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Selective Adsorption and Purification of the Acteoside in Cistanche tubulosa by Molecularly Imprinted Polymers-Part Ⅱ

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RESULTS AND DISCUSSION

Characterization of MIPs on acteoside in cistanche tubulosa

The SEM images of MIPs and NIPs are shown in Figure 1. As can be seen from Figure 1, MIPs and NIPs show different loose structures in different monomers, cross-linking agents, and solvents. MIP1-MIP4 and NIP1-NIP4 are heterogeneous particles with irregular shapes and different sizes. MIP5 and NIP5 are heterogeneous lamellar with irregular shapes and different sizes. The structure of MIPI prepared in EGDMA is looser than that of MIP4 prepared in DVB (Figures IA, G). The structure of MIPI with ACN and DMF as solvent(1;15,y/y) is looser than that of MIP5 with methanol as solvent (Figures 1A, ). The loose structure can speed up the molecular mass transfer and improve the binding speed of MIPs and ACT(acteoside).

acteoside in cistanche tubulosa

Figure 2 shows FT-IR spectra of MIPs with different functional monomers(A), cross-linkers(B), and solvents(C). As shown in Figure 2A, the C=N stretching vibration peak and the C=C stretching vibration peak appeared in 4-VP at 1,637 cm-1 and 1,456 cm-l, respectively(Figure 2A, trace 1). The C=Ostretching vibration peak and the C-O stretching vibration peak appeared in MAA at 1,730 and 1,260 cm-1, respectively (Figure 2A, trace 2). The stretching vibration peak of β-hydroxy and the stretching vibration peak of methylene appeared in HEMA at 3,436 and 2,958 cm-1, respectively (Figure2A, trace 3). The results indicate that 4-VP, MAA, and HEMA were successfully polymerized into the MIPS. As shown in Figure 2B, the stretching vibration absorption peaks of unsaturated C-H bond and skeleton vibration absorption peaks of benzene ring appeared in DVB at 3,021 and 1,600 cm-1, respectively (Figure 2B, trace 1). The stretching vibration absorption peaks of C=O and the stretching vibration absorption peaks of O-C-O appeared in EGDMA at 1,730 and 1,160 cm-I, respectively (Figure 2B, trace 2). The results indicate that MIP4 and MIPI were successfully polymerized. As shown in Figure 2C, it can be seen that MIP1 and MIP5 had no peculiar characteristic peak, indicating their chemical structure is similar.

cistanche tubulosa acteoside

Figure 2D is the FT-IR spectra of NIPI, MIP1-ACT(acteoside), and MIP1. The infrared spectrum of the NIP1 and the infrared spectrum of the MIP1 have the same characteristic peak, indicating that its chemical structure is similar (Figure 2D, trace 1 and trace 3). The infrared spectrum of the MIPI-ACT(acteoside) produced a new peak at 2,956 cm-'compared to the infrared spectrum of the MIP1 (Figure 2D, trace2 and trace3), due to the acyl group of ACT(acteoside) that interacts with 4-VP to form a hydrogen bond association on the MIPI, indicating that the template molecule has been adsorbed to the MIP1 through hydrogen bonding.

The hysteretic curve and pore size distributions of MIPs and NIPs are shown in Figure 3.The hysteretic curves of MIPs and NIPsexhibited" type IV" isotherm (Figures 3A, B). The pore size distributions of MIPs and NIPs were distributed in the range of 5-50nm, which indicated that the pores of MIPs and NIPs belonged to mesopores (Figures 3C, D).

The data of the specific surface area, pore-volume, and pore size for MIPs and NIPs are listed in Table 4. The specific surface area of MIPI was 593.91 m²/g with a pore size of 10.91nm. The specific surface area of NIPI was 427.12 m2/g with a pore size of 7.94 nm. Obviously, the specific surface area and pore size of the MIPI was larger than that of the NIP1. This can be attributed to the presence of imprinted holes on the surface of MIP1. The high specific surface area and large pore size of MIPI are favorable for increasing the adsorption capacity of MIP1 for ACT(acteoside).

