Stability of Chlorogenic Acid from Artemisiae Scopariae Herba Enhanced by Natural Deep Eutectic Solvents as Green and Biodegradable Extraction Media.

A green and inexpensive natural deep eutectic solvent (NADES) was screened and integrated with an ultrasonic technique for extracting chlorogenic acid (CGA) from artemisiae scopariae herba. Response surface methodology was employed to investigate significant factors and optimize their influence. Proline-malic acid exhibited an excellent extraction capacity with a yield of 28.23 mg/g under the optimal conditions of water content of 15% (wt), solid-liquid ratio of 1.0/10 (g/mL), ultrasonic power of 300 W, and extraction time of 25 min. Simultaneously, the stability and antioxidant activity analysis exhibited a better performance of CGA in NADES than that in water and ethanol. The hydrogen-bonding interaction between CGA and natural deep eutectic molecules enhanced the stability and meanwhile protected the antioxidant activity of CGA.

Introduction

Artemisiae scopariae herba (ASH), the dried sprout of Artemisia scoparia Waldst. et Kit. or Artemisia capillaries Thunb,[1] originated in China as a seasonal vegetable and is considered an edible and medicinal source.[2] ASH is rich in chlorogenic acid (CGA), flavonoids, triterpenoids, polyphenols, and some other bioactive substances.[3] In Chinese medicine, ASH exhibits outstanding antioxidants,[4] has antibacterial, antiviral, and antitumor properties, provides liver protection, and is antidiabetic, antiseptic, and anti-inflammatory.[5] ASH turns into a specific advantage wild plant resource in several provinces of China and also has good utilization prospects based on the folk medicine, herbal beverage, and folk cosmetic application experience.[3]

Among the various plant metabolites, CGA is an important bioactive polyphenol, which combines with quininic acid and trans cinnamic acid through ester bonding. CGA is the major bioactive component in ASH extract[6] and exhibits potential applications in cosmetics,[7] the food industry, and pharmacology[8] with the characteristics of antioxidant, antimicrobial, anti-inflammatory, and liver cell protection.[3]

Owing to the large demand of CGA, many preparation methods were explored, including new extraction media inventions[9] and efficient equipment assistance,[10,11] which were employed to improve yields. Nevertheless, the traditional approaches of extracting CGA mainly adopted water and conventional organic solvents,[12] and various intensification processes assisted by continuous heating, boiling, or reflux[13] were still mainstream in the industry. The lack of biocompatibility of conventional solvents was always the focus of environmental concern.[14] Additionally, the phenolic hydroxyl group, unsaturated double bond, and ester bond structure in CGA ensured better antioxidant activity and other functional activities,[15] and also these functional groups made CGA easily hydrolyzed and oxidized under normal or high temperatures, which is disadvantageous for the stability of CGA. Although the research studies of acquiring CGA have made certain progress until now,[16] further research is still needed to increase the quality and stability of CGA and to make the operation more economical and simpler.

Natural deep eutectic solvents (NADESs) have been proposed as effective media[17] and explored for extracting bioactive substances with the targets of reducing toxic waste[18] and improving the selectivity[19] and extraction efficiency.[20,21] Meanwhile, the composition of NADESs may appear in every single organism, coexisting with metabolically active substances in cells.[22] This stable coexistence relationship makes NADESs excellent process solvents for gentle extraction[23,24] (lower temperature and shorter time) to protect and enhance the stability of active ingredients at the same time.

Herein, this work proposed NADESs integrated with an ultrasound technique for extracting CGA from ASH. NADESs were designed through cheap, simple, and natural compounds[18] for the efficient and biodegradable scheme. Meanwhile, ultrasonic assistance and NADES assistance were employed as a gentle pathway for obtaining CGA to ensure better stability. The response surface method was applied to acquire the optimal extraction conditions. Finally, the antioxidant capacity and stability of CGA extracted by NADESs were compared to traditional solvents (water and ethanol). This study is of great importance for the utilization and development of CGA and ASH, which are extensively applied in the food, cosmetic, and pharmacological industries.

