Polymers enantioseparation

Fig. 9.1.. BSA immobilized on convcciivc macroporous polymer. Enantioseparation of Ketoprofen by perfusion chromatography (reprinted with permission from Ref. [221]).

Enantioseparation is typically achieved as a result of the differences in interaction energies A(AG) between each enantiomer and a selector. This difference does not need to be very large, a modest A(AG) = 0.24 kcal/mol is sufficient to achieve a separation factor a of 1.5. Another mechanism of discrimination of enantiomers involves the preferential inclusion of one into a cavity or within the helical structure of a polymer. The selectivity of a selector is most often expressed in terms of retention of both enantiomers using the separation factor a that is defined as ... 

A further modification is the creation of polymers out of CDs, which are also used for enantioseparations (20-22). They are obtained by radical copolymerization and often exhibit enormous solubility (23). 

L Ma, J Han, H Wang, J Gu, R Fu. Capillary electrophoresis enantioseparation of drugs using (3-cyclodextrin polymer Intramolecular synergistic effect. Electrophoresis 20 1900-1903, 1999. 

In addition to the classification of liquid chromatographic enantioseparation methods by technical description, these methods could further be classified according to the chemical structure of the diverse CSPs. The chiral selector moiety varies from large molecules, based on natural or synthetic polymers in which the chirality may be based on chiral subunits (monomers) or intrinsically on the total structure (e.g., helicity or chiral cavity), to low molecular weight molecules which are irreversibly and/or covalently bound to a rigid hard matrix, most often silica gel. 

Yashima, E., Matsushima, T., Nimura, T., and Okamoto, Y. (1996) Enantioseparation on optically active stereoregular polyphenylacetylene derivatives as chiral stationary phase for HPLC, Korea Polym. J. 4, 139-146. 

Fig. 9.11. Reaciion scheme for the synthesis of network-polymeric CSPs and representative chromatograms, (a) Derivatization of A, A -diallyl-(R.R)-tartaric acid diamide (DATD) to give the bifunctional monomers used as chiral SO units, (b) Cross-linking and immobilization by hydrosilylation with multifunctional hydrosilane (alternatively, cross-linking and immobilization can be performed first with DATD followed by O-derivatization). (c) Enantioseparation of 2-(octylsulphinyl)benzoic acid. The chromatograms illustrate the column performance under non-overloadcd (left) and overloaded conditions (right). CSP network polymer from /V. -diallyl-i/il.Rl-tartaric acid diamide fc/.s-. i.S-dimethylbenzoatc bound to. ) pm 1.50 A Kromasil. Mobile phase hexane-THF (80 20 v/v) with 0.0.55 - of TFA (reprinted with permission from Ref. [194]).

The use of a convective macroporous polymer as an alternative support material instead of silica for the preparation of protein-based CSPs has successfully been demonstrated by Hofstetter et al. [221]. Enantioseparation was performed using a polymeric flow-through-type chromatographic support (POROS-EP, 20 pm polymer particles with epoxy functionalities) and covalently bound BSA as chiral SO. Using flow rates of up to 10 ml/min, rapid enantiomer separation of acidic compounds, including a variety of amino acid derivatives and drugs, could be achieved within a few minutes at medium efficiencies, typical for protein chiral stationary phases (Fig. 9.13). 

CEC was demonstrated in the analysis of /i-adrenergic antagonists using capillaries modified with propranolol-imprinted polymer. In situ molecular imprinting was performed via photo-polymerisation at — 20°C within a capillary that was premodified with 3-methacryloxypropyltrimethoxysilane. The time for UV irradiation was carefully determined so as to obtain a polymer coating of appropriate thickness. Enantioseparation of the racemate of propranolol was successfully demonstrated with a separation factor of 1.12 and a resolution factor of 1.26. 

Guaran is a polygalactomannan containing T-OH groups. It is well known that the tetrahedral borate ions readily form tetrahedral complexes with the cw-OH groups. Thus the borate immobilized polymer (boron content 0.7 mmol/g) was used for enantioseparation of racemates such as 1,2- or 1,3-dihydroxy compounds or a-hydroxy acids.This appears to be the first example of the use of boron as a complexing ion in CLEC. 

Immobilization can be achieved by adsorption or covalent fixation of the biocatalyst to a solid support (e.g. surface-modified polymer or glass beads), by entrapment or by encapsulation in gel beads (e.g., agarose, polyacrylamide, alginate, etc.). Hundreds of immobilization methods have been described and reviewed in the literature [83-89], but only a limited set of methods has found real technical applications. The first large-scale applications of immobilized enzymes were established for the enantioseparation of D- and L-amino acids by Chib-ata, Tosa and co-workers at Tanabe Seiyaku Company. The Japanese achievements in the large-scale application of immobilized systems are very well documented in an excellent multi-author publication edited by Tanaka, Tosa and Kobayashi [90] (see also section 7). Some enzyme suppliers sell important industrial enzymes not only in the free form (solution or powder) but also immobilized on solid supports. 

Lin, J.M. Nakagama, T. Uchiyama, K. Hobo, T. Enantioseparation of D,L-phenylala-nine by molecularly imprinted polymer particles filled capillary electrochromatography. J. Liq. Chrom. Rel. Technol. 1997, 20, 1489 1506. 

The most extensive studies on molecular-imprinted polymers as stationary phases for enantioseparations were conducted using amino acids and their derivatives as the template, but many other classes of compounds have been studied, including peptides, p-blockers, and nonsteroidal anti-inflammatory drugs (see Table 3). Examples of separations are reported in Figs. 6-8. 

However, among the drawbacks, it is necessary to take into the account that the intrinsically polyclonal nature of the noncovalently imprinted polymers causes the presence of a small number of highly enantioselective binding sites and a large number of less selective binding sites, and it is associated to nonlinear adsorption isotherms. As a consequence, enantioseparations are frequently affected by asymmetric peaks and slow association-dissociation kinetics that result in low peak resolution and a marked sensitivity to the amount of sample introduced in the column. 

Ansell, R.J. (2005) Molecularly imprinted polymers for the enantioseparation of chiral drugs. Adv. Drug Deliv. Rev., 57,1809-1835. 

Okamoto Y, Noguchi J, Yashima E (1998) Enantioseparation on 3,5-dichloro- and 3,5-dimethylphenylcarbamates of polysaccharides as chiral stationary phases for high-performance liquid chromatography. React Funct Polym 37 183-188... 

Lin JM, Nakagama T, Uchiyama K et al. Molecularly imprinted polymer as chiral selector for enantioseparation of amino acids by capillary gel electrophoresis. Chromatographia 1996 ... 

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