The LH NMR spectra of ACT(acteoside), monomers, and prepolymers are shown in Figure4. The proton peaks of different phenolic hydroxyl groups on the ACT(acteoside) appeared at 7.50,6.75,6.20,5.02, 4.35, and 3.52 ppm(Figures 4A-C). As shown in Figure 4A, the proton peaks on the pyridine groups of 4-VP exhibited at 8.64,7.45,6.75,6.17, and 5.54 ppm. Compared with the proton peaks of ACT (acteoside) and 4-VP, a prepolymer of MIP1 appeared new proton peaks at 8.52,8.01,7.49,6.67,6.13, and 5.54 ppm, which resulted from the formed hydrogen bonds between ACT(acteoside) and 4-VP. As shown in Figure 4B, the proton peaks of MAA exhibited at 5.94, 5.53, and 1.47 ppm. Compared with the proton peaks of ACT(acteoside) and MAA, a prepolymer of MIP2 presented new proton peaks at 7.95,5.98,5.67, and 1.03 ppm, which resulted from the formed hydrogen bonds between ACT(acteoside) and MAA. As shown in Figure 4C, the proton peaks of HEMA exhibited at 6.04,5.56, 4.85,4.1l, and 3.75 ppm. Compared with the proton peaks of ACT(acteoside) and HEMAA, a prepolymer of MIP3 exhibited new proton peaks at 7.95,6.15,5.76,4.32,and3.66 ppm, which resulted from the formed hydrogen bonds between ACT and HEMA.

acteoside of cistanche tubulosa

Adsorption Experiment

In the process of MIPs preparation, the appropriate functional monomers determine whether molecularly imprinted polymers have excellent recognition ability. This is because different functional monomers contain different functional groups and the interaction between template molecules is different. According to the acidity and alkalinity, functional monomers can be further divided into acidic functional monomers, basic functional monomers, and neutral functional monomers, and acidic template molecules should be selected as basic functional monomers. Hydrogen bonds can be formed between different functional groups of monomers and hydroxyl groups of the template molecules in MIPs(Hammam et al.,2018; Panjan et al,2018).4-VP contains pyridine groups, and the pyridine groups can form hydrogen bonds with the hydroxyl groups in the template molecule of ACT(acteoside).MAA contains carboxyl groups, and carboxyl groups can form hydrogen bonds with the hydroxyl groups in the template molecule of ACT(acteoside). HEMA contains hydroxyl groups, and hydroxyl groups can form hydrogen bonds with template molecules of ACT(acteoside)(Yesilova et al.,2018; Haginaka et al., 2019; Luo et al, 2019). The static adsorption data of MIPs and NIPs are listed in Table 5. According to the data listed in Table 5, MIP1 has the highest adsorption capacity and the highest imprinting factor. The adsorption capacity of MIP1 was 168.05 mg/g, and the imprinting factor was 2.69. On one hand, MIP1 had large amounts of imprinted holes, which can match the spatial configuration of ACT(acteoside), achieving selective adsorption for ACT(acteoside). On the other hand, the hydroxyl groups on ACT(acteoside) contacted with the N-H groups of MIP1 in the adsorption process to form hydrogen bonds, which can increase the adsorption capacity of ACT(acteoside).

The effect of pH on the adsorption performance of MIP1 and NIPI for ACT(acteoside) is shown in Figure 5. It can be seen from Figure 5 that the optimal pH value was 7 for adsorption of ACT(acteoside) by MIP1 and NIP1. The reasons are as follows: ACT(acteoside) had a large number of phenolic hydroxyl groups and belonged to the weak acidic molecules, and the different pH values affected the stability of ACT(acteoside). The stability of phenolic hydroxyl groups on ACT(acteoside) molecules would decrease at pH>7 and pH<7. This will reduce the amount of ACT(acteoside) adsorbed by MIP1 and NIP1. Thus, the optimum pH was 7 for MIP1 and NIP1 adsorption.

