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Tyrosine imprinted polymer beads with different functional monomers via seed swelling and suspension polymerization.

Molecularly imprinted polymers (MIPs) in bead form were prepared by seed swelling and suspension polymerization with trimethylolpropane trimethacrylate (TRIM) as crosslinker arid tyrosine as template. The appearance of the beads was found to be greatly influenced by the concentration of polyvinyl alcohol (PVA) used as dispersant and the ratio of water to TRIM (W/T) during polymerization. When W/T was 46:1 (V/V) and the concentration of PVA-water solution was 4.6 (%m/m), the result was more desirable size distribution with fewer misshapen beads. A series of imprinted polymers with methacrylic acid (MAA), acrylamide (AM), 2-vinylpyridine (VP) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as functional monomers (FM), respectively, were prepared, where the molar ratio of tyrosine, FM and TRIM was chosen as 1:16:17. The molecule adsorbing and selecting analysis of the polymers showed that although all the imprinted polymers adsorbed the templates following the rule of Langmuir and exhibited an obvious molec ule selecting capability compared with their control polymers (CP), the adsorbing constant and the separating factor were much different when the functional monomers varied. AMPS worked better than the other FM in the present work with a higher value of asymptotic maximum solid phase concentration (0.282 mol/kg), adsorbing constant (198.8 1/mol), and a larger separating factor (1.93) when phenylalanine was used as the competitive molecule.

INTRODUCTION

Molecular imprinting is a powerful technique for designing and producing artificial binding sites in a synthetic polymer, exhibiting selective rebinding of the printing molecules used during polymerization (1, 2). The polymers prepared in this way are usually named molecularly imprinted polymers (MIPs). MIPs have been reported for a variety of substances including amino acids (3), proteins (4), drugs (5) and metal ions (6), and have been used for a tailor-made chiral separation phase (7, 8), enzyme mimic (9), antibody mimic (10), recognition units in sensors (11, 12), byproduct removal (13), equilibrium shifting (14) and combinatorial library screening materials (15).

So far, most MIPs have been prepared by bulk polymerization, which yields polymer monoliths. These monoliths have to be ground and sieved to produce particles of the desired dimensions for further use. This process is both wasteful and time consuming, and also provides irregularly shaped fragments (16-18), which are unfavorable for applications. Therefore, uniformly sized MIPS in bead form are preferable. A few reports have described methods for producing imprinted polymers in bead form. Among these methods, suspension polymerization has been utilized to prepare tert-butoxycarbonyl-L-phenylalanine imprinted polymers, resulting in polymer beads with good molecule recognition properties, when a special perfluorocarbon liquid is used as the dispersing solvent and a particular perfluorinated polymeric surfactant as stabilizer (19). Perfluorocarbon liquid is expensive and difficult to acquire, however, which limits the development of this method. Precipitation polymerization, described by Ye et al. (20), was compa ratively easier for preparing imprinted microspheres of sub-micron size without the particular dispersing liquid or any stabilizers, but the imprinted polymers were too small for some applications, and polymer fines were unavoidable during the polymerization, which limits the application of such polymers. Recently, multi-step swelling and polymerization was introduced to prepare MIPs in bead form in a water system by Hosoya et al. (21,22). This method proved to have the advantages of high yields and good size monodispersity compared with the suspension polymerization method (23) when ethylene dimethacrylate was used as crosslinker, and the naproxen imprinted polymers prepared with this method had more desirable bead size monodispersity and exhibited a high selectivity for naproxen and moderate selectivity for other 2-acryipropionic acid derivatives (24). Although this method has many advantages, it is a bit complex and time-consuming, and therefore needs to be improved, and at the same time other more feasibl e methods are certainly desirable.

The goal of the present work was to introduce a novel method, seed swelling and suspension polymerization, which is partially similar to multi-step swelling and polymerization (21,22) but easier to carry out and comparatively time saving. During the polymerization, the selected ligand or template is first allowed to interact via non-covalent bond formation with functional monomers freely in solution to form host-guest complexes. The resulting host-guest complexes are subsequently copolymerized with a large excess of crosslinking monomers to give a rigid insoluble polymer. After extraction of the template, specific recognition sides are left in the polymer network where the spatial arrangement of the functional groups in the polymer matrix, together with the shape image, is complementary to the template, which endows the polymers with molecule selectivity capabilities. Therefore, the functional monomer (FM) is one of the key factors influencing the molecule adsorption and molecule selectivity of the MIPs. To d iscuss such influences and find a more feasible FM, a series of imprinted polymer beads were prepared using tyrosine as printing molecule and trimethylolpropane trimeth-acrylate as crosslinker, and the FM was varied from methaerylic acid (MAA), acrylamide (AM) and 2-vinylpyridine (VP) to 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

