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Enantioselective permeability of membranes prepared from polyacrylonitrile-graft-(1→6)-2,5-anhydro-d -glucitol
REACTIVE & FUNCTIONAL POLYMERS
ELSEVIER
Reactive
& Functional
Polymers
37 (1998) 293-298
Short communication
Enantioselective permeability of membranes prepared from polyacrylonitrile-guaft-( l-,6)-2,5anhydro-D-glucitol Toshifumi Satoh a, Yoshiki Tanaka a, Kazuaki Yokota a,*, Toyoji Kakuchi b ’ Division of Molecular Chemistry Graduate School of Engineen’ng, Hokkaiab University, Hokkaido, Sapporo 060, Japan b Division
of Bioscience, Graduate School of Environmental Earth Science, Hokkaido Universiiy, Hokkaido, Sapporo 060, Japan Received
20 June 1997; revised version received 28 August
1997; accepted
11 September
1997
Abstract Enantioselective permeation of racemic amino acid perchlorates has been studied using a solid membrane prepared from polyacrylonitrile-grujr-aft-(l-+ 6)-2,5-anhydro-3,4-d-O-methyl-D-glucitol. The permeation rates of the amino acids decreased in the order of phenylglycine > phenylalanine > tryptophane, according to the molecular size of the guest compounds. For all the amino acids tested, the permeation rate of the D-isomer was greater than that of the L-isomer. When Dand L-phenylglycine perchlorates were permeated, the D-isomer permeated 1.17 times faster than the L-isomer. During transport using racemic phenylglycine perchlorate, the D-isomer was permeated first and the optical purity was 75% ee (max.). For the adsorption experiment for racemic phenylglycine, the L-isomer was more strongly adsorbed than the D-isomer (5.8% ee). From these data, it was directly made clear that the (1+6)-2,5-anhydro-3,4-di-O-methyl-D-glucitol moieties in the membrane preferentially interacted with the L-isomer rather than with the D-isomer. 0 1998 Elsevier Science B.V. All rights reserved. Keywords:
Enantioselective permeability; Membrane; Optical resolution; Macromonomer; (l-+6)-2,&knhydro-D-glucitol
1. Introduction The anionic cyclopolymerization of 1,2 : 5,6dianhydro-3,4-di-0-methyl-D-m&to1 (1) using potassium tert-butoxide proceeded regio- and stereoselectively to produce a well-defined polymer consisting of Only 2,5-anhydro-3,4-di-0-methyl-D-glucitol moieties, that is, (1+6)-2,5-anhydro-3,4-di-o-methyl-D-glucitol (2) [1,2]. The structural characteristic of 2 is a lack of the anomeric linkage which is found in the naturally occurring polysaccharides, * Corresponding author. Tel.: +81 (11) 706-6602, 706-6602, E-mail: [email protected]
Fax: +81 (11)
1381-5148/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII S1381-5148(97)00129-6
and hence, the polymer is a new type of polycarbohydrate (see Scheme 1). Polymer 2 acted as a host in the host-guest complexation and exhibited cation-binding selectivity toward metal ions [3-51. This host-guest complexation mechanism is similar to that for naturally occurring acyclic ionophores which accommodate guest metal cations in their pseudocyclic cavities and transport a guest across the biomembranes [6-81. Polymer 2, therefore, is a new class of host polymer, namely, a macromolecular ionophore. There are a few limited studies describing acyclic types of host polymers whose chit-al recognition is not based on that of the macrocyclic structure [9-131. Optical resolution of racemates us-
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2
1 Scheme 1.
o 1) t-BuOK, THF, 6O”C, 14h
1
2) +I,
“uO/WjCjWf
6h
3
AIBN
3+ CN
DMF, 6O’C, 2.5h
4 Scheme 2.
ing 2 as a chiral selector, therefore, is of interest. We reported the enantioselective transport of racemic amino acid derivatives through a liquid membrane containing 2 [ 12,131. For the experiment using racemic phenylglycine picrate, the L-isomer was eluted first and the optical purity was 10.9% ee as a maximum value. However, the optical resolution of the racemates by the liquid membrane is not a practical method because the transport efficiency is low. On the other hand, optical resolution by a solid membrane is a promising method because a large amount of racemates could be handled in one operation [ 14181. Here we report the enantioselective permeation of racemic amino acid perchlorates through a solid membrane of polyacrylonitrile-gr@( 1+6)-2&nhydro-$4~di-O-methyl-D-glucitol(4) which was prepared by a macromonomer method [ 191, as shown in Scheme 2.