Materials and Methods

Materials

Standard CGA was purchased from J&K Scientific Ltd. (Beijing). The biomaterials used in the NADES preparation for central composite design (CCD) extraction were obtained from Beijing MYM Biological Technology Co., Ltd. (malic acid and l-proline). Chromatographically pure acetonitrile, butylated hydroxyl toluene (BHT), and phosphate acid were purchased from Yonghua Chemical Technology (Jiangsu) Co., Ltd. Vitamin C (Vc) standard was obtained from Guangdong Guangxian Technology Co., Ltd. Macroporous resin NKA-9 was purchased from Yuanye Biotechnology Co., Ltd. Ethanol (95%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Deionized water was prepared in the laboratory.

ASH Material

The raw material (ASH) used was a tender seedling of the dry ground part of artemisia capillaries thumb collected in spring, which was produced in Xinjiang, China. ASH was pretreated via a process of picking (removing weeds, dead branches, stone impurities, etc.), crushing, and removing dust to obtain granular samples with a length of 2–4 mm. The formed materials were stored in a desiccator for further use.

NADES Preparation

The NADES synthesis route refers to previously described methods.[25] According to previous research studies, NADESs were divided into five groups on the basis of components that were obtained from nature:[26] sugar-based mixtures with an acid, a base, or amino acid, ionic types composed of a base or an acid, and those with neutral compounds. As a classification of NADESs, the selected and designed solvents are shown in Table . All raw materials were dried in a vacuum drying oven at 60 °C for 12 h. The mixtures containing two or three components were continuously stirred and heated at 90 °C, until stable, transparent, and uniform liquids formed. The solvents were stored in capped plastic bottles at room temperature for use.

Table 1

Composition Design of NADESs

no.	abbreviations	composition A	composition B	composition C	molar ratio
N1	MCH	malic acid	ChCl		1:1
N2	LGH	lactic acid	glucose		5:1
N3	PMH	proline	malic acid		1:1
N4	FGSH	fructose	glucose	sucrose	1:1:1
N5	SoCH	sorbitol	ChCl		1:3
N6	GCH	glucose	ChCl		1:1

NADES-Based Ultrasound-Assisted Extraction of CGA

The extraction of CGA from ASH was assisted by ultrasound to reduce the process time. 10 mL of NADES was injected into a beaker with 1 g of ASH particles for ultrasonic extraction in an ultrasonic cell crusher. Solid particles of the mixture were removed through a filter. The turbid extracts obtained after filtering were packed in tubes and were centrifuged for 15 min at 1500 rpm; the supernatant was collected for a HPLC test. Water content, solid–liquid ratio, ultrasonic power, and ultrasonic time were designed for optimizing the extraction efficiency. The parallel experiments were repeated three times on average.

Experimental Design and Statistical Analysis

Single-Factor Screening

Many variables will affect the phytochemical contents and the extraction yield obviously according to previous research studies.[27] Therefore, the extraction temperature, extraction time, solid–liquid ratio, water content, ultrasonic power, and ultrasonic time were considered as the initial design variables. Because of the influence of the rapid action of ultrasound assistance, the extraction temperature showed a weak effect on the extraction process in the pre-experimental stage. Finally, the water content (X1), solid–liquid ratio (X2), ultrasonic power (X3), and ultrasonic time (X4) were selected for the response surface design.

Experimental Design and Statistical Analysis

Based on single-factor screening experiments, proline–malic acid (PMH) was selected as the extraction solvent for optimization. A three-level-four-factor design of the CCD was carried out in this extraction experiment.[28] The experiments performed include 6 at the central point, 8 at the axial point, and 16 at the factorial point, which consist of 30 experimental runs for the optimization. Coded level parameters of each variable were designed and are presented in Table .

Table 2

Design of Coded and Actual Values’ Distribution for CCD

	levels
factors	–2	–1	0	1	2
water content (%, wt)	5	15	25	35	45
solid–liquid ratio (g/mL)	0.4:10	0.6:10	0.8:10	1.0:10	1.2:10
ultrasonic power (W)	150	200	250	300	350
extraction time (min)	10	15	20	25	30

The experimental data were fitted into a second-order polynomial equation, which includes all items to analyze the response surfacewhere Y is the response, k is the number of variables (k = 4), intercept β0 is the offset term, β is the linear effect, β is the squared effect, β is the interaction effect, and X and X are independent variables.

