가교된 키토산 멤브레인을 이용한 □-amino산 광학 이성질체의 입체선택적 분리

김지혜, 김장훈, 제갈종건, 이규호

한국화학연구원 분리막다기능소재연구센터

ENANTIOSELECTIVE PERMEATION OF □-AMINO ACID OPTICAL ISOMERS THROUGH CROSSLINKED CHITOSAN  MEMBRANES

Ji Hye Kim, Jang Hoon Kim, Jonggeon Jegal and Kew-Ho Lee

Membrane and Separation Research Center, Korea Research Institute of Chemical Technology, P. O. Box 107, Yusung, Taejon, 305-606, South Korea

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

INTRODUCTION

Nowadays, solid membrane separation process has naturally been considered as an attracting method for the optical resolution because of its several advantages such as ease of handling, instrumental simplicity and efficiency in energy as compared to the conventional methods, including preferential crystallization, chemical modification by an optical resolution agent, and high-performance liquid chromatography (HPLC) with a chiral stationary phase. Although there have been several reports on the optical resolutions through solid enantioselctive membranes by aoki. et al. and other groups.2-5, solid membrane process for optical resolution is just on the beginning stage and has to be much more improved for them to be used for practical use.

The large content of chiral center is important factor for the formation of chiral environment in the membrane that is critical for the separation of optical isomers. Without chiral environment, it is almost impossible for the membrane to have enantioselectivity. In previous study, we tried sodium alginate, one of the polysaccharide containing anionic groups, to prepare enantioselective membranes for the separation of a-amino acids such as tryptophan and tyrosine, because it has excellent hydrophilicity and a large content of chiral centers in the backbone. Good enantioselectivity was measured.

Chitosan has a large content of chiral centers in the backbone likely to sodium alginate, but these polysaccharides have different backbone structure and different ionic group: chitosan has a cationic amide group, but sodium alginate an anionic carboxylic group. In this study, we have selected chitosan to prepare enantioselective membranes for the separation of a-amino acids such as tryptophan and tyrosine. Likely to the sodium alginate, the excellent hydrophilicity and a large content of chiral centers of the chitosan seemed important for the membranes to have proper membrane properties for the separation of such water-soluble a-amino acid isomers as tryptophan and tyrosine. Because of its excellent hydrophilicity, chitosan also has often been used for the formation of hydrophilic membranes for the separation of water/alcohol or MTBE/methanol mixtures and showed good separation performances.

Likely to the sodium alginate, chitosan membranes crosslinked with glutaraldehyde were prepared and used for the optical resolution of tryptophan and tyrosine, controlling the degree of crosslinking of the membranes, operating pressures and feed concentrations. The details of the results obtained are elaborated in this article.

화학공학의 이론과 응용 제8권 제1호 2002년

EXPERIMENTAL

Materials. Chitosan, D,L-tryptophan (Trp) and D,L-tyrosine (Tyr) purchased from Sigma Aldrich were used for the formation of feed solution, without further purification. Glutaraldehyde (GA) (25 wt% content in water), hydrochloric acid (35.5 wt% content), acetic acid and acetone were purchased from Junsei Chemical (Tokyo, Japan). Ultra pure deionized water was used as a solvent of feed solutions. All other chemicals were used without any further purification.

Membrane preparation. 5 wt% chitosan solution in water was prepared by dissolving 5 g of chitosan into 1 kg of water containing 5 wt% acetic acid at 50 ℃. The chitosan solution was cast onto a polyester film attached to a glass plate, using a Gardner casting knife, and dried at room temperature in a fume hood for 4 days. After which, the dry chitosan films formed were peeled off the polyester film and immersed into an acetone solution containing 5.0 wt% GA and 1.0 wt% HCl for the crosslinking of the chitosan film. To control the degree of crosslinking of the chitosan films, the immersion time in the acetone solution was adjusted from 6 to 48 hrs. The crosslinked chitosan films were then taken out of the solution and washed out several times with an excess amount deionized water at 50 ℃ for 24 hours, and then dried under vaccum for 24 hours. The chitosan films prepared so were used as membranes. The thickness of the membranes was in the rage of 50 to 60□.

Figure 1. Preparation scheme of crosslinked chitosan.