2. Experimental
2.1. Materials Potassium terf-butoxide (t-BuOK) was obtained from Tokyo Kasei Co. and was purified by sublimation under reduced pressure. Dry tetrahydrofuran (THF) was used as commercial product (Kant0 Chemical Co.). N,N-Dimethylformamide (DMF) was distilled over CaS04. Acrylonitrile (AN) obtained from Kanto Chemical Co. was dried for 24 h over CaC12 and distilled under N2. 2,2’-azobis(2-methylpropionitrile) (AIBN) was purified by recrystallization from methanol. Amino acid perchlorates were prepared by reacting phenylglycine (from Tokyo Kasei Co.), phenylalanine (from Tokyo Kasei Co.) and tryptophan (from Kanto Chemical Co.) with 60% perchloric acid in methanol. 1,2: 5,6-Dianhy-
T Satoh et al. /Reactive
& Functional Polymers 37 (1998) 293-298
drO-3,4-di-O-methyl-D-maMitO1 (l), prepared according to the reported procedure, was freshly distilled from calcium hydride just before use [ 1,2,20]. 2.2. Measurements The molecular weights of the resulting macromonomer and copolymer were measured by gel permeation chromatography (GPC) in THF on a Jasco HPLC system equipped with three polystyrene gel columns (Shodex KF-840L) and in DMF on a Tosoh system equipped with three columns (TSK-GEL), respectively. The number-average molecular weight (M,) was calculated on the basis of a polystyrene calibration. The thickness (L) of the membrane was measured with a precision micrometer (Mitutoyo Co., Ltd). UV absorbance was recorded using a JASCO UVIDEC-660 instrument. The optical purity during an optical resolution experiment was measured by means of a HPLC (JASCO 880-PU) equipped with a CROWNPAK CR(+) (Daisel Chemical Industries, Ltd) as an optical resolution column and a UV detector (JASCO UVIDEC-100~III). 2.3. Synthesis of macromonomer The polymerization was carried out in an H-shaped glass ampoule. t-BuOK (193.2 mg, 1.72 mmol) and dry THF (17.0 ml) were added to the one side of ampoule, and compound l(3.0 g, 17.2 mmol) was added to the other side of the ampoule under a nitrogen atmosphere. After sealing, this ampoule was placed in a thermostated oven at 60°C. The monomer and the catalyst were then mixed and stirred at 60°C. After 14 h, methacryloyl chloride (0.25 ml, 2.58 mmol) as a terminating agent was added to the polymerization system by syringe, and the mixture was then allowed to react at 60°C for 6 h. The mixture was poured into a large amount of methanol and the solution was neutralized with CO*. After evaporating the solvent, the residual was purified by repeating reprecipitation from chloroform solution into n-hexane. The yield of the resulting macromonomer was 94%. ‘H NMR (CDCls): S = 6.14-6.10 (m, =CHz cis), 5.58-5.55 (m, =CHz trans), 4.13-4.06 (m, H5), 3.96-3.90 (m, H2), 3.78-3.62 (m, H3, H4 and H6), 3.54-3.46 (m, II’), 3.42-3.34 (m, OCH3), 1.95 (s, CH3) and 1.13 ppm (s, C(CH&).