HPLC Analysis

The analysis of extracts was performed resorting to high-performance liquid chromatography equipment Agilent 1260 (Germany) with a UV–vis detector. The column of Agilent Eclipse Plus C18 (USA) was used, with 4.6 mm × 250 mm and 5 μm particle sizes. The column oven was run at 40 °C. The mobile phase solution was composed of 10% acetonitrile and water acidified with 0.4% phosphoric acid, irrigating the system with 0.8 mL·min–1 for 2 h and stabilizing with 0.5 mL·min–1 for 40 min. A UV–vis detector was used at 327 nm, the injection volume was 0.8 μL, and the flow was 0.5 mL·min–1. The identification of CGA was handled by comparing with standard data (retention time and absorption spectrum of the peaks) detected under the same condition. The quantification was gained by a linear regression equation of CGA standards by transferring different volumes (0.1, 1, 3, 5, and 7 mL) of 0.16 mg·mL–1 standard to 10 mL brown volumetric flasks. The regression equation showed an outstanding linear relationship under 10.40–728.00 μg·mL–1 (y = 0.022 + 0.0001x, R2 = 0.98), where y is the concentration and x is the peak area.

Recovery of CGA

The recovery of CGA from crude extract was realized by NKA-9[29] macroporous resin technology. Dried and pretreated resin (80 g) and CGA crude extracts (20 mL diluted to 100 mL) were mixed in a flask. The mixture was agitated at room temperature for 24 h until an adsorption equilibrium state.[30] The adsorption process of clean resin was repeated three times. The filtered resin was washed with deionized water to remove the residual NADES. The CGA adsorbed on NKA-9 was desorbed with a 300 mL 40% ethanol solution (pH = 4) three times, agitating at room temperature for 2 h, and the desorption solutions were collected. After removal of most of the solutions by vacuum spinning, the residue was freeze-dried for 12 h to obtain the purified CGA for FT-IR and 1H NMR analyses.

Determination of the Antioxidant Property

The antioxidant ability of purified CGA was characterized by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging capacity. The DPPH radical scavenging capacities of CGA extracted by water, ethyl alcohol, and NADESs were evaluated as described previously[31] with slight modifications. The recovered CGA was redissolved by pure NADESs and traditional solvents. Briefly, 1.0 mL of the CGA solution was mixed with 1.0 mL of a 0.2 mmol·L–1 DPPH solution in ethanol. Then, the mixtures were shaken sharply for 30 s and kept in the dark for 30 min at room temperature. The absorbance of the samples was measured at 517 nm. The mixture of 1.0 mL of ethanol and 1.0 mL of DPPH solution was prepared as the control. In addition, 1.0 mL of ethanol was added to 1.0 mL of the CGA solution as the blank. Vc and BHT were selected as the reference standards. The DPPH scavenging activity was calculated using the following formulawhere As, Ab, and Ac are the absorbance of the sample, blank, and control, respectively. All the tests were performed in triplicate.

Stability Test

The stability of CGA extracts in solvents was considered in terms of heating, water content of NADESs, and storage time similar to that mentioned above. All 1.5 mL CGA solutions were poured into capped glass vials, treated with water baths at 25 and 85 °C for different times (10, 30, 50, 70, 90, and 110 min), and rapidly cooled to room temperature for the test. The effect of varying water contents (25, 40, 55, 70, and 85%, v/v) in NADESs on the stability of CGA was determined under PMH, LGH, and ethyl alcohol at 25 and −4 °C, and it was kept away from light for 60 min. The CGA solutions were stored in the dark at 25 and −4 °C for 0, 3, 8, 15, 24, and 35 days to investigate the effect of storage time. All the experiments were performed in triplicate. The degradation rate is calculated by the following formulawhere A0 is the initial content of CGA and AT is the test content of CGA.

