxy-3,7,4'-trimethoxyflavone, platyphyllone, 5-O-caffeoylquinic acid, a mixture of β-sitosterol and stigmasterol, and glycerol monostearate. Furthermore, these compounds were isolated for the first time from the rhizome of D. citrea. The results suggest that D. citrea is a good source of antidiabetic, antioxidant, and anti-inflammatory agents, but further research is needed to elucidate their mechanism of action.

Materials and Methods

General Experimental Procedures

See Supplemental Material for further information. Plant Materials

The rhizomes of D. citrea M.F. Newman were collected in Bach Ma National Park, Thua Thien Hue Province, Vietnam in October 2019 and identified by Assoc. Prof. Truong Thi Bich Phuong (Department of Biology, University of Sciences, Hue University). A voucher specimen (DC-10.19) has been stored at the Laboratory of the Traditional Medicine, University of Medicine and Pharmacy at Ho Chi Minh City, Vietnam.^5,53

Extraction and Isolation

See Supplemental Material for further information. 5-hydroxy-3,7,4'-trimethoxyflavone (1): Yellow needle crystal.

HR-ESI-MS m/z: 329.1029 [M + H]⁺ (calcd. for C₁₈H₁₇O₆⁺, 329.1020).

¹H NMR (δ_H, DMSO-d₆, 600 MHz): 12.63 (1H, s, 5-OH), 8.06 (2H, d, J = 9.0 Hz, H-2′/H-6′), 7.14 (2H, d, J = 9.0 Hz, H-3′/H-5′), 6.76 (1H, d, J = 1.8 Hz, H-8), 6.38 (1H, d, J = 2.4 Hz, H-6), 3.86 (6H, s, 7-OCH₃/4′-OCH₃), and 3.81 (3H, s, 3-OCH₃); ¹³C NMR (δ_C, DMSO-d₆, 150 MHz): 178.1 (C-4), 165.2 (C-7), 161.4 (C-4′), 160.9 (C-5), 156.3 (C-9), 155.5 (C-2), 138.1 (C-3), 130.0 (C-2′/C-6′), 122.1 (C-1′), 114.2 (C-3′/C-5′), 105.2 (C-10), 97.8 (C-6), 92.4 (C-8), 59.7 (3-OCH₃), 56.1 (7-OCH₃), and 55.4 (4′-OCH₃).

Kaempferol (2): Yellow powder.

ESI-MS m/z: 284.8 [M-H]^–.

¹H NMR (δ_H, DMSO-d₆, 600 MHz): 12.46 (1H, s, 5-OH), 10.79 (1H, s, 7-OH), 10.10 (1H, s, 3-OH), 9.35 (1H, s, 4′-OH), 8.04 (2H, d, J = 9.0 Hz, H-2′/H-6′), 6.92 (2H, d, J = 9.0 Hz, H-3′/H-5′), 6.44 (1H, d, J = 1.8 Hz, H-8), and 6.19 (1H, d, J = 1.8 Hz, H-6); ¹³C NMR (δ_C, DMSO-d₆, 150 MHz): 175.9 (C-4), 163.9 (C-7), 160.7 (C-9), 159.2 (C-4′), 156.2 (C-5), 146.8 (C-2), 135.6 (C-3), 129.5 (C-2′/C-6′), 121.7 (C-1′), 115.4 (C-3′/C-5′), 103.1 (C-10), 98.2 (C-6), and 93.5 (C-8).

Platyphyllone (3): Light yellow powder.

ESI-MS m/z: 315.0 [M + H]⁺.

¹H NMR (δ_H, CD₃OD, 600 MHz): 7.01 (4H, d, J = 8.4 Hz, H-2′/H-6′/H-2′′/H-6′′), 6.71 (2H, d, J = 8.4 Hz, H-3′′/H-5′′), 6.70 (2H, d, J = 8.4 Hz, H-3′/H-5′), 4.04-4.00 (1H, m, H-5), 2.78-2.73 (4H, m, H-1/H-2), 2.67-2.64 (1H, m, H_a-7), 2.62-2.53 (2H, m, H-4), 2.56-2.52 (1H, m, H_b-7), and 1.70-1.66 (2H, m, H-6); ¹³C NMR (δ_C, CD₃OD, 150 MHz): 211.9 (C-3), 156.6 (C-4′′), 156.4 (C-4′), 134.1 (C-1′′), 133.3 (C-1′), 130.3 (C-2′/C-6′/C-2′′/C-6′′), 116.2 (C-3′′/C-5′′), 116.1 (C-3′/C-5′), 68.3 (C-5), 51.3 (C-4), 46.4 (C-2), 40.5 (C-6), 31.9 (C-7), and 29.8 (C-1).

5-O-caffeoylquinic acid (4): White powder.

ESI-MS m/z: 191.8 [M-caffeoyl-H] ^–.