EXPERIMENTAL

Materials

Tyrosine and phenylalanine were purchased from Beijin Xingjin S&T Biological Technology Co. and Zhengxian Applied Science Service Section (Beijing, China) respectively, and their structures are illustrated in Fig. 1. 2-Vinylpyridine (VP) was supplied by Jilin Petrochemicals Co. (China). Trimethylolpropane trimethacry-late (TRIM) was purchased from Tianjin Chemical Reagent institute; styrene from Tianjin No. 2 Chemical Reagent Factory was purified by general distillation. N.N-dimethylaniline (NDMA), methacrylic acid (MAA), toluene, acrylamide (AM), 2-acrylamido-2methyipropanesulfonic acid (AMPS, structure is illustrated in Fig. 1.), benzoyl peroxide (BPO), polyvinyl alcohol 1788 (PVA), sodium dodecylsulfate (SDS) and other reagents were analytical reagents and were used without further purification.

Preparation of Polystyrene Seed Emulsion

To a 250 ml beaker, 18.12 g (0.174 mol) of the purified styrene, 200 mg (0.840 mmol) of potassium peroxidisulfate and 200 ml of distilled water were added, and the mixture was then disposed by sonication for 20 min. The solution was poured Into a 250 ml flask with three necks: after purging with nitrogen gas for 5 min, all the necks were sealed and polymerization was carried out at 70[degrees]C for 18 h, stirring at 200 rpm. The resulting emulsion, containing polystyrene microspheres about 1.0 [micro]m in average size, was used as the seeds for the next preparation.

Preparation of Spherical Molecularly Imprinted Polymers

An emulsion prepared from 17.3 g toluene (0.188 mol), 1.13 g BPO (4.67 mmol) and 80 ml of PVA-water solution with the percentage concentration varying from 3.10 to 5.00 (% m/m) through 20 min sonication (JY-BII sonication apparatus) was added to 10 ml of the seed emulsion prepared above, and this first-step swelling was carried out at room temperature, stifling at 300 rpm for 2 h. A solution of 323.9 mg (1.83 mmol) of tyrosine and 29.28 mmol of functional monomers in 5 ml distilled water and 9.6 g (31.37 mmol) of TRIM was mixed by stirring at 150 rpm for 1.5 h. The mixture was dispersed into 100 ml of water with the same dosage of PVA as above, and sonicated until the oil phase disappeared to form a complete emulsion. Then this emulsion was diluted with a certain amount of distilled water and added into the seed emulsion and the second-step swelling was carried out at room temperature for 2 h with stirring at 150 rpm. Finally, the solution was poured into a one-neck 500 ml flask. After adding the reducer emul sion, which was prepared from 2.4 g (7.84 mmol) of TRIM, 0.382 g (3.16 mmol) NDMA, 80 mg (0.36 mmol) SDS and 20 ml water by sonicating for 20 min, the polymerization was carried out at room temperature for 24 h in a nitrogen atmosphere, stirring at 80 rpm with an electromagnetic stirrer. After polymerization, the resulting dispersoid of polymerized beads was poured into 600 ml of boiling water; the water temperature was maintained for 15 min while stirring at 150 rpm, and then filtrated to obtain the polymer beads. After washing again with 1000 ml water, the beads were dispersed into 34.676 g (0.3769 mol) of toluene for 2 h, followed by filtration. Last, the beads were washed and filtered for 2 times by 78.99 g (1.36 mol) of acetone and 39.14 g (0.955 mel) acetonitrile, respectively, followed by drying at room temperature. The control polymer beads were prepared in the same recipe without the addition of tyrosine.