295
2.4. Copolymerization of macrOllZonomerwith acrylonitrile The radical copolymerization of acrylonitrile and macromonomer was carried out in a glass ampoule. The macromonomer (1.0 g, 0.417 mm01 based on M, value measured by GPC), acrylonitrile (2.28 g, 42.9 mmol), AIBN (72.9 mg, 0.437 mmol) and dry DMF (8.3 ml) were added to the ampoule. After deaeration and replacement by nitrogen, the ampoule was placed in the thermostated oven at 60°C. After 2.5 h, the mixture was poured into a large amount of methanol. The obtained copolymer was purified by reprecipitation from DMF solution into methanol. The copolymer was filtered and dried in vacua at 30°C. The yield of copolymer was 38.7%. The M, and I&,/M,, were 1.3 x lo5 and 1.8, respectively. The mole fraction of macromonomer in the copolymer was 0.01 which was determined by elemental analysis. 2.5. Preparation of membrane The membrane was obtained by casting the solution of the copolymer in DMF. A 35 wt% solution of copolymer in DMF (2 ml) was cast on a glass plate and the DMF evaporated in vacua at 25°C for 2 h. The glass plate was then soaked in water, and the polymer membrane was detached from the glass plate. A thickness of the membrane was 27 pm. The area of the membrane for the permeation experiment was 7.05 cm2. 2.6. Adsorption experiment Copolymer powder (50 mg) was added to a 0.05 mol/l perchloric acid solution of the racemic phenylglycine perchlorate (40 ml) and the mixture was stirred for 24 h. The copolymer with the adsorbed compounds was filtered and stirred in water for 24 h to desorb the absorbed compounds. The optical purity was determined by HPLC. 2.7. Permeation experiment through the copolymer membrane The polymer membrane was placed between two glass cells with silicone-rubber packing. A 0.05-mol/l
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perchloric acid solution of D-, L- and DL-amino acid perchlorates (20 ml) and water (20 ml) was supplied to the feed- (a-phase) and permeate-side (B-phase) cells, respectively. The permeation experiment was carried out at 25°C with stirring. The amount of ammo acids permeated from the CX-to the p-phase were determined by monitoring the W absorbance of the solution in the p-phase. The optical purity was determined by HPLC. The permeation rate was estimated from the slope of the amount of permeate-time plot before more than 5% of amino acid was permeated. The permeation coefficient [Po, PL (cm2 mm-‘)] was calculated from P = QL/AtAC, where Q is the amount of permeate, L and A are the thickness and the area of the membrane, AC is the difference of the concentration between a+ and B-phase and t is permeation time. 3. Results and discussion 3.1. Preparation of polymer membrane (1+6)-2,5-Anhydro-3,4-di-O-methyl-D-glucitol (2) obtained by the anionic cyclopolymerization of 1,2 : 5,6-dianhydro-3,4-di-O-methyl-D-man&o1 (1) was a sticky semisolid and had an amphiphilic property. The polymer membrane prepared from 2, therefore, was not a durable material for the permeation experiment. The problem was resolved by synthesizing the graft copolymer having polyactylonitrile in the backbone and (1+6)-2,5-anhydro-3,4-diQ-methyl-D-glucitol in the side chain. The synthetic route is shown in Scheme 2. The end-functionalized (l-+6)-2,5-anhydro-3,4di-O-methyl-D-glucitol (3) was synthesized by the anionic cyclopolymerization of 1 using t-BuOK, followed by treatment with methacryloyl chloride as a terminating agent. The M, and M,/M, values were 2400 and 1.3, respectively. The degree of end-functionalization was about 96% which was determined by estimating the peak intensity ratio of the vinyl protons in the methacryloyl group to the methine protons in the 2,ktUhydrO-D-glUCitO1 uuits in the ‘H-NMR spectrum. The copolymerization of 3 with acrylonitrile was carried out using AIBN in DMF to produce orange powdery polymers (4) which were soluble in DMF, but insoluble in chloroform, THF and methanol. For the copolymerization
at a [AN]/[3] molar ratio of 0.99/0.01 in the feed, copolymer 4 had a high molecular weight (M, of 1.3 x 105) and showed a good membrane-forming ability. 3.2. Enantioselective permeability of the polymer membrane
Fig. 1 shows the plots of the quantity of the Dand L-amino acids permeated into B-phase vs. permeation time. The permeated amount of D-amino acids transported for 660 min was 4.8 x low2 mm01 for phenylglycine @a), 3.7 x lop2 mm01 for phenyh&nine (5b) and 2.4 x 10m2 mm01 for tryptophan (5~). The permeation rates were determined from the slope of the straight lines in Fig. 1 and are summarized in Table 1. The permeation rates decreased in the order of 5a > 5b > 5c, which may be related to the molecular size of the amino acids. A similar tendency was also observed in the permeation experiment of the L-isomer. The rate of permeation was faster in the D-isomer than in the L-isomer for all the ammo acids tested. This result indicated
0
200
400
600
800
Time I min Fig. 1. Permeation behavior of D- and L-amino acid perchlorates through a membrane prepared from copolymer 4 at 25°C: the a-phase, 20 ml of amino acid perchlorate solution (0.05 mol/l); the B-phase, 20 ml of water; (~,a) D- and L-phenylglycine (Phgly); (A,A) D- and Lphenylalanine (Phe); (0,m) D- and L-tryptophan (Trp).
