Anal. Chem. 1996, 68, 3769-3774
Molecularly Imprinted Polymer Beads: Suspension Polymerization Using a Liquid Perfluorocarbon as the Dispersing Phase Andrew G. Mayes† and Klaus Mosbach*
Pure and Applied Biochemistry, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden
A suspension polymerization technique suitable for molecular imprinting is described, based on the use of a liquid perfluorocarbon as the dispersing phase. This dispersant does not interfere with the interactions between functional monomers and print molecules required for the recognition process during molecular imprinting. The method produces polymer beads, with almost quantitative yield, which can be used after only a simple washing step. An acrylate polymer with perfluorocarbon and poly(oxyethylene) ester groups was used to stabilize an emulsion of functional monomer, cross-linker, print molecule, initiator, and porogenic solvent in perfluoro(methylcyclohexane). Initiation of polymerization by UV irradiation resulted in polymer beads. The average bead size could be controlled between about 50 and 5 µm by varying the amount of stabilizing polymer. SEM of the beads indicated spherical particles with morphology typical of beads made by suspension polymerization. The technique was applicable to a range of conditions typically used for molecular imprinting. A detailed chromatographic study of the polymer beads confirmed that r values and resolution factors were similar to those achieved with traditional ground and sieved imprinted polymers. Small (5 µm) beaded packings gave low back pressure and rapid diffusion, giving good separation even at high flow rates. Molecular imprinting involves arranging polymerizable functional monomers around a print molecule. This is achieved by utilizing either noncovalent interactions such as hydrogen bonding, ion-pair interactions, etc. (noncovalent imprinting) or reversible covalent interactions (covalent imprinting) between the print molecule and the functional monomers. The complexes formed are then incorporated by polymerization into a highly cross-linked macroporous polymer matrix. Extraction of the print molecule leaves sites in the polymer with specific shape and functional group complementarity to the original print molecule. The technique is now well established and has been recently reviewed.1-4 Molecular imprinting has considerable potential in the area of chiral separations. Many different racemic compounds have been † Present address: University of Cambridge, Institute of Biotechnology, Tennis Court Road, Cambridge CB2 1QT, England. (1) Mosbach, K.; Ramstro ¨m, O. Bio/Technology 1996, 14, 163-170. (2) Wulff, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1812-1832. (3) Shea, K. J. Trends Polym. Sci. 1994, 2, 166-173. (4) Steinke, J.; Sherrington, D. C.; Dunkin, I. R. Adv. Polym. Sci. 1995, 23, 81-125. (5) Andersson, L. I. Ph.D. Thesis, University of Lund, Sweden, 1990.
S0003-2700(96)00363-0 CCC: $12.00
© 1996 American Chemical Society
successfully resolved, including amino acid derivatives,5,6 drugs,7,8 and sugars.9,10 Baseline resolution has been achieved in many cases. In a recent study in our laboratory, an R value of 17.8 was achieved for enantiomeric resolution of a dipeptide using only noncovalent, nonionic interactions.11 A major advantage of molecularly imprinted polymers, in contrast to other chiral stationary phases, is the predictable order of elution of enantiomers. To date, most imprinting studies have been made using polymers prepared by bulk polymerization methods, using a porogenic solvent to create a block of macroporous polymer. This is subsequently crushed, ground, and sieved to an appropriate particle size for the particular studies which will be undertaken. For chromatographic evaluation of the polymers, for example, particles of less than 25 µm are generally used. The grinding process is unsatisfactory for several reasons. It inevitably produces irregular particles as well as a considerable quantity of “fines” which have to be removed by sedimentation. The process is also labor intensive and wasteful. Typically less than 50% of the ground polymer is recovered as useable particles. Irregular particles generally give less efficient column packing for chromatography and often prove troublesome in process scaleup; hence, beaded polymers would be preferable in most cases. Suspension and dispersion polymerization methods for producing beads from acrylic monomers similar to those usually used in imprinting are well established.12-19 In principle, these approaches offer a very attractive alternative to bulk polymers since (6) Andersson, L. I.; O’Shannessy, D. J.; Mosbach, K. J. Chromatogr. 1990, 513, 167-179. (7) Fischer, L.; Mu ¨ ller, R.; Ekberg, B.; Mosbach, K. J. Am. Chem. Soc. 1991, 113, 9358-9360. (8) Kempe, M.; Mosbach, K. J. Chromatogr. 