¹H NMR (δ_H, CD₃OD, 600 MHz): 7.58 (1H, d, J = 16.0 Hz, H-7′), 7.08 (1H, d, J = 1.8 Hz, H-2′), 6.98 (1H, dd, J = 7.8, 1.8 Hz, H-6′), 6.80 (1H, d, J = 7.8 Hz, H-5′), 6.28 (1H, d, J = 16.0 Hz, H-8′), 5.35 (1H, td, J = 9.0, 4.2 Hz, H-5), 4.20-4.17 (1H, m, H-3), 3.75 (1H, dd, J = 8.4, 3.0 Hz, H-4), 2.26-2.23 (1H, m, H_a-6), 2.21-2.18 (1H, m, H_a-2), 2.12-2.09 (1H, m, H_b-6), 2.08-2.05 (1H, m, H_b-2); ¹³C NMR (δ_C, CD₃OD, 150 MHz): 177.0 (C-7), 168.7 (C-9′), 149.6 (C-4′), 147.1 (C-7′), 146.8 (C-3′), 127.8 (C-1′), 123.0 (C-6′), 116.5 (C-5′), 115.2 (C-2′), 115.3 (C-8′), 76.1 (C-1), 73.5 (C-4), 72.0 (C-5), 71.3 (C-3), 38.8 (C-6), 38.2 (C-2).

A mixture of β-sitosterol (5) and stigmasterol (6): white amorphous powder.

ESI-MS, ¹H NMR (δ_H, CDCl₃, 600 MHz), ¹³C NMR (δ_C, CDCl₃, 150 MHz) data shown in Figures S29 to S32 (Supplemental Material).

Glycerol monostearate (7): White crystal.

ESI-MS m/z: 356.8 [M-H]^– ; 338.6 [M-H₂O-H]^– ; 320.6 [M-2H₂O-H]^–.

¹H NMR (δ_H, CDCl₃, 600 MHz): 4.21 (1H, dd, J = 11.4, 4.8 Hz, H_a-1′), 4.15 (1H, dd, J = 11.4, 6.0 Hz, H_b-1′), 3.93 (1H, t, J = 5.0 Hz, H-2′), 3.69 (1H, dd, J = 11.4, 5.0 Hz, H_a-3′), 3.60 (1H, dd, J = 11.4, 5.0 Hz, H_b-3′), 2.35 (2H, t, J = 7.5 Hz, H-2), 1.64-1.60 (2H, m, H-3), 1.38-1.20 (28H, brm, H-4 to H-17, overlapped), 0.88 (3H, t, J = 7.2 Hz, H-18); ¹³C NMR (δ_C, CDCl₃, 150 MHz): 174.4 (C-1), 70.3 (C-2′), 65.2 (C-1′), 63.4 (C-3′), 34.2 (C-2), 31.9-29.1 and 22.7 (C-4 to C-17, overlapped), 24.9 (C-3), and 14.1 (C-18).

Antioxidant Activity Assay

See Supplemental Material for further information. α-Glucosidase Inhibitory Assay

See Supplemental Material for further information. Conclusions

This study provides information on the isolated chemical compounds from D. citrea rhizomes and their biological activities. Specifically, seven known compounds, 5-hydroxy-3,7,4'-trimethoxyflavone (1), kaempferol (2), platyphyllone (3), 5-O-caffeoylquinic acid (4), β-sitosterol (5), stigmasterol (6), and glycerol monostearate (7), were firstly isolated and tested for antioxidant activity (compounds 2 > 4 > 1 > 3 > 5/6 > 7 with IC₅₀ values ranging from 14.08 ± 5.60 μM to 794.71 ± 2.68 μM) and α-glucosidase inhibitory effect (compounds 2 > 3 > 1 > 5/6 > 7 > 4 with IC₅₀ values ranging from 31.86 ± 5.38 μM to 189.40 ± 1.55 μM) from D. citrea rhizomes. Specifically, this is the first report on the α-glucosidase inhibition of platyphyllone (3). These results may be used for standardization of medicinal herbs as well as contribute the information to carrying on further studies on this D. citrea.

Supplemental Material

sj-docx-1-npx-10.1177_1934578X231177878 - Supplemental material for Phytochemical Constituents Isolated From Distichochlamys citrea M.F. Newman Rhizomes and Their Antioxidant and α-glucosidase Inhibitory Activities

Supplemental material, sj-docx-1-npx-10.1177_1934578X231177878 for Phytochemical Constituents Isolated From Distichochlamys citrea M.F. Newman Rhizomes and Their Antioxidant and α-glucosidase Inhibitory Activities by Tran Van Chen, Cao Ly Tan Thong, Hieu Tran-Trung and Nguyen Thi Ai Nhung, Nguyen Thanh Triet in Natural Product Communications

Footnotes

Acknowledgments

The authors would like to thank the partial facility support of the Faculty of Traditional Medicine, University of Medicine and Pharmacy at Ho Chi Minh City, and ChenPharm's Lab for this study.

Author Contribution

Research idea was provided by NT Triet and TV Chen. Isolation and in vitro experiments were carried out by TV Chen and CLT Thong. Strutural elucidation and writing were done by TV Chen, TT Hieu and CLT Thong. Reading and revising the manuscript were done by NT Triet, TV Chen, TT Hieu and NTAN.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Tran Van Chen

Cao Ly Tan Thong

Hieu Tran-Trung

Nguyen Thi Ai Nhung

Nguyen Thanh Triet

Supplemental Material

Supplemental material for this article is available online.

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