291
lY Satoh et al. /Reactive & Functional Polymers 37 (1998) 293-298
Table 1 Permeation coefficient of D- and L-amino acid perchlorates @a-c) through a membrane prepared from copolymer 4 at 25°C a
5a 5b 52
D-isomer
L-isomer
6.41 5.24 3.66
5.49 4.89 3.29
(cm2 min-‘)
PL X10’s (cm’ mint)
4.91 4.01 2.80
4.2 1 3.15 2.52
PD XlO’b
Permeation rate x lo5 (mm01 min-‘)
No.
1.17 1.07 1.11
a The o-phase contained a 0.05 mol/l perchloric acid solution (20 ml) of ammo acid perchlorate; the /J-phase was 20 ml of water. b PO and PL are permeation coefficient for D- and L-isomer, respectively.
that (1+6)-2,5-anhydro-3,4-di-0-methyl-D-glucitol moieties in the membrane preferentially interacted with the L-isomer rather than with the D-iSOIDer. The ability of optical resolution is estimated by the ratio of the permeation coefficients between the faster moving D-isomer and the slower moving L-isomer. For the transports of D- and IAa, the ratio of permeation coefficients was calculated to be 1.17, and thus, the D-isomers permeated faster than the L-isomer. The ratio decreased in the order 5a > 5c > Sb, which is caused by the participation of the bulkiness of the substituent (R) in the vicinity of the chiral center of RCH(C02H)NHs+C104-.
For the transport using racemic 5a, the D-isomer was permeated first and the optical purity was 75% ee for the permeated amount of 9.2 x 10e3 mmol, as shown in Fig. 2. Therefore, the optical resolution of the racemic amino acid was realized by using the polymer membrane. However, the optical purity sharply decreased to a constant value of 3.0% ee with the progress of permeation. Enantioselectivity during adsorption on copolymer 4 was examined for racemic phenylglycine in water. As a result, L-isomer was more strongly adsorbed than the D-iSOIDer (5.8% ee), though the latter showed a faster permeation than the former in the polymer membrane experiments. Therefore, it was suggested that the optical resolution between the Dand L-isomers was caused not by selective adsorption at the membrane surface, but mainly by relative diffusion in the membrane. 4. Conclusions
L. 0
I
0
I 2
Onon_ I
1-1 4
Amount of DL-phenylglycine
I 6
I
I ”
8
permeated / lO%rmol
Fig. 2. Optical resolution of racemic phenylglycine perchlorate through a membrane prepared from copolymer 4 at 25°C: the o-phase, 20 ml of DL-phenylglycine perchlorate solution (0.05 ml/l); the B-phase, 20 ml of water.
A racemic amino acid perchlorate was enantioselectively separated through a solid membrane prepared from polyacrylonitrile-graft-( 1+6)-2,5-anhydro-3,4-di-0-methyl-D-glucitol. The permeation rates of the amino acids decreased in the order of phenylglycine > phenylalanine > tryptophan, according to the molecular size of guest compounds. For the transport using racemic phenylglycine, the D-isomer was permeated first and the optical purity was 75% ee (max.). For the adsorption experiment on copolymer 4 for racemic phenylglycine, the L-isomer was more strongly adsorbed than the D-isomer (5.8% ee). Therefore, the enantioselective permeation is suggested to be caused by the difference in diffusion between the D- and L-isomers in the membrane.
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References [l] T. Satoh, K. Yokota, T. Kakuchi, Macromolecules 28 (1995) 4762. [2] T. Satoh, T. Hatakeyama, S. Umeda, H. Hashimoto, K. Yokota, T. Kakuchi, Macromolecules 29 (1996) 3447. [3] H. Hashimoto, T. Kakuchi, K. Yokota, J. Org. Chem. 56 (1991) 6471. [4] T. Kakuchi, T. Satoh, J. Mata, S. Umeda, H. Hashimoto, K. Yokota, J. Macromol., Sci. Chem. 3 (1996) 325. [5] T. Kakuchi, T. Hatakeyama, H. Kanai, T. Satoh, K. Yokota, Polymer, submitted. [6] E.M. Choy, D.F. Evans, E.L. Cussler, J. Am. Chem. Sot. 96 (1974) 7085. [7] B.G. Cox, N. Truong, J. Rzeszotarska, H. Schneider, J. Am. Chem. Sot. 106 (1984) 5965. [8] H. Tsukube, K. Takagi, T. Higashiyama, T. Iwachido, N. Hayama, J. Chem. Sot., Chem. Commun. (1986) 448. [9] W.J. Schultz, M.C. Etter, A.V. Pocius, S. Smith, J. Am. Chem. Sot. 102 (1980) 7981. [lo] U. Koert, M. Stein, K. Harms, Angew. Chem. Int. Ed. Engl.