1994, 664, 276-279. (9) Wulff, G.; Schauhoff, S. J. Org. Chem. 1991, 56, 395-400. (10) Mayes, A. G.; Andersson, L. I.; Mosbach, K. Anal. Biochem. 1994, 222, 483-488. (11) Ramstro ¨m, O.; Nicholls, I. A.; Mosbach, K. Tetrahedron Asymmetry 1994, 5, 649-656. (12) Svec, F.; Fre´chet, J. M. Anal. Chem. 1992, 64, 820-822. (13) Guyot, A. In Synthesis and Separations using Functional Polymers; Sherrington, D. C., Hodge, P., Eds.; John Wiley and Sons: London, 1988; pp 1-42. (14) Munger, M.; Trommsdroff, E. In Polymerization Processes; Schildknecht, C. E., Skeist, I., Eds.; Wiley-Interscience: New York, 1977; pp 106-142. (15) Nilsson, H.; Mosbach, R.; Mosbach, K. Biochem. Biophys. Acta 1972, 268, 253-256. (16) Reinholdsson, P.; Hargitai, T.; Isaksson, R.; To¨rnell, B. Angew. Macromol. Chem. 1991, 192, 113-132. (17) Dispersion Polymerization in Organic Media; Barrett, K. E. J., Ed.; John Wiley and Sons: London, 1975. (18) Williamson, B.; Lukas, R.; Winnik, M. A.; Croucher, M. D. J. Colloid Interface Sci. 1987, 119, 559-564. (19) Paine, A. J. J. Polym. Sci. A: Polym. Chem. 1990, 28, 2485-2500.
Analytical Chemistry, Vol. 68, No. 21, November 1, 1996 3769
they should produce a higher yield of particles with better chromatographic characteristics. In almost all cases, however, water or a highly polar organic solvent (e.g., an alcohol) is used as the continuous phase for the relatively hydrophobic monomers. These solvents are incompatible with most covalent and noncovalent imprinting mixtures due to the competition between solvent and functional monomers for specific interaction with the print molecule. Since the dispersing solvent is present in large molar excess, it saturates the monomer phase and drastically reduces the number and strength of the interactions between functional monomers and print molecule. In addition, the high solubility of acidic monomers in water means that random copolymerization of monomers and cross-linker is probably not achieved. Watersoluble print molecules would also be lost due to partitioning into the aqueous phase. Not unexpectedly, attempts to make molecularly imprinted polymer beads by suspension polymerization in water have led to only very poor recognition. A few attempts have been made to produce composite beaded particles by imprinting in the pore network of preformed beaded silica20,21 or tris(hydroxymethyl)propane trimethacrylate (TRIM).22 While the results showed some promise, the preparation requires careful handling, and the volume of imprinted polymer per unit column is inevitably reduced by the beads themselves. Two recent publications23,24 report the use of an unstabilized dispersion polymerization in a polar solvent mixture for molecular imprinting. The process produces random precipitates rather than regular beads and gives good results only for rather highly charged print molecules, such as pentamidine and tri-O-acetyl adenosine, presumably due to the presence of competing solvent effects. An attempt has also been made to use the “aqueous two-step swelling” procedure to make monodisperse imprinted particles.25 The quality of the beads produced was excellent, and because of the high efficiency of the column packing, chromatographic performance was much improved compared with previous data for the same print molecules (aminonaphthalene derivatives).26 Unfortunately, this technique also suffers from the need for an aqueous phase during the swelling procedure, so it is likely to show the same limitations of applicability as suspension and dispersion methods. For most imprinting situations, a different approach is required, avoiding the use of dispersants which interfere with the interactions used for recognition between print molecule and functional monomers. We have developed an entirely new suspension polymerization technique based on emulsions of noncovalent imprinting mixtures formed in liquid perfluorocarbons. The latter are largely immiscible with most organic compounds and hence form an appropriate inert phase for suspension polymerization. To create reasonably stable emulsion droplets containing monomers, cross-linkers, print molecules, and porogenic solvents, fluorinated surfactants and a range of surface-active polymers containing fluorinated units were tested to find a suitable candidate for use as a stabilizer. This paper reports the suspension (20) Norrlo¨w, O.; Glad, M.; Mosbach, K. J. Chromatogr. 1984, 299, 29-41. (21) Wulff, G.; Oberkobusch, D.; Minarik, M. React. Polym. 1985, 3, 261-2757. (22) Glad, M.; Reinholdsson, P.; Mosbach, K. React. Polym. 1995, 25, 47-54. (23) Sellergren, B. J. Chromatogr. A 1994, 673, 133-141. (24) Sellergren, B. Anal. Chem. 1994, 66, 1578-1582. (25) Hosoya, K.; Yoshizako, K.; Tanaka, N.; Kimata, K.; Araki, T.; Haginaka, J. Chem. Lett. 1994, 1437-1438. (26) Matsui, J.; Kato, T.; Takeuchi, T.; Suzuki, M.; Yokoyama, K.; Tamiya, E.; Karube, I. Anal. Chem. 1993, 65, 2223-2224.