Theoretical Studies for NADESs Enhancing the Stability and Antioxidant Ability of CGA

Theoretical simulations for proline, malic acid, PMH, and CGA were conducted by Gaussian 09 programs. The optimization of the corresponding geometric structures was carried out under the density functional theory method at the M062x/6-31+g level. To eliminate the dispersion effect, empirical dispersion = gd3 was added as the keyword. Frequency analysis was conducted to verify the energy minima. The reduced density gradient (RDG) quantitative analysis of the output files was performed for investigating the hydrogen-bonding interaction.[32]

Results and Discussion

Evaluation of NADESs in the Extraction of CGA

NADESs have impressive performances in natural product extraction, which revealed a strong solubility for a wide range of compounds.[33] Six types of NADESs were investigated for extracting CGA from ASH in this work. As shown in Figure , the special strong dissolving capacity made NADESs exhibit a better extraction effect of CGA than a traditional organic solvent. Simultaneously, not all the NADESs were suitable for CGA extraction; the design of NADESs was critical. PMH (N5) showed a more perfect capacity in extracting CGA from ASH, which provides guidance for the further use of CGA and ASH.

Figure 1

Screening of NADESs and conventional solvents in extracting CGA from ASH; the structure of CGA (a).

The strong interaction of NADESs provided a strong solubility of CGA.[34] Simultaneously, CGA was the secondary metabolite of the herb, and NADESs had the ability of breaking down the three components (cellulose, lignin, and hemicellulose) of a cell wall,[35] which greatly facilitates the release of metabolites from plant cells to the solvent environment. In addition, the compositions of NADES have a hydrotrope effect on dissolving biomolecules.[36] Therefore, NADESs exhibited a better CGA extraction efficiency than traditional solvents. Proper moisture can be added in NADES for optimization as a contribution of water.

Statistical Analysis

Selection of Single Factors

PMH was selected as the suitable NADES for process optimization. Pre-experiments were performed to confirm the upper and lower limits of the factors. The ranges of X1, X2, X3, and X4 were pre-designed for 5–45% (wt), 0.2/10 to 1.0/10 (g·mL–1), 150–350 (W), and 10–30 (min), respectively. The upper and lower levels of the parameters in the response surface design were confirmed as the water content (35 and 15 wt %), solid–liquid ratio (1:10 and 0.6:10, g·mL–1), ultrasonic power (300 and 200, W), and ultrasonic time (25 and 15, min).

CCD Analysis and Model Fitting

A quadratic polynomial model expressed the effects of each variable and the interaction between them most ideally. The optimization was carried out by applying a second-order polynomial equation, and the model was shown to be highly significant. Six replicates were designed in the 30 sets of experiments. The experimental data were fitted with a variety of models, and then an analysis of variance was performed. All the calculations were performed with the Design Expert program (version 8.0.6), and the regression coefficients for dependent variables were acquired by multiple linear regressions. The CGA content was best expressed using a quadratic polynomial model

The quality of fit to the quadratic polynomial equation was tested by the coefficient of determination (R2), which reached 0.93. The model for the response variable was highly significant with a very low p-value, and the F-test (7.66) explained the reliability of the model, simultaneously. The model also described a statistically insignificant lack of fit, (p-value = 0.012), indicating the adequate fit to the experimental data. A 3D view of the response surface generated for the response factor was exhibited, from which one can observe the interaction of the variables intuitively.

The experimental and theoretical information with respect to the CGA content is shown in Table . Experiment 3 [water content, 15% (wt); solid–liquid ratio, 0.1/10 (g·mL–1); ultrasonic power, 300 W; and ultrasonic time, 15 min] contributed the highest CGA content (18.82 μg·mL–1). The theoretical calculation by a quadratic polynomial model showed that the optimum conditions were water content, 10.43% (wt); solid–liquid ratio, 1.19/10 (g·mL–1); ultrasonic power, 238.77 W; and ultrasonic time, 8.09 min with the CGA concentration of 20.22 μg·mL–1.