Isothermal Adsorption Experiment

The isotherm adsorption curves of MIP1 and NIP1 for ACT(acteoside) were shown in Figure 6. It can be seen that the adsorption capacity of MIP1 for ACT(acteoside) increases with the increase of the initial ACT(acteoside) concentration, and this might be the reason that the amount of ACT(acteoside) was not enough to saturate the specific binding cavities. The adsorption curve reached the saturation and tended to be stable when the initial concentration exceeded 1.50 mg/ml, and the maximum adsorption capacity of MIP1 was 250.00 mg/g, which indicated that a great many ACT(acteoside) specific binding sites were produced during the imprinting process. Figure 4 also shows that the amounts of ACT(acteoside) bound to the MIPs were at a high level compared with those of the NIPs under the same conditions.

Scatchard Plot Analysis

The Scatchard plot of MIP1 and NIP1 for ACT (acteoside)is shown in Figure 7. The binding properties of MIPs were determined by Scatchard plot analysis, which was based on the following equation:

where c is the equilibrium concentration of ACT(acteoside) in the solution, he is the amount of ACT (acteoside) bound to the MIPs at equilibrium, Bmax is the apparent maximum binding amount, and K₀ is the dissociation constant.

The Scatchard plot of MIP1 contained two different linear regression lines, suggesting two types of binding sites. As shown in Figure7A, the left line suggested that the MIP1 had high binding affinity with ACT(acteoside) in the concentration range of 0.005-0.25 mg/ml. The Kaand Bmax was found to be 1.94 mg/ml and 18,080.15 mg/g for dry polymer, respectively, and they were calculated from the intercept and slope of the regression equation g_e/c_e=-86.54g_e+9310.66（R²=0.98）. The right line indicated that MIP1 had low binding affinity in the concentration range of 0.25-4.00 mg/ml. The Ka and Bmax were found to be 127.31 mg/ml and 54,316.81 mg/g for dry polymer, respectively, and they were calculated from the regression equation g_e/c_e=-1.32q_e ＋ 426.65(R²=0.91). Meanwhile, it can be seen from the two equations that the slope of the straight line on the left side was small, and the slope of the straight line on the right side was large. The small slope had high binding affinity with ACT(acteoside) for MIP1.

The Scatchard plot of NIP1 shows a straight line, indicating that there is only one type of binding site in NIP1. The Ka and Bmax were found to be 12.19 mg/ml and 10,689.41 mg/g for dry polymer, respectively, and they were calculated from the regression equation q_e/c_e = −5.13q_e+ 876.91 (R² = 0.89).

Adsorption Kinetics Study Figure 8 shows the adsorption kinetics curve of MIP1 for ACT(acteoside). The dynamic adsorption experiments were carried out in ACT(acteoside) solution with an initial concentration of 0.50 mg/ml; it can be seen that the adsorption capacity of MIP1 for ACT(acteoside) increases rapidly in 20 min and slowly in 100 min, but it does not change much after 150 min. Therefore, the equilibrium adsorption time of MIP1 is 150 min and the equilibrium adsorption capacity is 204.08 mg/g. At the beginning of dynamic adsorption, there are more free ACT(acteoside) molecules in ACT(acteoside) solution and more specific recognition sites in MIP1, so the hydrogen bonding rate between MIP1 and ACT(acteoside) is fast. After 20 min, the number of specific recognition sites of MIP1 and the number of free radicals in the ACT(acteoside) solution decreased, which reduced the binding rate of MIP1 and ACT(acteoside), eventually reaching dynamic equilibrium. Figure 9 shows the quasi-first-order kinetic model and quasi-second-order kinetic model of ACT(acteoside) adsorption on MIP1, and Table 6 shows the data fitted by the kinetic model. The equilibrium adsorption capacity in the dynamic adsorption equilibrium experiment of MIP1 was 204.08 mg/g. The experimental data are consistent with the pseudo-second-order kinetic fitting data, which proves that the adsorption behavior of MIP1 conforms to the pseudo-second-order kinetic model (R2 > 0.99). The dynamic adsorption equilibrium accords with the quasi-secondorder kinetics; it indicates that chemical adsorption is a speed-control step in the adsorption process. Therefore, the adsorption behavior of ACT(acteoside) on MIP1 may be hydrogen bonding.