Chromatography

A small volume of methanol was added to the prepared beads, and the slurry was dispensed into 1/16inch stainless steel tubing (45 cm x 40 mm ID). The column was connected to an instant liquid chromatographic instrument (made by us), composed of a LC9A pump and a 756MC UV-VIS spectrophotometer (the 2nd Analytical Apparatus Factory of Shanghai, China). Removal of the tyrosine molecule from the column was accomplished at a flow rate of 2.5 ml/min with a 10% acetic acid in methanol solution until elution of tyrosine could be no longer detected (X [lambda] = 276 nm). Then the column was washed by water until neutral. For detection of adsorbing dynamics behavior, 5 ml of 0.05 mol/1 of tyrosine solution was injected into the column, and the solution was made circularly flowing in the column maintaining the flow-rate at 1.5 ml/min. The concentration of the tyrosine solution ([C.sub.t]) at certain time was then detected every 5 min at 276 nm until the decrease of [C.sub.t] was imperceptible (less than 0.001 mol/l in 2 0 min), and the amount of tyrosine adsorbed at the time was calculated from the function [Q.sub.t] = V([C.sub.o] - [C.sub.t])/W, where [C.sub.o] is the original concentration of tyrosine solution, V is the volume of the solution and W is the mass of solid phase. For detection of thermodynamics behavior, 20 ml of tyrosine solution with different concentrations were injected and the relative [C.sub.R] (which is defined as the [C.sub.t] when the adsorbing is in a relative equilibrium stage, that is, when the change of [C.sub.t] in 20 min was less than 0.001 mol/l) was detected, and QR was calculated as well for drawing [Q.sub.R] vs. [C.sub.R] curves. For molecular selecting detection, 5 ml of 0.05 mol/l tyrosine and phenylalanine mixed solution was added to the column, and the [C.sub.R]S of tyrosine and phenylalanine were detected at 276 nm and 206 nm, respectively, and then the [Q.sub.R] of tyrosine and phenylalanine on the solid phase were calculated, based on which the molecular selecting properties of MIPs w ere evaluated (described in detail in the following section).

RESULTS AND DISCUSSION

Preparation of Imprinted Polymers With Different Dosages of PVA

In the previous work, non-imprinted poly-TRIM beads were prepared by suspension polymerization in water with diameters in the range of 10 [micro]m to 150 [micro]m, but their diameter dispersion was very wide (25), which is not convenient for use. In the present study, the seed swelling and suspension polymerization method was introduced to prepare imprinted poly-TRIM beads with better size distribution in order to make it more convenient for use. According to our previous study (26), BPO and NDMA were used as the oxidant and reducer, respectively, for the red-ox initiating system, and toluene as porogenous solvent (porogen) during polymerization. In the swelling steps, a homogeneous phase of the compound of porogen, initiator and polymerizing monomers was needed, which was helpful to make the swelling smooth and complete. In the present work, PVA was used as the dispersant in the compound of toluene and BPO in the first swelling step, and the compound of TRIM, tyrosine and functional monomers in the second st ep of swelling. The results showed that the size distribution of the polymer beads was greatly influenced by the dosage of the PVA (see Fig. 2). It can be clearly seen from the figure that the beads produced with the lower concentration of PVA have a wider size distribution, but are on average larger and are more often misshapen than the beads prepared with the higher concentration of PVA. When the concentration of PVA-water solution was as low as 3.10 (% in/in), the polymers were rarely in bead form (see Fig. 2a). On the other hand, when the concentration of PVA-water solution was higher than 4.6 (% rn/in), again many misshapen beads appeared (see Fig. 2d). So, under the conditions of the present work, 4.6 (% in/in) of PVA-water solution was needed to prepare the imprinted beads with more desirable size distribution (see Fig. 2c).

Preparation of Imprinted Polymers With Different Ratios of Water to TRIM

In a typical suspension polymerization, a certain ratio of water to oil is needed to prepare the polymer beads with the expected physical properties. In the seed swelling and suspension polymerization utilized in this paper, the ratio of water to TRIM (WIT) proved to have considerable influence on both the size distribution and the number of the misshapen beads of the result. SEM pictures of some beads prepared with different WIT are shown in Fig. 2c and Fig. 3. It can be seen that the size distribution gets narrower and the number of misshapen beads decreases with an increase of the WIT in the scope of the present work. When W/T was up to 46:1 (v/v), misshapen beads could be almost avoided (see Fig. 3b), and polymeric beads of 126 [micro]m in average size were obtained, which are much more desirable than beads prepared with 30:1 or 40:1 W/T (see Fig. 2c and Fig. 3a, respectively) for classic liquid chromatography or solid phase ex-fraction units.