33 (1994) 1180. [ll] B.M. Novak, R.H. Grubbs, J. Am. Chem. Sot. 110 (1988) 960. [12] T. Kakuchi, Y. Harada, T. Satoh, K. Yokota, H. Hashimoto, Polymer 35 (1994) 204. [13] T. Kakuchi, T. Satoh, S. Umeda, J. Mata, K. Yokota, Chirality 7 (1995) 136. [14] A. Maruyama, N. Ada&i, T. Takatsuki, M. Torii, K. Sanui, N. Ogata, Macromolecules 23 (1990) 2748. [15] T. Aoki, K. Shinohara, T. Kaneko, E. Oikawa, Macromolecules 29 (1996) 4192. [16] T. Masawaki, M. Sasai, S. Tone, J. Chem. Eng., Jpn. 25 (1992) 33. [17] A. Higuchi, M. Hara, T. Horiuchi, T. Nakagawa, J. Membr. Sci. 93 (1994) 157. [18] M. Yoshikawa, J. Izumi, T. Kitao, S. Koya, S. Sakamoto, J. Membr. Sci. 108 (1995) 171. [19] T. Satoh, T. Miura, T. Hatakeyama, K. Yokota, T. Kakuchi, Macromol. Rapid Commun. 18 (1997) 1041. [20] J. Kuszmann, Carbohydr. Res. 71 (1979) 123.
ELSEVIER
Reactive
& Functional
Polymers
37 (1998) 293-298
Short communication
Enantioselective permeability of membranes prepared from polyacrylonitrile-guaft-( l-,6)-2,5anhydro-D-glucitol Toshifumi Satoh a, Yoshiki Tanaka a, Kazuaki Yokota a,*, Toyoji Kakuchi b ’ Division of Molecular Chemistry Graduate School of Engineen’ng, Hokkaiab University, Hokkaido, Sapporo 060, Japan b Division
of Bioscience, Graduate School of Environmental Earth Science, Hokkaido Universiiy, Hokkaido, Sapporo 060, Japan Received
20 June 1997; revised version received 28 August
1997; accepted
11 September
1997
Abstract Enantioselective permeation of racemic amino acid perchlorates has been studied using a solid membrane prepared from polyacrylonitrile-grujr-aft-(l-+ 6)-2,5-anhydro-3,4-d-O-methyl-D-glucitol. The permeation rates of the amino acids decreased in the order of phenylglycine > phenylalanine > tryptophane, according to the molecular size of the guest compounds. For all the amino acids tested, the permeation rate of the D-isomer was greater than that of the L-isomer. When Dand L-phenylglycine perchlorates were permeated, the D-isomer permeated 1.17 times faster than the L-isomer. During transport using racemic phenylglycine perchlorate, the D-isomer was permeated first and the optical purity was 75% ee (max.). For the adsorption experiment for racemic phenylglycine, the L-isomer was more strongly adsorbed than the D-isomer (5.8% ee). From these data, it was directly made clear that the (1+6)-2,5-anhydro-3,4-di-O-methyl-D-glucitol moieties in the membrane preferentially interacted with the L-isomer rather than with the D-isomer. 0 1998 Elsevier Science B.V. All rights reserved. Keywords:
Enantioselective permeability; Membrane; Optical resolution; Macromonomer; (l-+6)-2,&knhydro-D-glucitol
1. Introduction The anionic cyclopolymerization of 1,2 : 5,6dianhydro-3,4-di-0-methyl-D-m&to1 (1) using potassium tert-butoxide proceeded regio- and stereoselectively to produce a well-defined polymer consisting of Only 2,5-anhydro-3,4-di-0-methyl-D-glucitol moieties, that is, (1+6)-2,5-anhydro-3,4-di-o-methyl-D-glucitol (2) [1,2]. The structural characteristic of 2 is a lack of the anomeric linkage which is found in the naturally occurring polysaccharides, * Corresponding author. Tel.: +81 (11) 706-6602, 706-6602, E-mail: [email protected]
Fax: +81 (11)
1381-5148/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII S1381-5148(97)00129-6
and hence, the polymer is a new type of polycarbohydrate (see Scheme 1). Polymer 2 acted as a host in the host-guest complexation and exhibited cation-binding selectivity toward metal ions [3-51. This host-guest complexation mechanism is similar to that for naturally occurring acyclic ionophores which accommodate guest metal cations in their pseudocyclic cavities and transport a guest across the biomembranes [6-81. Polymer 2, therefore, is a new class of host polymer, namely, a macromolecular ionophore. There are a few limited studies describing acyclic types of host polymers whose chit-al recognition is not based on that of the macrocyclic structure [9-131. Optical resolution of racemates us-