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Analytical Chemistry, Vol. 68, No. 21, November 1, 1996
polymerization technique, the physical characterization of the resulting beads, and a study of the performance of the imprinted beads in HPLC. EXPERIMENTAL SECTION Ethylene glycol dimethacrylate (EDMA) and methacrylic acid (MAA) (Merck, Darmstadt, Germany) were distilled under reduced pressure prior to use. Tris(hydroxymethyl)propane trimethacrylate (TRIM), styrene, methyl methacrylate, and benzyl methacrylate (Aldrich Chemie, Steinheim, Germany) were used as received. 2,2′-Azobis(isobutyronitrile) (AIBN) came from Janssen Chimica (Geel, Belgium). 2-(N-Ethylperfluoroalkylsulfonamido)ethanol (PFA-1) (Fluorochem, Old Glossop, UK) and PEG2000 monomethyl ether (MME) (Fluka Chemie A. G., Buchs, Switzerland) were converted to their acrylates by reaction with acryloyl chloride and triethylamine in dichloromethane, but they are also available commercially (from Fluorochem and Polysciences, respectively). Perfluoro(methylcyclohexane) (PMC) and fluorad FC430 were also obtained from Fluorochem. Boc-D-Phe, Boc-L-Phe, and Boc-DL-Phe came from Bachem A.G. (Bubendorf, Switzerland). Chloroform (HPLC grade) was passed down a basic alumina column to remove ethanol and stored over molecular sieves for use as porogenic solvent during imprinting. For HPLC it was used as received. Toluene was dried with sodium and acetone with molecular sieves prior to use. Other solvents were of analytical grade or better and were used as received. Synthesis of Perfluoro Polymeric Surfactant (PFPS). Acryloyl PFA-1 (4 g, 7.2 mmol) and acryloyl PEG2000MME (0.76 g 0.36 mmol) were dissolved in 10 mL of chloroform. AIBN (24 mg, 76 mmol) was added, and dissolved oxygen was removed by nitrogen sparging for 5 min. The tube was then sealed and polymerized at 60 °C for 48 h in a shaking water bath. The resulting solution was slightly turbid and became much more turbid on cooling. Most of the solvent was removed by slow evaporation at 30 °C under reduced pressure (to avoid foaming), and the remainder was removed under reduced pressure at 60 °C. The resulting polymer was a sticky pale yellow paste which was used without further treatment. Other random and graft copolymers were synthesized similarly using the appropriate ratios of monomers and 1 mol % AIBN as initiator. Suspension Polymerizations. The amount of porogenic solvent required to just saturate 20 mL of PMC was determined. The required amount of PFPS was dissolved in this volume of porogenic solvent in a 50 mL borosilicate glass tube, and 20 mL of PMC was added and the mixture shaken to give a uniform white to opalescent emulsion. Five milliliters of “imprinting mixture” (see Table 1 for compositions) was added and emulsified by stirring at 2000 rpm for 5 min. The emulsion was purged with nitrogen for 5 min and then polymerized by UV irradiation at 366 nm at room temperature under a gentle nitrogen stream with stirring at 500 rpm. Polymerization was continued for 3 h. The resulting polymer particles were filtered on a sintered glass funnel, and the PMC was recovered. The beads were washed extensively with acetone, sonicating to break up loose aggregates of beads (large aggregates were broken up by gentle crushing with a spatula), before drying and storing. The simple polymerization apparatus used for all polymerizations in this study comprised a 50 mL borosilicate glass tube with screw lid, the center of which was drilled to allow the shaft of a stainless-steel flat blade stirrer to pass through. The stirrer blade was about half the length of