Table 3

CCD of a Response Variable with Experimental and Theoretical Values

	factor 1	factor 2	factor 3	factor 4	CGA content (μg/mL)
experiment	water content	solid–liquid ratio	ultrasonic power	ultrasonic time	experimental value	theoretical value
1	2	0	0	0	12.56	11.00
2	0	0	0	0	16.30	16.04
3	–1	1	1	1	18.82	18.59
4	1	1	–1	–1	13.17	11.96
5	0	0	0	0	15.31	16.04
6	–1	–1	1	1	14.09	12.78
7	–1	–1	1	–1	8.31	8.85
8	1	1	–1	1	14.33	15.05
9	1	–1	–1	–1	5.10	6.59
10	0	–2	0	0	5.98	6.26
11	–1	1	–1	1	18.35	18.25
12	0	0	2	0	13.94	13.75
13	0	0	0	2	15.80	15.34
14	–1	–1	–1	1	10.38	9.65
15	–1	1	1	–1	16.42	14.78
16	1	1	1	–1	10.61	12.60
17	0	0	0	0	17.37	16.04
18	1	–1	–1	1	10.67	9.79
19	0	0	0	–2	6.58	8.32
20	1	–1	1	1	8.17	11.28
21	0	0	0	0	15.94	16.04
22	1	–1	1	–1	12.44	10.01
23	0	0	–2	0	8.53	9.98
24	–1	–1	–1	–1	5.41	3.80
25	0	0	0	0	16.05	16.04
26	1	1	1	1	14.69	13.77
27	–2	0	0	0	10.20	13.03
28	0	0	0	0	15.28	16.04
29	0	2	0	0	16.45	17.44
30	–1	1	–1	–1	14.36	12.50

Response Surface Analysis of the CGA Content

The relationship between extraction variables and the CGA content was explored by response surface plots (Figure ).

Figure 2

Response surface plots of the interaction of the water content and solid–liquid ratio (a), water content and ultrasonic power (b), water content and ultrasonic time (c), solid–liquid ratio and ultrasonic power (d), solid–liquid ratio and ultrasonic time (e), and ultrasonic power and ultrasonic time (f) on the CGA content.

The 3D response surface graphs provided a direct visualization of the interaction of different variables. The interactions of the water content and solid–liquid ratio, water content and ultrasonic time, and solid–liquid ratio and ultrasonic power showed a significant positive effect on the CGA content. At a lower water content and with increasing solid–liquid ratio, the CGA content increased rapidly (Figure a). Furthermore, at a lower water content and a lower ultrasonic time, the CGA content increased but as both increased, the ultrasonic time predominated (Figure c). Also, the solid–liquid ratio exhibited a more significant effect than ultrasonic power.

Characterization of the Purified CGA Extracts

The chemical structures of the CGA extracts were tested using 1H NMR, which aided in the identification of the structure by the information of the type and numbers of hydrogen atoms under the atomic resonance. The signal of the 1H NMR spectra is shown in Figure . Analysis of the spectrum showed good clarity, wherein the major material peaks were confirmed by comparing with the standard spectra (Figure b). The signals of 2–2.14 ppm were the contribution of protons in the 1, 2 position signed in the structure picture, where the other protons on the six-membered ring responded to different signals due to the coupling of adjacent hydrogen protons and attracting electronic influence of oxygen atoms. The numbers 3, 4, and 10 might be assigned at 3.74, 4.15, and 5.14 ppm, respectively. The intense signals at 6.77–6.98 ppm were attributed to the aromatic ring protons (protons 5 and 6). Considering the attracting electronic influence of oxygen atoms, the signal of 7.41 ppm was found to be of proton 7 and that of 6.14 ppm of proton 8. The peak location and characteristics were very similar to the standard. The 1H NMR spectrum analysis provided the evidence for the identification of purified CG substances. FI-IR spectrum analysis of CGA also provided the primary feature peaks of the CGA structure (Figure S2 in the Supporting Information), and the characteristic peaks were also consistent with the CGA structure.

Figure 3

1H NMR profile of the CGA sample (a) and the standard (b).

Antioxidant Capacity and Stability of CGA Extracts from NADESs

Antioxidant Capacity

The antioxidant capacity is a significant property of natural active substances, which plays a crucial role in the utilization and storage process. With the DPPH radical as the stable free radical in vitro, the scavenging activity was analyzed for evaluating the antioxidant capacity of CGA.

As shown in Figure a, the CGA extracted under NADESs and traditional solvents both have the free radical scavenging effect at a certain concentration. The scavenging activities increased first and reached a stable trend gradually. The antioxidant capacity of CGA under PMH and LGH approaches the Vc and BHT when the concentration was greater than 400 μg·mL–1, and it is noteworthy that the DPPH radical scavenging capacity was stable at a high level at a concentration of 200 μg·mL–1. This study showed that the CGA extracted by PMH and LGH exhibited better DPPH radical scavenging capacity than that extracted by EtOH and water. It is because PMH and LGH have a rich hydrogen bonding network, which causes CGA to be surrounded by a more stable solvent environment, and the active groups of CGA were better protected.