294
i’I Satoh et al. /Reactive & Functional Polymers 37 (1998) 293-298
2
1 Scheme 1.
o 1) t-BuOK, THF, 6O”C, 14h
1
2) +I,
“uO/WjCjWf
6h
3
AIBN
3+ CN
DMF, 6O’C, 2.5h
4 Scheme 2.
ing 2 as a chiral selector, therefore, is of interest. We reported the enantioselective transport of racemic amino acid derivatives through a liquid membrane containing 2 [ 12,131. For the experiment using racemic phenylglycine picrate, the L-isomer was eluted first and the optical purity was 10.9% ee as a maximum value. However, the optical resolution of the racemates by the liquid membrane is not a practical method because the transport efficiency is low. On the other hand, optical resolution by a solid membrane is a promising method because a large amount of racemates could be handled in one operation [ 14181. Here we report the enantioselective permeation of racemic amino acid perchlorates through a solid membrane of polyacrylonitrile-gr@( 1+6)-2&nhydro-$4~di-O-methyl-D-glucitol(4) which was prepared by a macromonomer method [ 191, as shown in Scheme 2.
2. Experimental
2.1. Materials Potassium terf-butoxide (t-BuOK) was obtained from Tokyo Kasei Co. and was purified by sublimation under reduced pressure. Dry tetrahydrofuran (THF) was used as commercial product (Kant0 Chemical Co.). N,N-Dimethylformamide (DMF) was distilled over CaS04. Acrylonitrile (AN) obtained from Kanto Chemical Co. was dried for 24 h over CaC12 and distilled under N2. 2,2’-azobis(2-methylpropionitrile) (AIBN) was purified by recrystallization from methanol. Amino acid perchlorates were prepared by reacting phenylglycine (from Tokyo Kasei Co.), phenylalanine (from Tokyo Kasei Co.) and tryptophan (from Kanto Chemical Co.) with 60% perchloric acid in methanol. 1,2: 5,6-Dianhy-
T Satoh et al. /Reactive
& Functional Polymers 37 (1998) 293-298
drO-3,4-di-O-methyl-D-maMitO1 (l), prepared according to the reported procedure, was freshly distilled from calcium hydride just before use [ 1,2,20]. 2.2. Measurements The molecular weights of the resulting macromonomer and copolymer were measured by gel permeation chromatography (GPC) in THF on a Jasco HPLC system equipped with three polystyrene gel columns (Shodex KF-840L) and in DMF on a Tosoh system equipped with three columns (TSK-GEL), respectively. The number-average molecular weight (M,) was calculated on the basis of a polystyrene calibration. The thickness (L) of the membrane was measured with a precision micrometer (Mitutoyo Co., Ltd). UV absorbance was recorded using a JASCO UVIDEC-660 instrument. The optical purity during an optical resolution experiment was measured by means of a HPLC (JASCO 880-PU) equipped with a CROWNPAK CR(+) (Daisel Chemical Industries, Ltd) as an optical resolution column and a UV detector (JASCO UVIDEC-100~III). 2.3. Synthesis of macromonomer The polymerization was carried out in an H-shaped glass ampoule. t-BuOK (193.2 mg, 1.72 mmol) and dry THF (17.0 ml) were added to the one side of ampoule, and compound l(3.0 g, 17.2 mmol) was added to the other side of the ampoule under a nitrogen atmosphere. After sealing, this ampoule was placed in a thermostated oven at 60°C. The monomer and the catalyst were then mixed and stirred at 60°C. After 14 h, methacryloyl chloride (0.25 ml, 2.58 mmol) as a terminating agent was added to the polymerization system by syringe, and the mixture was then allowed to react at 60°C for 6 h. The mixture was poured into a large amount of methanol and the solution was neutralized with CO*. After evaporating the solvent, the residual was purified by repeating reprecipitation from chloroform solution into n-hexane. The yield of the resulting macromonomer was 94%. ‘H NMR (CDCls): S = 6.14-6.10 (m, =CHz cis), 5.58-5.55 (m, =CHz trans), 4.13-4.06 (m, H5), 3.96-3.90 (m, H2), 3.78-3.62 (m, H3, H4 and H6), 3.54-3.46 (m, II’), 3.42-3.34 (m, OCH3), 1.95 (s, CH3) and 1.13 ppm (s, C(CH&).