the tube. A rubber seal was used to reduce evaporation through this hole. An additional small hole in the lid allowed a nitrogen stream to be fed into the tube via a syringe needle. The tube was held vertically in a retort stand and stirred with an overhead stirrer. A UV lamp was placed about 5 cm away from the tube, and the lamp and tube were surrounded with aluminum foil to maximize reflected light. Bead Size Distributions. Suspensions of beads in acetone were dried onto microscope slides, and about 150 beads were measured at random using a calibrated graticule in an optical microscope. Either 100× or 400× magnification was used, depending on the particle size. Some samples were also imaged by SEM. Measurements made from these images compared well with the results from optical determinations. SEM. Polymer beads were placed on aluminum pegs and sputter coated with 15 nm of gold using a Polaron E5150 gold coater. Images were then obtained using an ISI 100A SEM at 25 kV in order to compare the sizes, surfaces, and pore structures of beads produced under different conditions. HPLC. Beads were suspended in chloroform/acetone (17:3) by sonication and slurry packed into 10 cm × 0.46 cm or 25 cm × 0.46 cm stainless-steel columns at 300 bar using an air-driven fluid pump and acetone as solvent. The columns were washed with 250 mL of methanol/acetic acid (9:1) and then equilibrated with chloroform containing 0.1% or 0.25% acetic acid. 10 µg BocD- or L-Phe or 20 µg of the racemate in 20 µL of solvent was injected, and chromatograms were recorded at 254 nm at a flow rate of 0.5 mL/min. Some separations were also run at higher flow rates and with larger amounts of compound loaded. Chromatographic parameters were calculated using standard theory.27 RESULTS AND DISCUSSION Making a stable emulsion of an imprinting mixture in a liquid perfluorocarbon proved to be quite difficult. It was hindered by the high density of the dispersant, which caused rapid “creaming” of the emulsion, thus favoring coalescence of dispersed droplets. Two commercial fluorinated surfactants (polyfluoro alcohol PFA1 and fluorad FC430) were tried, followed by a homopolymer of acryloyl PFA-1, a range of random copolymers of acryloyl PFA-1 with styrene, methyl methacrylate or benzyl methacrylate, and graft copolymers containing perfluoro ester groups and PEG groups attached to the main acrylate chain. Most of these, however, proved to be ineffective. Poly(acryloyl PFA1) gave sufficiently stable emulsions for suspension polymerization and gave good-quality beads. Unfortunately, due to its poor solubility, this surfactant proved to be difficult to remove from the surface of the beads after polymerization, resulting in extremely hydrophobic surfaces. The best emulsion stability was achieved using a copolymer of acryloyl PFA1 and acryloyl PEG2000MME (mole ratio 20:1, termed PFPS), and this polymer was used in all the bead polymerizations presented in this paper. The structures of PFPS and its precursor monomers are shown in Figure 1. Table 1 summarizes the compositions of the polymerization mixtures for the polymers presented in this paper. The resulting beads are referred to in the text using the PF code numbers from column 1. Varying the amount of PFPS used during the polymerization gave control over the bead sizes which were obtained. Figure 2 (27) Snyder, L. R.; Kirkland, J. J. Introduction to Modern Liquid Chromatography, 2nd ed.; Wiley: New York, 1979.