Figure 4

DPPH scavenging activity (a) and the effect of temperature (b), water content (c), and storage time (d) in the degradation of CGA in different solvents.

Stability

Natural products have special application value in industry and life. However, the particular chemical structure made it easy for them to change during processing and storage. In this work, we evaluated the stability of CGA in PMH and LGH compared to traditional solvents (water and EtOH) in terms of heating, water content, and storage time.

The degradations of CGA extracts under different solvents at 25 and 85 °C are exhibited in Figure b. Degradations of CGA in both NADESs and conventional solvents increased obviously, accompanied by the temperature raise from 25 to 85 °C. The degradation was distributed between 1 and 4% at 25 °C within 100 min. However, the tendency increased to 3–10% at 85 °C. This study illustrated that temperature can significantly affect the stability of CGA, and the stability became worse under high-temperature conditions. Nevertheless, PMH and LGH can suppress the degradation of CGA under high temperature.

Figure c shows the effect of water content of NADESs on the stability of CGA at −4 and 25 °C. Each degradation curve showed an increasing tendency with the content of water. The stabilities of CGA in PMH and LGH were better than in ethanol at both −4 and 25 °C. When the water content was at a relatively low level, the degradation of CGA proceeded slowly, but when the water content increased to a higher level (more than 50%), the degradation of CGA became accelerated. It could be explained that a certain amount of water addition can promote the formation of NADESs as hydrogen bond donors, but excess water weakened the strong molecular interaction of the PMH and LGH solvent system. These changes broke the suitable surrounding environment in which the active substance existed before. Moreover, the addition of water also decreased the viscosity of the solvents, which contributed to preserving the antioxidant activity of the compounds. Hence, the addition of water should be reduced during the extraction and storage of CGA. Simultaneously, it also verified that PMH and LGH can enhance the stability of CGA.

The storage time was also one of the important factors affecting the stability of active substances. As shown in Figure d, the degradation of CGA gradually tends to become steady with an increase of storage time. −4 °C was better for storing CGA than 25 °C. Although PMH and LGH demonstrated better resistance to degradation than EtOH and water, there was still 40% degradation after 20 days of storage even at −4 °C. Hence, PMH improved the stability of CGA and contributed to the extraction process, but it was not perfect in storage; other auxiliary storage experiences were also needed.

Mechanism Analysis of NADES Enhancing the Stability

PMH was explored as a representative to reveal the mechanism of NADES enhancing the CGA stability. Theoretical simulation was carried out to explain the hydrogen bond interactions of PMH and CGA compounds. These interactions were confirmed by RDG analyses (Figure ). The RDG scatter diagram of PMH displayed an obvious hydrogen bond interaction region (the vertical bar gathering area on the left in Figure b), and strong and clear hydrogen bond interaction signals are exhibited in Figure c. When CGA was added, not only the intramolecular hydrogen bond interaction of PMH enhanced, but also abundant intermolecular hydrogen bond interaction formed between the most stable group with CGA (Figure d). According to the topological criteria for the hydrogen bond, the electron density value (ρBCP) is assigned in Figure a. Hydrogen bond interactions occurred between PMH and CGA molecules; especially a ρBCP of 0.063 was identified as a strong hydrogen bond. The existence of these hydrogen bond interactions maintained the active groups of CGA around them, which protected the active site of CGA and was conducive to maintaining the structural stability of CGA simultaneously.

Figure 5

Optimized configuration and hydrogen bonding description for CGA and PMH calculated at the m062x/6-31+g level (a), RDG scatter diagram of PMH (b), 3D RDG analysis of the hydrogen bonding interaction in PMH (c) and PMH + CGA (d).

Conclusions

This work provided a new insight into the extraction of CGA from ASH using a designed green NADES. PMH showed higher extraction capacities with an extraction ratio of 28.23 mg/g than traditional solvents. Furthermore, the obtained CGA exhibited excellent antioxidant activity and better stability. Also, the mechanism of PMH enhanced the stability of CGA, which was exhibited with molecular simulation in this work. The hydrogen bonding interaction between CGA and natural deep eutectic molecules enhanced the stability and meanwhile protected the antioxidant activity of CGA.