Extraction of flavonoids from Glycyrrhiza residues using deep eutectic solvents and its molecular mechanism

https://doi.org/10.1016/j.molliq.2022.119848Get rights and content

Highlights

  • Fourteen deep eutectic solvents (DESs) were used to extract flavonoids from Glycyrrhiza residues.

  • ChCl-glycolic acid had the greatest capacity for extracting four flavonoids.

  • Optimal extraction rate was 83.03% higher than that using 60% ethanol.

  • Molecular mechanism associated with DES-flavonoid interactions were analyzed.

  • Hydrogen bond strength was significant for the DES-flavonoid extraction process.

Abstract

The re-extraction of bioactive compounds from Chinese herb residues is significant for the maximum utilization of biomass resources. However, conventional alcoholic and aqueous solvents are unsuitable for recovering those compounds. In this study, 14 deep eutectic solvents (DESs) were used to improve the efficiency of flavonoid extraction from Glycyrrhiza residues. The maximal total extraction rate (10.68 mg/g) for four flavonoids (liquiritin, isoliquiritin, liquiritigenin, and isoliquiritigenin) was achieved using choline chloride-glycolic acid as the DES under optimal conditions. The extraction rate was 83.03 % higher than that by 60 % ethanol, a traditional solvent. Analysis of the associated molecular mechanism based on density functional theory showed that interactions between the solvent and liquiritin were dominated by hydrogen bonds followed by Van der Waals forces, whereas the bonding between the solvent and liquiritigenin only involved Van der Waals forces, thereby verifying the significance of the strength of hydrogen bonding in the DES-flavonoid extraction process.

Introduction

Herbal medicines are significant components of Chinese medicine and they play critical roles in the daily lives of Chinese people [1]. However, the growth of the Chinese herbal medicine industry has led to the annual production of over 50 million tons of undesirable Chinese herb residues, thereby leading to the waste of resources and the high burden imposed by their treatment [2]. Herbal medicines are prepared by boiling for a long time to extract water-soluble medical constituents containing considerable amounts of bioactive compounds, such as flavonoids, alkaloids, polysaccharides, amino acids, and trace elements, but large amounts still remain in the Chinese herb residues [3]. Glycyrrhiza is one of the most widely used traditional Chinese medicinal herbs and it is employed in over 60 % of Chinese medicine prescriptions. As a consequence, it has been estimated that 12 million tons of Glycyrrhiza residues are produced each year. Thus, recycling the flavonoids that remain in Glycyrrhiza residues has attracted much attention because of the large amounts of these valuable compounds that remain in medicinal herb residues. In addition to their applications in the pharmaceutical field, flavonoids are common ingredients of other products, including agrochemicals, nutraceuticals, and cosmetics [4]. Flavonoids are usually recovered with water because its characteristics satisfy the obligatory requirements for applications in the fields mentioned above [5]. However, water extraction is not suitable for water-insoluble and nonpolar molecules, and thus, the recovery rates are low for the total bioactive compounds [6]. Moreover, after the first round of extraction in boiling water, the remaining bioactive compounds are mostly surrounded by lignocellulose and difficult to isolate from the herb residues. Thus, there is an urgent need to develop more suitable efficient solvents to improve the utilization of bioactive compounds in natural biomass resources.

