Chapter 5
Recent Advances in the Use of Molecularly Imprinted Materials in Separation and Synthesis
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Olof Ramström, Lei Ye, Cong Yu, and Per-Erik Gustavsson Department of Pure and Applied Biochemistry, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
Molecularly imprinted materials, prepared using self-assembly imprinting protocols using only non-covalent interactions, can be used as chromatography media in aqueous phase. The recognition properties are highly dependent on the interacting species used in the imprinting protocol. Ionic interactions, together with strong hydrogen bonding, represent useful means of obtaining recognition. With increasing levels of water in the recognition media, the hydrophobic effect comes into play. Molecularly imprinted materials can furthermore be used as auxiliary agents in enzymatic syntheses in water-saturated organic phases. When materials molecularly imprinted for the reaction product were applied to the thermolysin-catalyzed aspartame synthesis, an increase in yield was observed. Introduction of a thermodynamic trap provided by the molecularly imprinted matrices allowed a non-favorable equilibrium for the thermolysin reaction to be pushed in the forward direction.
Molecularly imprinted materials possess a high potential for use in a variety of applications, such as chromatographic stationary phases, immunoassay-type analyses, and sensor technologies (1-5). The stability and endurance of these materials are attractive features entailed by these materials which are focusing attention on their use as recognition elements besides naturally occurring matrices, such as antibodies and receptors. One of the current challenges for molecularly imprinted materials is their preparation and/or usage in aqueous environments (6). When self-assembly imprinting protocols using only non-covalent interactions are employed, water reduces the binding strength of such interactions. Since self-assembly protocols have been shown to be of great importance, which produce faster binding kinetics and a wider range of appropriate systems, this challenge needs to be overcome.
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©1998 American Chemical Society
In Molecular and Ionic Recognition with Imprinted Polymers; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
Downloaded by PENNSYLVANIA STATE UNIV on July 31, 2012 | http://pubs.acs.org Publication Date: May 7, 1998 | doi: 10.1021/bk-1998-0703.ch005
83 One approach to meeting the challenge is to prepare the materials in an organic phase and subsequently use them in an aqueous phase (7-9). Although the aqueous phase challenge is an important one, these materials may also be employed in various applications in water-poor phases. A multitude of applications has been envisioned for such materials in organic phases, the foremost being in the separation area. In some biotechnological applications, water-poor phases are prerequisite to the success of the process. For example, some enzymatic syntheses proceed well in water-poor phases, which control or even reverse the reaction equilibrium (10). In these cases, molecularly imprinted polymers (MIPs) may serve as auxiliary recognition elements for an enhanced control of the process. In this work we describe some recent advances in the use of imprinted materials in recognition and separation applications. Molecularly imprinted polymers are used as stationary phases in aqueous-phase chromatographic systems and as equilibrium traps in enzymatic synthesis. Recognition Properties of Molecularly Imprinted Polymers in Aqueous Phases In biological systems, water plays a dominant role as the surrounding medium. Therefore, mimics of natural binding events are most effectively demonstrated in aqueous systems. However, the influence of water as a porogen in the imprinting process or recognition media greatly diminishes the energy of non-covalent interactions (77). For techniques currently in use, where small, often monofunctional, monomers are employed for recognition of the imprint species in solution prior to polymerisation, the disrupting effect of water on ionic, hydrogen bonded, and van der Waals' interactions intrinsically leads to very weak recognition effects. Also, the solubility of the necessary ingredients, such as crosslinkers and monomers, may be too low in an aqueous or partially aqueous medium to allow for the formation of solid polymers. To overcome the bond-breaking effects of water, molecularly imprinted polymers have first been prepared in organic media, where the interactions are strong, and subsequently been used in aqueous environments (12-13). With this protocol, the designed sites formed in the imprinting process may lead to a concerted action of participating functional groups to selectively recognize the ligands. The recognition properties of molecularly imprinted polymers in aqueous phases are dependent on the nature and quality of the interacting species. The choice of functional monomers together with the arrangement of functional groups of the imprint species are two of the main factors responsible for recognition. This was clearly demonstrated when molecularly imprinted polymers were prepared with the protected amino acid benzyloxycarbonyl-L-tyrosine (Cbz-L-Tyr) and either 2-vinylpyridine (2VPy) or methacrylic acid (MAA), or a combination of both, as functionally interacting monomers in a self-assembly imprinting protocol which involved only non-covalent interactions. These systems have previously been shown to possess excellent recognition properties in organic media (14). In particular, a terpolymer system, which incorporated both functional monomers, exhibited a very high discrimination effect between the enantiomers of Cbz-Tyr. The separation factor (a) was 4.32 in acetonitrile containing 1% acetic acid (v/v). A combination of ionic interactions between the pyridinyl moieties of the polymer and the carboxyl functionalities of the imprinted species, together with hydrogen bonding capabilities of the carboxy functionalities of the matrix are responsible for recognition. In the present study, these polymer systems were analyzed in the HPLCIn Molecular and Ionic Recognition with Imprinted Polymers; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
84 mode for their performance to discriminate in binding between the enantiomers of the imprint species in aqueous media. The influences of the pH of the mobile phase and the level of added organic solvent on the separation behavior were investigated. It was found that the polymers containing 2-vinylpyridine performed well in aqueous media at pH