Structure of the Hydrophobic/Oleophobic Side Chain Unit CnF2n+1SO2N(C2H5)CH2CH2OH CnF2n+1SO2N(C2H5)CH2CH2O
CO
CH
PFA-1 CH2
acryloyl PFA-1
(where n averages 7.5) Structure of the Hydrophilic/Oleophilic Side Chain Unit CH3O(CH2CH2O)mCH2CH2OH CH3O(CH2CH2O)mCH2CH2O CO
CH
PEG2000MME CH2
acryloyl PEG2000MME
(where m averages approximately 43) Structure of Polymeric Surface-Active Agent PFPS CH2CH
p
CH2CH
C O
q
C PFA-1
O
PEG2000MME
(where p averages 19 and q averages 1)
Figure 1. General structure of the polymeric surface-active agent (PFPS) used to stabilize dispersions of “imprinting mixtures” in perfluoro(methylcyclohexane) (PMC). The structures of the two acrylate precursors from which the polymer was synthesized are also shown. Polymerizations were done for 48 h at 60 °C in chloroform using 1 mol % AIBN as initiator.
is a graph of the mean and standard deviation for beads made in the presence of different amounts of PFPS, for a “standard” recipe containing 2 g of monomers in 5 mL total volume (see Table 1). Ten milligrams of PFPS is at the lower limit of the range where stable emulsions can be formed, and quite a lot of aggregate was also present in this sample. Using 150 mg or more of PFPS gave only very small irregular particles of 1-2 µm. No beads were apparent in these samples. The method of emulsion formation was also found to influence the outcome of the polymerizations markedly. The method adopted, stirring at 2000 rpm for 5 min, appeared to be very efficient and gave good uniformity and reproducibility in the required size range (2-25 µm). Five separate polymerizations performed on different days using 25 mg of PFPS as emulsifier gave a mean of 19.7 µm and a standard error of 0.6 µm. Emulsification in an ultrasonic bath for 5 min gave much broader size distributions with many very small particles, while simply shaking the tube three or four times gave good results if larger beads were required (40-100 µm). Clearly, the balance of emulsification method and PFPS amount needs to be optimized for a particular application. The conditions reported here give beads suitable for HPLC. Temperature also affected the outcome of the polymerizations. Attempts to use thermal initiation at 45 °C using ABDV as initiator gave only small irregular fragments. UV initiation of polymerization at 4 °C led to a large amount of aggregation. Most polymerizations were performed at ambient temperature (about 20 °C), although some temperature increase occurred during polymerization due to the proximity of the UV lamp. Polymerizations carried out during a spell of very warm weather, when ambient temperature reached 30 °C, gave slightly smaller beads, indicating that polymerization temperature is important and should be optimized and controlled in order to achieve completely reproducible results. Test polymerizations were carried out in a range of solvents commonly used in molecular imprinting: chloroform, toluene, acetonitrile, and acetone. The polymerization method is applicable to all of these solvents except acetonitrile, which produced only irregular aggregates, and hence should be appropriate for most imprinting situations. The size and surface structure of the beads produced depend on the porogenic solvent used. Both toluene Analytical Chemistry, Vol. 68, No. 21, November 1, 1996
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Table 1. Composition Data for the Polymers Made in This Study polymer
print molecule (amount, mg)
MAA (g)
EDMA (g)
solvent (amount, g)
PFPS (mg)
PF1 PF2 PF3 PF4 PF5 PF6 PF7 PF8 PF9 PF10 PF11 PF12 PF13 PF14 PF15 PF16
Boc-L-Phe (120) Boc-L-Phe (120) Boc-L-Phe (120) Boc-L-Phe (120) Boc-L-Phe (120) Boc-L-Phe (120) Boc-L-Phe (120) Boc-L-Phe (120) Boc-L-Phe (120) Boc-L-Phe (120) Boc-L-Phe (68) Boc-DL-Phe (120) none none Boc-L-Phe (308) Boc-L-Phe (308)
0.16 0.16 0.16 0.16 0.16 0.16 0.16 0.32 0.48 0.64 0.265 0.16 0.16 0.16 0.4 0.4
1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.57 (TRIM) 1.57 (TRIM)
chloroform (4.2) chloroform (4.2) chloroform (4.2) chloroform (4.2) chloroform (4.2) chloroform (4.2) chloroform (4.2) chloroform (4.2) chloroform (4.2) chloroform (4.2) chloroform (4.2) chloroform (4.2) toluene (2.46) acetone (2.25) chloroform (4.6) chloroform (4.6)
10 25 50 75 100 200 500 25 25 25 25 25 25 25 100 25
Figure 2. Graph of bead diameter obtained against amount of PFPS added for a “standard polymerization” containing 1.84 g of EDMA, 0.16 g of MAA, 4.2 g of chloroform, and 20 mg of AIBN emulsified in 20 mL of PMC. (For polymerization method, see text.)
