Green and efficient extraction of four bioactive flavonoids from Pollen Typhae by ultrasound-assisted deep eutectic solvents extraction
Introduction
Solvent extraction remains one of the most widely used sample preparation techniques for chromatographic analysis. Several types of solvents including ionic liquids (ILs) have been suggested as “green” solutions to replace volatile organic solvents. In recent years, much attention has been paid to ILs as sustainable alternatives to hazardous organic solvents [1], [2], [3], [4]. However, most ILs have the disadvantages of high price and toxicity [5], [6]. To overcome the limitations of ILs, a novel class of sustainable solvents with similar properties to ILs have emerged [7], [8], [9], Abbott et al. presented a new solvent system, termed “deep eutectic solvents (DESs)” which are formed by mixing natural and renewable starting materials with high melting points [10]. DESs are mixtures prepared by combining two or more components from hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs) with a lower melting point than that of its individual components mainly due to the generation of inter molecular hydrogen bonds [11], [12], [13], [14]. Meanwhile, DESs have attracted more attention as a rapidly emerging new green solvent that replaces traditional solvents or even ILs owing to their unique advantages of biodegradability, non-toxicity, and low costs for synthesis [10], [11], [12]. They have been widely used in catalysis, organic synthesis, electrochemical materials and solvents of extraction because of the green property and variety of DESs [14], [15], [16], [17], [18]. In previous studies, DESs have been successfully applied for efficient extraction of different types of bioactive compounds including quercetin [19], ginsenosides [20], anthocyanins [21], glaucarubinone [22] and catechins [23], [24].
Pollen Typhae, the dried pollen of Cattail plants, is commonly used in Chinese medicine in the treatment of stranguria, hematuria, dysmenorrhea, and metrorrhagia [25], [26]. Pharmacological studies have shown that isorhamnetin-3-O-neohesperidin, isorhamnetin, quercetin and other flavonoids are the main active constituents in Pollen Typhae which possess antioxidant, antiinflammatory, antigenotoxic, and antiprotozoal activities [27], [28]. The flavonoid constituents in Pollen Typhae, which cover a wide range of polarities, contribute to its pharmacological activities and therapeutic efficacy. A series of methods have been reported for the efficient extraction of bioactive flavonoids from Pollen Typhae [26], [27], [28], [29]. Because the solubility of flavonoids in water is generally low, various organic solvents including ethanol, methanol, acetone, and ethyl acetate have been commonly used as extraction solvents. The selection of analytical methods for sample pre-treatment is also crucial since the method of extraction not only affects the extraction efficiency, but also determines accuracy of developed method [30]. These commonly used extraction methods mainly include immersion extraction (ME), soxhlet extraction (SE) [31], heat reflux extraction (HRE), pressurized liquid extraction (PLE), ultrasound-assisted extraction (UAE) [32], [33], [34], [35], [36], [37] and microwave assisted extraction (MAE) [38], [39], [40], [41]. However, some of these reported methods are time consuming, labor intensive and require a large number of organic solvents [21].
In present study, we aimed to develop a simple and green method for extraction of four bioactive flavonoids including quercetin, kaempferol, isorhamnetin and naringenin in Pollen Typhae by using DESs as extraction solvent. Simultaneously, we also established a simple and rapid method of acid hydrolysis, through the acid hydrolysis method, the flavonoid glycosides which were not easy to be quantified in the extracts could convert to their corresponding aglycons forms [42], [43] so as to accurately reflect the extraction efficiency of each extraction solvent. The ultrasound assisted-deep eutectic solvent extraction (UAE-DES) method was applied to extract the analytes, and the high performance liquid chromatography (HPLC) method was developed for their sensitive and selective quantification. The efficiencies of DESs were thoroughly investigated and optimized in this study. Finally, the Pollen Typhae extraction yields of the optimized method were compared with those of conventional extraction methods. To the best of our knowledge, no extraction procedures based on these DESs have been reported for extraction of Pollen Typhae.
Section snippets
Materials
Pollen Typhae was purchased from the local medicine market in Liaoning Province, China. The samples were dried to constant weight and stored in the shade to use. Choline chloride (>98.0%), 1,4-butanediol (>98.0%), 1,2-propanediol (>98.0%), glucose (>98.0%), glycerol (>98.0%), L-proline (>99%), lactic acid (>98.0%) and ethylene glycol (>99.5%) were purchased from Shanghai Aladdin Chemistry Co., Ltd (Shanghai, China). Analytical standard quercetin (≥98.5%, E1712092), kaempferol (≥98.0%,
Characterization of DES
Most DESs can be prepared with various ratios at room temperature or at a certain temperature to form a transparent and uniform liquid. The prepared DESs have a lower freezing point than their individual constituents, which is attributed to the reduction of the coulomb forces of DESs with the large volume and asymmetric charge distribution of molecular ions [18]. It has been proved that the densities of mostly prepared DESs are higher than that of water [11]. DES of ChCl/1,2-propanediol (1:4)
Conclusion
In the present study, a green and efficient extraction method using DESs was established for the extraction of flavonoids from Pollen Typhae. DESs showed greater extraction efficiency for extraction of quercetin, naringenin, kaempferol and isorhamnetin from Pollen Typhae comparing with conventional solvents such as methanol and 75% of aqueous ethanol. ChPri (1:4) with 30% water content showed the highest extraction efficiency among all prepared DESs. This study suggests the excellent properties
Acknowledgment
This work was supported by the National Natural Science Foundation of China (No. 81503029) and Young and Middle-aged Backbone Personnel Training Program of Shenyang Pharmaceutical University (ZQN2016011).
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