Xiao Dingshu ¹, Hu Jiwen^1,2,*, Zhang Mingqiu², Li Mingwei¹, Feng Liang¹
(¹Key Laboratory of Polymer Materials for Electronics of Guangdong Province, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650; ²Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Zhongshan University, Guangzhou 510275)

Abstract In this work crosslinked sodium alginate (SA) membrane was prepared and its enantioselective transport of a-amino acids was studied. It was found that the resultant thin membrane exhibits high permeability coupled with encouraging enantioselectivity. Low temperature helps to enhance the enantioselectivity but leads to reduced permeation rate. The different enantioselective transports of tyrosine and phenylalanine enantiomers through the chiral membranes confirmed the significance of hydrogen bond in enantioseparation.
Keywords Sodium alginate, membrane, enantioselective, hydrogen bond.

Techniques for preparative-scale enantiomer resolution are urgently needed because of the growing interests of the pharmaceutical industry in single-enantiomer drugs. The newly developed resolution procedures based on enantioselective polymeric membranes, characterized by their continuous operation ability, convenient up-scaling and energy-saving fashion, have been recognized as the most challenging and attractive way for purification of chiral drugs and natural products^[1]. The optical resolution through enantioselective membranes is right at the beginning stage. The efficiency, enantioselectivity and stability, controlled by many factors (e.g., structure parameters of the polymer, interactions between the chiral environment and enantiomers, size and shape of chiral penetrant molecules), are not high enough for practical usage^[2-4]. Therefore, a comprehensive understanding of the enantioselective transport behavior is necessary for designing highly efficient and economically viable membranes.
    Sodium alginate (SA), comprised of linear chains of 1,4-linked b-D-mannuronate and L-guluronate, has been widely employed for separations in food and daily-used chemical productions. In the present work, SA was selected as an enantioresolution membrane material because it can assemble with chirally active carbons on the ring backbone structure to form certain chiral recognition sites, very similar to that of cellulose and its derivatives which have been widely used for chiral stationary in HPLC and reported for optical resolution membranes^[3,5]. The aim of this work is focused on elaborate preparation of SA based chiral membranes and the influence of temperature on their enantioselectivity and permeability. Furthermore, the role of hydrogen bonding between SA chiral sites and enantiomers is also studied by selecting tyrosine (Tyr) and phenylalanine (Phe) as the model enantiomers.

1 EXPERIMENTAL
Commercial sodium alginate (analytical grade, Sigma-Aldrich Co.) was applied for the formation of enantioselective membranes. The coupling aqueous solution, containing dissolved SA at 40ºC and a trait of glycerol, was cast onto a glass plate, allowing evaporation of the solvent under infrared radiation for several minutes and then naturally drying. The resultant membranes were peeled off from the glass plate and immersed in an acid glutaraldelyde-acetone solution (10.0 % GA and 1.5% HCl) for crosslinking. Afterwards, the membranes were washed with an excess amount of the de-ionized water and then dried in vacuum at 20-22ºC for 4 days. Tensile performance of the membranes was determined using a universal test machine CMT 7503 (AVS, China) at a crosshead rate of 5.0 mm/min.
    Permeation tests were carried out in the way similar to the procedures reported in the literature^[5]. The membrane is placed between two chambers in water bath at a constant temperature. Tyrosine or phenylalanine (biochemical grade, HuiXing Biochemical Co. Ltd., China) solution (500 ml, 1.5 mM) was poured into one of the chambers and the solvent (500 ml de-ionized water) into the other. Permeation rate (P) was estimated from the permeated quantity (q) in single-enantiomer permeation, which was obtained by monitoring concentration using SP-2100 UV-visible spectrophotometer at maximum absorbance 272 nm and 253 nm for tyrosine and phenylalanine, respectively.
P (g·m·m^-2·h^-1) = (q×L) / (A×t)
  where t denotes the permeation time, L and A the thickness and area of the membrane, respectively. Enantiomeric excess (% e.e.) and separation factor (a) were obtained from the permeated quantity in racemic permeation, which was reckoned by the concentration of the permeated solution and its specific rotation measured by a Model W22-1S automatic polarimeter.
% e.e. = (q_D - q_L) / (q_D + q_L) × 100%                    a= q_D / q_L
   where q_D and q_L are the isomer permeated quantities in recemic permeation, respectively.

2 RESULTS AND DISCUSSION
Table 1 shows the membrane formation ability and membrane quality at various SA concentrations in the solution. It can be seen that the membranes 10-15 mm thick can be easily formed at a SA concentration of 1.5 wt.% without loss in surface flatness and membrane strength.
    So far, it is known that improvement of permeability is of the same importance as that of enantioselectivity, as the former determines the scale and efficiency of the given enantiomers resolution. According to the mechanism of transport, there are lots of routes for enhancing permeability including the application of host membranes with high porosities, permeation under high pressure and / or temperature, reduction of membrane thickness^[3]. However, chiral polymeric membranes with both high enantioselectivity and permeability are difficult to be made^[4].

Table 1 Effect of SA concentration on membrane formation ability and membrane quality

    The data in Figure 1 and Table 2 were obtained from the SA membrane with a thickness of 10-15 mm. It is seen that the fluxes of racemate are approximately half of the values of the two neat enantiomers. In addition, the separation factors coincide with the ratios of the two permeation rates in single-enantiomer permeation test. These results suggest that the separation factor can be converted into the permeation rate ratio (a= P_D/P_L) in the single-enantiomer permeation. Analogously, the permeation rate yielded from single-enantiomer permeation test can also represent the isomer transport in racemic permeation. The high L-phe flux clearly reveals the enantioselective trend of L-phenylalanine transport. In contrast, a contrary enantioselective pattern (i.e., the permeation rate of L-Tyr is lower than that of D-Tyr), is observed in Figure 1b, probably due to the different enantiorecognition mechanisms^[6].
    As expected, the flux increases with decreasing membrane thickness. The permeability rate of tyrosine of the current SA membrane is about 10 times higher than that measured under a pressure of 1-2 ㎏f/cm² as reported in the literature^[2], indicating that it is an effective way to improve the permeating rate by decreasing membrane thickness. What is more, it is interesting to find that the enantioselectivity can also be enhanced simultaneously. The results show that the values of %e.e. and a are 33.56 and 2.01 for the membrane, respectively. They are much higher than those reported in the literature^[2]. Therefore, the mechanism involved in the permeation behavior of the enantioselective membrane cannot be simply explained by the common model like solution-diffusion one, and need to be studied further.

Figure 1 Plots of the amount of the permeant permeated through the membrane versus time at 25 ºC and 40ºC. (a) phenylalanine, and (b) tyrosine

Table 2 Enantioselective permeation of amino acids through the SA membrane

手性多糖膜对光学异构体的对映体选择传递
肖定书¹, 胡继文^1,2,*, 章明秋², 李明威¹, 冯良¹
(¹中国科学院广州化学所，广东省电子高分子材料重点实验室，广州，５１０６５０;²中山大学教育部高分子复合与功能材料重点实验室，广州，５１０２７５)
摘要  本工作制备了交联的海藻酸钠膜，并研究了该膜对a－氨基酸对映体的选择性传递行为。结果显示：所制备的薄膜具有较高的渗透能力和较理想的选择性。此外，低温有利于提高选择性，但渗透率下降。手性膜对酪氨酸和苯丙氨酸的对映体选择性传递行为的差异揭示了氢键作用在手性分离中的意义。
关键词 海藻酸钠；膜；对映体选择；氢键