(26 µm ( 12 µm; mean ( SD) and acetone (52 µm ( 15 µm) gave larger beads than chloroform (18 µm ( 8 µm) for 25 mg of PFPS in a “standard” polymerization. Scanning electron micrographs (SEMs) of some of the beaded polymer preparations are shown in Figure 3. The method clearly produces reasonably spherical beads, both for EDMA (a-d)- and TRIM (e)-based polymers and for using a variety of porogenic solvents. The incidence of defects, such as surface indentations or small holes, is somewhat higher than is usually observed for water-based suspension polymerizations. A number of hollow beads were also observed, due to the fact that they had fractured. Such hollow beads have also been reported for conventional suspension polymerizations in water.28 The morphology of the beads is typical of beads made by suspension polymerization with a slightly denser and smoother surface layer covering a more open porous structure in the interior.29,30 The beads made using acetone as porogenic solvent (Figure 3d) differed from the others. They were larger and had much rougher surface morphology and more “debris” on their surfaces than those prepared using (28) Pelzbauer, Z.; Luka´sˇ, J.; Sˇvec, F.; Ka´lal, J. J. Chromatogr. 1979, 171, 101107. (29) Schmid, A.; Kulin, L.-I; Flodin, P. Makromol. Chem. 1991, 192, 1223-1234. (30) Hora´k, D.; Pelzbauer, Z.; Bleha, M.; Ilavsky, M.; Sˇvec, F.; Ka´lal, J. J. Appl. Pol. Sci 1981, 26, 411-421.
3772 Analytical Chemistry, Vol. 68, No. 21, November 1, 1996
Figure 3. Scanning electron micrographs of beads produced from suspension polymerization in PMC. Beads were placed on aluminum pegs and sputter coated with 15 nm of gold using a polaron E5150 coater. The images were obtained using an ISI 100A SEM at 25 kV. The magnification is 500×. (a, top left) PF2, (b, top right) PF10, (c, middle left) PF13, (d, middle right) PF14, and (e, bottom left) PF15.
chloroform or toluene. The beads made with toluene as porogenic solvent had a less dense surface shell and somewhat more porous interior structure than those made with chloroform. Figure 3b shows beads of polymer PF9, which has a lower proportion of cross-linker than that in Figure 3a (polymer PF2). The beads in Figure 3b are much more irregular and distorted, suggesting that these particles might remain softer and deformable for a longer time during polymerization and hence are more prone to distortion due to shear or collision. The internal morphology of these beads also appeared to be more open and porous than that of polymer PF2. This might contribute to the better HPLC performance of the latter (see below). To confirm that the polymer beads made by this method are really imprinted, and that the quality of the recognition sites is similar to that obtained by traditional bulk polymerization meth-
Table 2. HPLC Data for Separation of Boc-DL-Phe by 10 cm Columns of Beaded Polymers
polymer
print mol:MAA ratio
% cross-link
KD′
PF2 PF8 PF9 PF10 PF11 PF11b PF12
1:4 1:8 1:12 1:16 1:12 1:12 1:4
80 71 62.5 56 75 75 80
0.69 0.77 1.12 1.44 0.71 0.79 0.7
chloroform + 0.1% acetic acid K L′ R Rs 1.44 1.42 1.88 2.61 1.27 1.43 0.7
2.09 1.86 1.68 1.81 1.8 1.82 1
0.59 0.43 0.83 0.49 0.67 1.23 0
f/g
K D′
0.51 0.37 0.73 0.5 0.63 0.94 0
0.43 0.54 0.76 1 0.4 0.58 0.54
chloroform + 0.25% acetic acid K L′ R Rs 0.78 0.97 1 1.89 0.71 1.11 0.54
1.81 1.79 1.7 1.89 1.78 1.91 1
0.26 0.31 0.69 0.46 0.48 1.08 0
f/g 0.23 0.28 0.68 0.44 0.39 0.88 0
a The value f/g is calculated by measuring the vertical distance (f) to the trough between two peaks, from a point on the line joining the two adjacent peak maxima. This value is then divided by the vertical distance (g) to the baseline from the same point (hence, by definition, baseline separation has a value of 1 regardless of peak shape). b 25 cm column.