Adsorption Behavior of Imprinted Polymers With Different Functional Monomers

Since functional monomers (FM) are theoretically important for effective imprinting, a series of imprinted polymers with methacrylic acid (MAA), acrylamide (AM), 2-vinylpyridine (VP) and AMPS were used as FM, respectively, and were prepared under the optimized dosage of PVA and WIT, in order to discern the influence of FM and find a feasible FM for molecule imprinting. The molar ratio of tyrosine, FM and TRIM was set at 1:16:17 during the preparation, according to our previous work (27). The curves of adsorption dynamic of some of the polymer beads with different functional monomers are shown in Fig. 4. It can be seen that the figures of the dynamic curve of all the MIPs were similar to each other. But the values of [Q.sub.R] (the solid phase concentration in a relative equilibrium stage] of the different MIPs were obviously different (see Table 1). The MIPs with MAA as functional monomers (polymer a) had the lowest [Q.sub.R] (0.18 mol/ kg], while the MIPs with AMPS as functional monomers (polymer d) had the highest (0.276 mol/ kg).

To evaluate the thermodynamics behavior of the imprinted polymers with different FM, the curves of the adsorption thermodynamics of the imprinted polymers are desirable. Figure 5 shows these curves of polymers a, b, c and d prepared using MAA, AM, 2-vinylpyridine and AMPS as FM, respectively. It can be clearly seen that the [Q.sub.R] at the same [C.sub.R] (concentration of solute in a equilibrium stage] increases in the sequence of polymer a, b, c and d, when [C.sub.R] is lower than 0.06 mol/l. Although all the [Q.sub.R] of the MIPs increased with the increase of the value of [C.sub.R], the [Q.sub.R] of polymer d increased more rapidly than that of the others when [C.sub.R] was lower than 0.33 mol/l, and the adsorption was up to saturation with the lowest [C.sub.R]. Since the adsorption of the beads follows the rule of Langmuir (26), according to the adsorption equation at certain temperature of Langmuir as Eq 1:

[C.sub.R]/[Q.sub.R] = [C.sub.R]/[Q.sub.0] + 1/(K[Q.sub.0]) (1)

where K is the adsorbing constant and [Q.sub.0] is the asymptotic maximum solid phase concentration.

The plots of [Q.sub.R]/ [C.sub.R] vs. [C.sub.R] were found to be basically linear (Fig. 6). According to these plots and Eq 1. the [Q.sub.0] and K of the polymers with different FM were calculated; see Table 2. As can be seen, the [Q.sub.0] of polymer a and c were very near to that of polymer b and d, respectively. However, the [Q.sub.R] at the same [C.sub.R] was much different when [C.sub.R] was lower than 0.05 mol/1. On the other hand, polymer d prepared using AMPS as FM had the highest value of [Q.sub.0] (0.282 mol/kg) as well as K (198.8 1/mol), and therefore had more desirable adsorption capabilities than the other three types of beads, which were prepared with MAA, AM, VP as FM, respectively.

Molecule Selectivity of Imprinted Polymers With Different Functional Monomers

Usually, the static distributing coefficient [K.sub.D], separating factor a and the ratio of separating factor [beta] are utilized to evaluate the molecule selectivity capability of MIPs. [K.sub.D], a and [beta] are defined as follows:

[K.sub.D] = [Q.sub.R]/[C.sub.R] (2)

where [K.sub.D] demonstrates the adsorbing capability of the sorbents.

[alpha] = [K.sub.Di]/[K.sub.Dj] (3)

[K.sub.Di] and [K.sub.Dj] are the static distributing coefficient of printing molecule and competitive molecule, respectively. [alpha] demonstrates the molecule selectivity of sorbents. The higher the value of [alpha], the better the selectivity. If [alpha] is 1.0 or lower than 1.0, the sorbent has no selecting property to the printing molecule.

[beta] = [[alpha].sub.M]/[[alpha].sub.C] (4)

[[alpha].sub.M] is the separating factor of MIPs, [[alpha].sub.C] is the separating factor of control polymers, and [beta] demonstrates the difference of molecule selectivity between MIPs and control polymers. The higher the value of [beta], the greater the difference. When [beta] is 1.0, this means there is no difference between MIPs and the control polymers.

