Chiral Solvent Imprinting

3 Pluinericin and Prismatomerin isolation and characterization of plumericin led Albers-Schonberg and Schmid to assign it structure 30. Subsequent analysis of its BCD spectrum and comparison with computed semiempirical spectrum suggested that the actual stracture is the enantiomer of 30.  

Stephens next examined the related iridoid prismatomerin 31. The OR of plumericin 30 is [aJo = -1-204. The structurally related prismatomerin 31 has 

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Mukhopadhyay, R Zuber, G. Wipf, P. Beratan, D. N. Contribution of a solute s chiral solvent imprint to optical rotation, Angew. Chem. Int. Ed. 2007, 46, 6450-6452. 

Molecular imprinted polymers MIPs exhibit predetermined enan-tioselectivity for a specific chiral molecnle, which is nsed as the chiral template dnring the imprinting process. Most MIPs are obtained by copolymerization from a mixture consisting of a fnnctional mono-nnsatn-rated (vinylic, acrylic, methacrylic) monomer, a di- or tri-nnsatnrated cross-linker (vinylic, acrylic, methacrylic), a chiral template (print molecnle) and a porogenic solvent to create a three-dimensional network. When removing the print molecnle, chiral cavities are released within the polymer network. The MIP will memorize the steric and functional binding featnres of the template molecnle. Therefore, inclusion of the enantiomers into the asymmetric cavities of this network can be assumed as... 

MIP assays can also be utilized in synthetic organic applications. For example, MIP-based assays have been used to measure the chiral purity of samples in organic solvents. An L-phenylalanine anilide (l-PAA) imprinted polymer was utilized as a recognition element to measure the enantiomeric excess (ee) of PAA samples (Chen and Shimizu 2002). The MIP displays greater capacity for l-PAA versus d-PAA samples of similar concentration, and this difference was used to estimate enantiomeric excess. The enantiomeric excess of an unknown solution was determined by comparing the UV absorbance of the PAA remaining in solution after equilibration against a calibration curve. This MIP assay was demonstrated to be rapid and accurate with a standard error of +5% ee. 

Conventionally, MlPs are obtained by bulk co-polymerization from a mixture consisting of a functional monomer, cross-linker, chiral template, and a porogenic solvent mixture. Nowadays, imprinting via non-covalent template binding is preferred over the covalent mode and involves three major steps (see Fig. 9.9). (i) Functional monomers (e.g. methacrylic acid, MAA) and a cross-linker (e.g. ethyleneglycol dimethacrylate, EDMA) assemble around the enantiomeric print molecule, e.g. (S)-phenylalanine anilide (1), driven by non-covalent intermolecular interactions, e.g. ionic interactions, hydrogen bonding, dipole-dipole interaction. Tr-rt-interaction. (ii) By thermally or photochemi-... 

The enantiomers of phenylalanine anilide were analyzed on an imprinted poly(EDMA-co-MAA] polymer grafted onto an ITO glass electrode based on the chiral discriminative gate effect of the MIP in organic solvents [49]. The chiral differentiation implied the evaluation of the effect of phenylalanine anilide enantiomers on the faradic current of redox species (ferrocene], effect caused by a... 

Sekine, S., Watanabe, Y, Yoshimi, Y., Hattori, K., and Sakai, K. (2007). Influence of solvents on chiral discriminative gate effect of molecularly imprinted poly(ethylene glycol dimethaciylate-co-methacrylic acidl Sens. Actuators B. 127, 512-517. 

The optimal cross-linker is also dependent upon the template and solvent. When imprinting proteins, for example, a water-soluble cross-linker is needed like the water-soluble cross-linker A,A -methylene diacrylamide (methyleneb-isacrylamide). Divinylbenzene (DVB) is also a popular cross-linker and has been shown to be efficient for certain imprinting systems. An example of this is in the second imprinting protocol in this chapter, where DVB is used as a cross-linker for imprinting the chiral template (R) -isoproterenol. 

Kondo, Y. Yoshikawa, M. Effect of solvent composition on chiral recognition ability of molecularly imprinted DIDE derivatives. Analyst 2001,126, 781-783. 

A MI membrane suitable for chiral separations was recently introduced by Yoshikawa et al They imprinted Boc-L-Ttp in THF using an anchor-tetrapeptide (DIDE, DLDE or DVNE) immobilized on a polymer (polystyrene) / copolymer (acrylonitrile-costyrene) by simply allowing the solvent to evaporate to yield the membrane. The sep>aration factors obtained with these membranes were between 1.0 and 1.4 and hence not all that impressive. The authors demonstrated that an increase of the Boc-L-Trp/DIDE ratio led to an increase of the adsorbed amounts of both the template (L-Ttp) audits enantiomer (D-Trp), probably due to... 

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