Swelling Index measurements. The swelling indices (SI) of the membranes were measured to compare the degree of crossliking of the membranes indirectly, using the following equation. The chitosan membranes crosslinked for different reaction time were fully swollen in deionized water at room temperature until there was no difference in the weight of the swollen membranes. After removing the water remained on the surface of the membranes, they were weighed to determine the weight of the swollen membranes (Ws). After which, they were dried under vacuum to a constant weight to determine the weight of the dried membranes (Wd).

SI = 100 x (Ws - Wd)/Wd

Permeation tests. For the permeation test, a test cell shown in the previous work used in this study. The chitosan membranes were kept in distilled water for more than 24 hours before loading into the cell. For the permeation test, aqueous solutions of tryptophan and tyrosine racemic mixtures were used as feed solutions. The concentration of the feed solutions was varied between 0.49 to 4.9 mmol/L. The operating pressures used were 1.0 and 2.0 kgf/cm2, and the operating temperature was 25 ℃. Flux for various racemic compounds (Flux, g/m2h) were determined by weighing the permeated compound after the water had been evaporated.

Flux = Q/(At)

Where Q is the quantity of the solute permeated, t is the permeation time, and A is the area of the membrane. The composition (contents of D and L isomers) of the feed and permeates was measured by means of a liquid chromatography equipped with a Chiralpak-WH column (Diacel Chemical Industries, Ltd., Japan) as a optical resolution column.  The enantiomeric excess (ee) of permeates was determined from the peak areas of their two enantiomers, D- (AD) and L-isomers (AL).

% ee = 100 x (AD - AL)/( AD + AL)

The flux was determined by weighing the amounts of permeate penetrated through the membrane per unit time and unit membrane area.

RESULTS AND DISCUSSION

permeation tests & effect of degree of crosslinking. Figure 2 shows the characteristics of the optical resolution of tryptophan racemates using a chitosan membrane with 70 % of swelling index. The concentration of the feed solution and operating pressure were 0.49 mmol/L and 1 kgf/cm2. Figure 3 shows the characteristics of the optical resolution of tryptophan racemates as a function of the swelling indices of the chitosan membranes. The concentration of the feed solution and operating pressure were 0.49 mmol/L and 1 kgf/cm2.

Figure 2. Characteristics of the optical resolution of tryptophan racemates using a chitosan membrane.

Figure 3. Characteristics of the optical resolution of tryptophan racemates as a function of the swelling indices.

Effect of operating pressures & feed solutions concentration. Figure 4 show the effect of the concentration of the feed solutions on the characteristics of the optical resolution of tryptophan racemates, using a chitosan membrane with 80 % of swelling index: (●,○) Concentration of feed solution = 0.49mmol/L, (△,▲) Concentration of feed solution = 4.9mmol/L; (●,▲) D-tryptophan, (○,△) L-tryptophan. Figure 6 shows the effect of operating pressure on the characteristics of the optical resolution of tryptophan racemates, using a chitosan membrane with 80 % of swelling index: (●,○) Pressure = 1kgf, (△,▲) Pressure = 2kgf ; (●,▲) D-tryptophan, (○,△) L-tryptophan.

Figure 4. Effect of the concentration of the feed solutions on the characteristics of the optical resolution of tryptophan racemates.

Figure 5. Effect of operating pressure on the characteristics of the optical resolution of tryptophan racemates.

CNCLUSIONS

Chitosan membranes crosslinked with GA are possible for the optical resolution of   □-amino acids, especially tryptophan and tyrosine, by a pressure driven process. The presence five chiral carbons located on the ring structure of chitosan seemed to make helical molecular structure and chiral environments in the membrane as cellulose and its derivatives did, making the membrane enantioselective. With increasing degree of crosslinking of the membrane was enabled to have better enantioselectivity by increasing the interaction between the functional groups of the chiral environment of the membrane and solutes penetrating through the membrane. The other factors that could decrease the intermolecular interaction between the membrane and solutes mentioned above, such as increase in operating pressure and increase in the concentration of a feed solution, and smaller size of solutes, acted against the enantioselectivity.

화학공학의 이론과 응용 제8권 제1호 2002년

REFERENCES

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