In the present work, phenylalanine was used as the competitive molecule to evaluate the molecule selectivity of the polymers, because the molecular structure of phenylalanine is the same as the tyrosine, except that the tyrosine has a hydroxyl on the opposite side of the phenylalanine (see Fig. 1b and c). The result of the molecule selecting analysis (see Table 3) shows that all the imprinted polymers exhibit an inherent selectivity for tyrosine, the printing molecule, indicated by their a values much higher than 1.0. The control polymers had no distinct selectivity for tyrosine or phenylalanine at all, indicated by the a value are 1.0 or so, and the considerable difference of molecule selecting between MIPs and control polymers is obviously demonstrated by the values of [beta], which are between 1.44 and 1.87. The MIPs prepared with different FM. however, have different molecule selecting capabilities, as can be seen from the [alpha] values of these polymers that the MIPs with AMPS and VP as functional monom ers exhibited the better molecule selecting capabilities in all the four imprinted polymers, whereas the MIPs with MAA and AM as FM had the poorer molecule selecting capacities. The difference of molecule selecting and adsorbing capabilities are considered to lie on the different combining capability of the FM to printing molecules. MAA and AM are likely to combine tyrosine through only hydrogen bond (see Fig. 7a, b), which is proved to be easily weaken by the polar solvent, especially water, and therefore the molecule adsorbing and molecule selecting properties of the imprinted polymers are weaker. Vinylpyridine, however, has been reported to interact with amino acid via ionic bond (13), which is more effective than hydrogen bond in water, so the relevant MIPs had better molecule adsorbing and selecting capabilities (Fig. 7c). As for AMPS, sulfonic group is involved in the structure (Fig. 1a), which is likely to more effectively interact with amidogen through ionic bond in aqueous system (Fig. 7d), so it worked better than the other three functional monomers in the present water system, and therefore the imprinted polymers using AMPS exhibited the most desirable properties including the adsorbing and selecting properties in this paper.

CONCLUSIONS

The seed swelling and suspension polymerization method was described for preparing molecularly imprinted polymers (MIPs) In bead form, using trimethylolpropane trimethacrylate (TRIM) as crosslinker. The appearance of the beads was influenced by the concentration of dispersant (polyvinyl alcohol), and the ratio of water to TRIM (W/T) during polymerization. When W/T was 46:1 (V/V) and the concentration of WA-water solution was 4.6 (% m/m). the result was more desirable size distribution with fewer misshapen beads.

Tyrosine was imprinted using methacrylic acid (MAA), acrylamide (AM), 2-vinylpyridine (VP) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as functional monomers (FM), respectively. All the MIPs exhibited an obvious molecule selecting capability to tyrosine, while the control polymers did not. The MIPs using AMPS had the more desirable adsorption and molecule selectivity than the others in the present work with the value of asymptotic maximum solid phase concentration [Q.sub.0] and the separating factor [alpha] up to 0.282 mol/kg and 1.93, respectively, when phenylalanine was used as the competitive molecule.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]
Table 1

The Value of [Q.sub.R] of the Imprinted Polymers With Different
Functional Monomers.

Imprinted polymers a b C d

Functional monomer MAA AM VP AMPS
[Q.sub.R] (mmol/g) 0.180 0.206 0.248 0.276

Table 2

The Value of [Q.sub.0] and K of the Imprinted Polymers With Different
Functional Monomers.

Imprinted polymers a b c d

Functional monomer MAA AM VP AMPS
K (I/mol) 95.4 127.7 155.5 198.8
[Q.sub.0] (mol/Kg) 0.234 0.234 0.274 0.282

Table 3

Molecule Selectivity of Tyrosine Imprinted Polymers With Different
Functional Monomers and the Relevant Control Polymers.

 [K.sub.D]I/kg
Sorbent * phenylalanine tyrosine [alpha] [beta]

MIPs-a 0.90 1.38 1.53 1.44
CP-a 0.81 0.86 1.06 /
MIPs-b 0.91 1.48 1.63 1.54
CP-b 0.84 0.89 1.06 /
MIPs-c 1.1 1.92 1.74 1.66
CP-c 1.31 1.38 1.05 /
MIPs-d 1.04 2.01 1.93 1.87
CP-d 1.37 1.41 1.03 /

* MIPS a, b, c and d were the tyrosine imprinted polymers using MAA, AM,
VP and AMPS as functional monomers, respectively, and CM were the
relative control polymers of the imprinted one.


ACKNOWLEDGMENT

The authors would like to thank Dr. Li Fang for liquid chromatography and the National Natural Science Foundation of China (grant number: 29906008) for supporting this research work.

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LTYONG ZHANG (a), GUOXIANG CHENG (*,a), CONG FU (a), and XIAOHANG LIU (b)

(a) School of Materials Science & Engineering Tianjin University, Tianjin 300072, China

(b) State Key Lab of Functional Polymer Materials, Nankai University, Tianjin 300071, China

* To whom correspondence should be addressed: gxcheng@tju.edu.cn.
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Author:Zhang, Liyong; Cheng, Guoxiang; Fu, Cong; Liu, Xiaohang
Publication:Polymer Engineering and Science
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Date:Apr 1, 2003
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