Figure 4. HPLC trace showing separation of Boc-DL-Phe by a 25 cm column of PF11. Conditions: the column was equilibrated with chloroform + 0.1% acetic acid; 20 µg of Boc-DL-Phe was injected in 20 µL of the same solvent and eluted at a flow rate of 0.5 mL min-1. The chromatogram was recorded at 254 nm.
ods, a range of polymers imprinted using Boc-L-Phe were evaluated by HPLC. This system was chosen since a great deal of information is available on the performance of traditional crushed bulk polymers imprinted with Boc-L-Phe. The results for HPLC evaluation of six of the polymers are summarized in Table 2. As the ratio of MAA to print molecule increased, the retention times and hence the capacity factors increased due to greater nonspecific interaction. The R values, however, stayed almost constant at about 1.8. This is very similar to the values obtained under similar conditions for ground and sieved bulk Boc-Phe polymers (range 1.77-2.17).31,32 The optimum resolution was found at a MAA:Boc-Phe ratio of 12:1, the Rs value of 0.83 being good for a 10 cm column. Using a longer column (25 cm) of polymer PF11 resulted in near baseline resolution of the enantiomers, as has previously been reported for bulk polymers.31,33 The chromatogram for this separation is shown in Figure 4. Interestingly, polymer PF9, which was only 62.5% cross-linked, performed better than polymer PF11, which had the same print molecule:MAA ratio but was 75% cross-linked. It has previously been shown that separations improve as the (31) Kempe, M.; Mosbach, K. Int. J. Pept. Protein Res. 1994, 44, 603-606. (32) Ramstro¨m, O.; Andersson, L. I.; Mosbach, K. J. Org. Chem. 1994, 58, 75627564. (33) Andersson, L. I.; Mosbach, K. J. Chromatogr. 1990, 516, 313-322.
degree of cross-linking increases within this range,34,35 but this was not observed in these experiments. Polymer PF11 was made using less print molecule (see Table 1), since it is not possible to vary these parameters independently (without introducing additional monomers into the mixture), and it is thus not clear whether the improved resolution was due to the larger number of binding sites or to changes in polymer morphology as a result of the lower cross-linking. Such observations indicate that significant improvements in separation can be achieved by careful optimization of the many compositional and operational variables. The simplicity and speed of the bead polymerization method makes extensive optimization possible. It has recently been reported36 that polymers based on the trifunctional cross-linker TRIM had much better resolution and load capacity than EDMA-based polymers for a range of di- and tripeptides. To further evaluate the suspension polymerization method, imprints of Boc-L-Phe were made in a TRIM-based polymer. Beads produced using 100 mg of PFPS (PF15) had an average diameter of 5.7 µm, and those using 25 mg of PFPS (PF16) had a diameter of 18.8 µm, very similar to what would have been expected for EDMA-based polymers with the same amount of stabilizing polymer (see Figure 1). Thus, in terms of bead size prediction, TRIM and EDMA seem to behave very similarly. A SEM picture of beads of PF15 is shown in Figure 3e. These beads were tested by HPLC and gave excellent resolution and high load capacities, as was also noted for the ground and sieved bulk polymers.36 The packed column had very low back pressure, and high resolution could be achieved, even at quite high flow rates. Figure 5 shows a series of chromatograms for flow rates between 0.5 and 5 mL min-1. Little difference was observed between 0.5 and 2 mL min-1, suggesting that diffusion rates are rapid for these small beads. The back pressure was very low and resolution excellent (f/g ) 0.89, 0.89, and 0.85 at 0.5, 1, and 2 mL min-1, respectively). Reasonable resolution (f/g ) 0.61) was still achieved at 5 mL min-1 (back pressure, 1300 psi). Ground and sieved random