Several concerns have been raised about the development of green and sustainable extraction methods for bioactive compounds, especially the usage of green solvents [5]. Conventional solvents, including methanol, ethanol, acetone, and chloroform, are highly effective at extracting bioactive compounds, and they can be readily used for preparing extracts from natural resources, but they also have some limitations due to their intrinsic properties in terms of toxicity, volatility, thermal instability, solubility, and poor selectivity [3]. In the last two decades, the development of deep eutectic solvents (DESs) has attracted increasing attention because of their suitability for extracting bioactive compounds due to their unique physicochemical characteristics [7]. DESs are eutectic mixtures comprising a hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA), mostly with liquid forms at temperatures below 100 °C. The significant lower melting point of DESs than that of their individual component is the consequence of the formation of hydrogen bond network during the hot blending process which decreases the lattice energy of the system [8]. Compared with conventional solvents, DESs have several attractive attributes such as non-flammability, low volatility, high stability, and short extraction times [9]. Moreover, DESs are considered green and sustainable solvents because they mostly comprise biodegradable natural products and they are easy to prepare by simply mixing the components with heating. In addition, the combination of various components facilitates the design of specific DESs for certain tasks, thereby providing a broad route for the efficient extraction of target products and avoiding the issue of low selectivity that affects conventional solvents [10]. Many studies have investigated the extraction of bioactive compounds from biomass using specifically designed DESs and considerable progress has been made in their use [11]. In particular, DESs composed of choline chloride and lactic acid obtained the best performance at extracting target catechins from Camellia sinensis leaves after extracting for only 8 min at 66 °C with the assistance of microwave heating [12]. The optimal extraction efficiency of four bioactive flavonoids from Pollen Typhae was obtained using a DES comprising choline chloride and 1,2-propanediol with recovery rates of 86.87–98.89 %, and the extraction efficiency was much higher than that using conventional solvents such as methanol, ethanol, or water [13]. DESs were also designed that exhibited high efficiency at extracting rutin, with a maximum extraction rate of 9.5 mg/g and an extraction efficiency of 95 % from buckwheat hull [14]. The simultaneous extraction of five aromatic acids from Ginkgo biloba leaves was achieved using a tailor-made DES prepared from xylitol, glycolic acid, and 1,5-pentanedioic acid with a water content of 50 %, where the extraction rates ranged from 25.90 to 94.15 mg/g [15]. Cao [16] found that the hydrophobic eutectic solvent composed of methyltri-n-octylammonium chloride/1-butanol could effectively extract artemisinins from Artemisia annua leaves, with the extraction rate of artemisinin compounds of 7.9936 ± 0.0364 mg/g under the optimal extraction conditions. Yan [17] found that the extraction rate of phenolic and flavonoids compounds from Ginkgo biloba L leaves by the hydrophobic deep eutectic solvent of menthol/levulinic acid was equivalent to that of methanol. Previous studies mainly focused on optimizing the extraction process, but a clear understanding of the detailed extraction mechanism is still lacking.

The results obtained in previous studies indicate that the extraction efficiencies of DESs can vary according to the type of DES and target compounds. These variations are related to the structures of the DESs and extracts, which determine by their physicochemical properties and influence the capacity to extract bioactive compounds. For example, a higher capacity for extracting alkaloids was observed using carboxylic acid-based DESs compared with sugar- and alcohol-based DESs [18]. By contrast, glycerol- or sucrose-based DESs obtained four times higher extraction yields of saponins from Camellia oleifera Abel compared with citric acid-based DESs [19]. Moreover, the addition of water to DES solvents had variable effects on the yields of different extracts, possibly due to its polarity affecting the extraction environment [20]. Thus, it is possible to design an optimal solvent for structurally similar compounds by tuning the properties of the solvent. Screening and comparing multiple solvents might lead to positive outcomes, but understanding the exact interaction between the solvent and extract will help to determine the key factors and eliminate redundant work.

In the present study, the efficiency of flavonoid extraction from Glycyrrhiza residues was compared for eight hydrophilic and six hydrophobic DESs. The most important factors such as the molar ratios of the DES components, solid loading, and water content were examined for the best DES after screening. Analysis was conducted based on density functional theory (DFT) to understand the molecular mechanism associated with the extraction process and the key factors that influenced the extraction rate. The results obtained in the present study may provide important references to facilitate the extraction of natural ingredients and the utilization of Chinese herb residues.