295
2.4. Copolymerization of macrOllZonomerwith acrylonitrile The radical copolymerization of acrylonitrile and macromonomer was carried out in a glass ampoule. The macromonomer (1.0 g, 0.417 mm01 based on M, value measured by GPC), acrylonitrile (2.28 g, 42.9 mmol), AIBN (72.9 mg, 0.437 mmol) and dry DMF (8.3 ml) were added to the ampoule. After deaeration and replacement by nitrogen, the ampoule was placed in the thermostated oven at 60°C. After 2.5 h, the mixture was poured into a large amount of methanol. The obtained copolymer was purified by reprecipitation from DMF solution into methanol. The copolymer was filtered and dried in vacua at 30°C. The yield of copolymer was 38.7%. The M, and I&,/M,, were 1.3 x lo5 and 1.8, respectively. The mole fraction of macromonomer in the copolymer was 0.01 which was determined by elemental analysis. 2.5. Preparation of membrane The membrane was obtained by casting the solution of the copolymer in DMF. A 35 wt% solution of copolymer in DMF (2 ml) was cast on a glass plate and the DMF evaporated in vacua at 25°C for 2 h. The glass plate was then soaked in water, and the polymer membrane was detached from the glass plate. A thickness of the membrane was 27 pm. The area of the membrane for the permeation experiment was 7.05 cm2. 2.6. Adsorption experiment Copolymer powder (50 mg) was added to a 0.05 mol/l perchloric acid solution of the racemic phenylglycine perchlorate (40 ml) and the mixture was stirred for 24 h. The copolymer with the adsorbed compounds was filtered and stirred in water for 24 h to desorb the absorbed compounds. The optical purity was determined by HPLC. 2.7. Permeation experiment through the copolymer membrane The polymer membrane was placed between two glass cells with silicone-rubber packing. A 0.05-mol/l
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perchloric acid solution of D-, L- and DL-amino acid perchlorates (20 ml) and water (20 ml) was supplied to the feed- (a-phase) and permeate-side (B-phase) cells, respectively. The permeation experiment was carried out at 25°C with stirring. The amount of ammo acids permeated from the CX-to the p-phase were determined by monitoring the W absorbance of the solution in the p-phase. The optical purity was determined by HPLC. The permeation rate was estimated from the slope of the amount of permeate-time plot before more than 5% of amino acid was permeated. The permeation coefficient [Po, PL (cm2 mm-‘)] was calculated from P = QL/AtAC, where Q is the amount of permeate, L and A are the thickness and the area of the membrane, AC is the difference of the concentration between a+ and B-phase and t is permeation time. 3. Results and discussion 3.1. Preparation of polymer membrane (1+6)-2,5-Anhydro-3,4-di-O-methyl-D-glucitol (2) obtained by the anionic cyclopolymerization of 1,2 : 5,6-dianhydro-3,4-di-O-methyl-D-man&o1 (1) was a sticky semisolid and had an amphiphilic property. The polymer membrane prepared from 2, therefore, was not a durable material for the permeation experiment. The problem was resolved by synthesizing the graft copolymer having polyactylonitrile in the backbone and (1+6)-2,5-anhydro-3,4-diQ-methyl-D-glucitol in the side chain. The synthetic route is shown in Scheme 2. The end-functionalized (l-+6)-2,5-anhydro-3,4di-O-methyl-D-glucitol (3) was synthesized by the anionic cyclopolymerization of 1 using t-BuOK, followed by treatment with methacryloyl chloride as a terminating agent. The M, and M,/M, values were 2400 and 1.3, respectively. The degree of end-functionalization was about 96% which was determined by estimating the peak intensity ratio of the vinyl protons in the methacryloyl group to the methine protons in the 2,ktUhydrO-D-glUCitO1 uuits in the ‘H-NMR spectrum. The copolymerization of 3 with acrylonitrile was carried out using AIBN in DMF to produce orange powdery polymers (4) which were soluble in DMF, but insoluble in chloroform, THF and methanol. For the copolymerization
at a [AN]/[3] molar ratio of 0.99/0.01 in the feed, copolymer 4 had a high molecular weight (M, of 1.3 x 105) and showed a good membrane-forming ability. 3.2. Enantioselective permeability of the polymer membrane
Fig. 1 shows the plots of the quantity of the Dand L-amino acids permeated into B-phase vs. permeation time. The permeated amount of D-amino acids transported for 660 min was 4.8 x low2 mm01 for phenylglycine @a), 3.7 x lop2 mm01 for phenyh&nine (5b) and 2.4 x 10m2 mm01 for tryptophan (5~). The permeation rates were determined from the slope of the straight lines in Fig. 1 and are summarized in Table 1. The permeation rates decreased in the order of 5a > 5b > 5c, which may be related to the molecular size of the amino acids. A similar tendency was also observed in the permeation experiment of the L-isomer. The rate of permeation was faster in the D-isomer than in the L-isomer for all the ammo acids tested. This result indicated
0
200
400
600
800
Time I min Fig. 1. Permeation behavior of D- and L-amino acid perchlorates through a membrane prepared from copolymer 4 at 25°C: the a-phase, 20 ml of amino acid perchlorate solution (0.05 mol/l); the B-phase, 20 ml of water; (~,a) D- and L-phenylglycine (Phgly); (A,A) D- and Lphenylalanine (Phe); (0,m) D- and L-tryptophan (Trp).