Purification of ACT(acteoside) From C. tubulosa Preparation of the Extracts of C. tubulosa Twenty grams of C. tubulosa powder was dispersed in a 50% ethanol solution at 70◦C. The extraction was carried out for 2 min under a high shear homogenizer at 16,000 rpm. The extracts were filtered through a 0.22-µm filter to obtain the extract of C. tubulosa. Preparation of Solid Phase Extraction Column and Solid Phase Extraction of ACT in the Extracts of C. tubulosa One hundred milligrams of MIP1 was dispersed in the 50% ethanol solution and loaded into a solid-phase extraction column, and the SPE column of MIP1 was then rinsed with a 50% ethanol solution at a fellow rate of 2.00 ml/min for 10 min. The extracts of C. tubulosa were injected into the SPE column at a fellow rate of 2.00 ml/min, and the sample concentration of the efficient was measured. The eluate was collected after the SPE column of MIP1 was eluted with 90% ethanol solutions and 10% ethanol solutions, respectively (Gao et al., 2016). The collected eluent was dried and dissolved in water for constant volume, and then the content of ACT was measured by HPLC. Figure 10 shows the relationship between efficient volume and efficient concentration in the adsorption process of MIP1 and NIP1, respectively. The contents of ECH and ACT(acteoside) in the extract of C. tubulosa were 2.44 and 0.53 mg/ml, respectively. As can be seen from Figure 10, when the efficient volume is 40.00 ml, the concentrations of ACT(acteoside) and ECH in the efficient are the same as those in the extract of Cistanche tubulosa, indicating that MIP1 reaches the adsorption equilibrium. When the efficient volume was 23.00 ml, the concentrations of ACT(acteoside) and ECH in the efficient were the same as those in the extract of C. tubulosa, indicating that NIP1 reached the adsorption equilibrium. The binding amount of ACT(acteoside) by the MIP1 solid-phase extraction column is larger than that by NIP1 solid-phase extraction column; it indicated that MIP1 has an excellent imprinting effect on ACT (acteoside). The content of ECH in the extract of Cistanche tubulosa was 4.58 times that of ACT. The adsorption capacity of MIP1 on ACT and ECH was 36.86 and 88.67 mg/g, respectively. After the eluent is eluted, the recovery rate of ACT was 90.09%. The purity of ACT increased from 1.99 to 27.88%, and the increasing amplitude of purity is 1301.00%, which was higher than the increment of 960.00% by adsorption of microporous resins (HPD300) (Liu et al., 2013). The increase in purity of ACT(acteoside) results from the selective adsorption of ACT(acteoside) by the molecularly imprinted cavity. CONCLUSIONS An imprinting material with a high selective adsorption capacity is used for simple and rapid separation of ACT(acteoside). MIPs were investigated in terms of static adsorption experiments, dynamic adsorption experiments, and selectivity experiments. The experimental results showed that MIP1 exhibited the optimal adsorption performance to ACT(acteoside). MIP1 was prepared with ACT(acteoside) as template molecule, 4-VP as a functional monomer, EGDMA as a cross-linking agent, the volume ratio of ACN and DMF of 1:1.5 (v/v) as a solvent, and AIBN as initiator. The adsorption results displayed that the adsorption capacity of MIP1 to ACT(acteoside) reached 112.60 mg/g, and the separation factor of ACT(acteoside)/ECH was 4.68. The dynamic adsorption of ACT(acteoside) accorded with the quasi second-order kinetics; it indicated that the adsorption process of MIP1 is the process of chemical adsorption to ACT(acteoside). MIPs with high selectivity makes it a potential adsorption material for the purification of plant active ingredients.

DATA AVAILABILITY STATEMENT All datasets generated for this study are included in the article/supplementary material.

AUTHOR CONTRIBUTIONS XZ carried out experiments and wrote the manuscript. WP designed the experiments, provided useful suggestions, and solved the problems in the experiments. RG provided the experiment tools. XL contributed to the study design, manuscript revision, and final version.

ACKNOWLEDGMENTS We gratefully acknowledge the support from the National Natural Science Foundation for Young Scientists of China (Grant No. 21706166), the Program for Young Innovative Talents of Shihezi University (CXRC201802), the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT_15R46), and the Yangtze River scholar research project of Shihezi University (Grant No. CJXZ201601).

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