Section snippets

Materials and chemicals

Glycyrrhiza roots were ground and sieved to collect a powder with a particle size of 20–40 mesh. The cooking process for Chinese herbal medicine preparation was mimicked to obtain the Glycyrrhiza residue, where 300 g of the Glycyrrhiza powder was boiled in 1.5–2.0 L deionized water for 0.5 h and the solid residue was dried to constant weight in an oven at 50 °C. The resulting dry Glycyrrhiza residue was used as the raw material in the subsequent experiments. Standards for four flavonoids

Optimization of DES configurations

Hydrogen bond interactions play important roles in the formation of DESs. However, it is difficult to directly quantify the interactions between hydrogen bonds in a DES because the intra- and intermolecular hydrogen bonds are delicate and complicated. In the present study, the hydrogen bond interactions in 14 DESs comprising six l-menthol-based hydrophobic DESs (DES-1 to DES-6) and eight ChCl-based DESs (DES-7 to DES-14) were estimated based on DFT calculations. To eliminate the dispersion

Conclusion

In the present study, the molecular configurations of 14 DESs and their hydrogen bonding interactions as well as their capacities of extracting four flavonoids from Glycyrrhiza residues were evaluated. The acidic hydrophilic DES comprising ChCl-GA had the highest extraction capacity with the maximum total flavonoids extraction rate of 4.23 mg/g, 5.88 mg/g, 0.56 mg/g, and 0.02 mg/g for liquiritin, isoliquiritin, liquiritigenin, and isoliquiritigenin, respectively. The results of molecular

CRediT authorship contribution statement

Qiang Yu: Conceptualization, Funding acquisition. Fan Wang: Methodology, Investigation. Yating Jian: Formal analysis, Data curation. Victor M. Chernyshev: Conceptualization, Writing – review & editing. Yu Zhang: Methodology, Visualization. Zhongming Wang: Supervision, Funding acquisition. Zhenhong Yuan: Supervision, Validation. Xiaoyan Chen: Conceptualization, Funding acquisition, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was supported by the National Key R&D Program of China (2019YFC1906602), National Natural Science Foundation of China (51876206), Science and Technology Program of Guangzhou (202103000011).

References (42)

  • Y. Dai et al.

    Tailoring properties of natural deep eutectic solvents with water to facilitate their applications

    Food Chem.

    (2015)
  • M. Wang et al.

    Fast environment-friendly ball mill-assisted deep eutectic solvent-based extraction of natural products

    J. Chromatogr.

    (2016)
  • B. Ozturk et al.

    Extraction of polyphenolic antioxidants from orange peel waste using deep eutectic solvents

    Sep. Purif. Technol.

    (2018)
  • P. Gong et al.

    Traditional chinese medicine on the treatment of coronary heart disease in recent 20 years

    J. Altern. Complement. Med.

    (2017)
  • S.P.M. Ventura et al.

    Ionic-liquid-mediated extraction and separation processes for bioactive compounds: Past, present, and future trends

    Chem. Rev.

    (2017)
  • F. Chemat et al.

    Green extraction of natural products: Concept and principles

    Int. J. Mol. Sci.

    (2012)
  • X. Xu et al.

    Direct flavonoid-focused chemical comparison among three epimedium plants by online liquid extraction–high performance liquid chromatography–tandem mass spectrometry

    Molecules

    (2021)
  • P. Yu et al.

    Extraction and analysis of six effective components in glycyrrhiza uralensis fisch by deep eutectic solvents (DES) combined with quantitative analysis of multi-components by single marker (QAMS) method

    Molecules

    (2021)
  • A.P. Abbott et al.

    Novel solvent properties of choline chloride/urea mixtures

    Chem. Commun. (Camb)

    (2003)
  • Q. Zhang et al.

    Deep eutectic solvents: Syntheses, properties and applications

    Chem. Soc. Rev.

    (2012)
  • M. Ruesgas-Ramón et al.

    Application of deep eutectic solvents (DES) for phenolic compounds extraction: Overview, challenges, and opportunities

    J. Agric. Food Chem.

    (2017)
  • Cited by (0)

    View full text