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Table 1 Permeation coefficient of D- and L-amino acid perchlorates @a-c) through a membrane prepared from copolymer 4 at 25°C a
5a 5b 52
D-isomer
L-isomer
6.41 5.24 3.66
5.49 4.89 3.29
(cm2 min-‘)
PL X10’s (cm’ mint)
4.91 4.01 2.80
4.2 1 3.15 2.52
PD XlO’b
Permeation rate x lo5 (mm01 min-‘)
No.
1.17 1.07 1.11
a The o-phase contained a 0.05 mol/l perchloric acid solution (20 ml) of ammo acid perchlorate; the /J-phase was 20 ml of water. b PO and PL are permeation coefficient for D- and L-isomer, respectively.
that (1+6)-2,5-anhydro-3,4-di-0-methyl-D-glucitol moieties in the membrane preferentially interacted with the L-isomer rather than with the D-iSOIDer. The ability of optical resolution is estimated by the ratio of the permeation coefficients between the faster moving D-isomer and the slower moving L-isomer. For the transports of D- and IAa, the ratio of permeation coefficients was calculated to be 1.17, and thus, the D-isomers permeated faster than the L-isomer. The ratio decreased in the order 5a > 5c > Sb, which is caused by the participation of the bulkiness of the substituent (R) in the vicinity of the chiral center of RCH(C02H)NHs+C104-.
For the transport using racemic 5a, the D-isomer was permeated first and the optical purity was 75% ee for the permeated amount of 9.2 x 10e3 mmol, as shown in Fig. 2. Therefore, the optical resolution of the racemic amino acid was realized by using the polymer membrane. However, the optical purity sharply decreased to a constant value of 3.0% ee with the progress of permeation. Enantioselectivity during adsorption on copolymer 4 was examined for racemic phenylglycine in water. As a result, L-isomer was more strongly adsorbed than the D-iSOIDer (5.8% ee), though the latter showed a faster permeation than the former in the polymer membrane experiments. Therefore, it was suggested that the optical resolution between the Dand L-isomers was caused not by selective adsorption at the membrane surface, but mainly by relative diffusion in the membrane. 4. Conclusions
L. 0
I
0
I 2
Onon_ I
1-1 4
Amount of DL-phenylglycine
I 6
I
I ”
8
permeated / lO%rmol
Fig. 2. Optical resolution of racemic phenylglycine perchlorate through a membrane prepared from copolymer 4 at 25°C: the o-phase, 20 ml of DL-phenylglycine perchlorate solution (0.05 ml/l); the B-phase, 20 ml of water.
A racemic amino acid perchlorate was enantioselectively separated through a solid membrane prepared from polyacrylonitrile-graft-( 1+6)-2,5-anhydro-3,4-di-0-methyl-D-glucitol. The permeation rates of the amino acids decreased in the order of phenylglycine > phenylalanine > tryptophan, according to the molecular size of guest compounds. For the transport using racemic phenylglycine, the D-isomer was permeated first and the optical purity was 75% ee (max.). For the adsorption experiment on copolymer 4 for racemic phenylglycine, the L-isomer was more strongly adsorbed than the D-isomer (5.8% ee). Therefore, the enantioselective permeation is suggested to be caused by the difference in diffusion between the D- and L-isomers in the membrane.
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