The Principle of Molecular Imprinting

Decades ago, when the very nature of the immune response was still under discussion, Linus Pauling proposed the so-called instructive theory of antibody creation. According to this theory antibody molecules containing flexible, functional groups were winding around the different antigens in question, thus capturing them in a specifically created interactive pocket . 

Synthetic Polymers for Biotechnology and Medicine, edited by Ruth Freitag. 

In case of covalent interactions the choice of the functional monomer, which has to be linked covalendy to the template prior to imprinting, depends on the chemistry of the template. Templates containing cis-diol groups are usually esterified with a vinyl-boronic acid component to yield a reversible bis-ester. If this is not possible, alternatives are the generation of Schiff-bases (azomethines). This requires an amino group on the template and a carboxyl acid group on the functional monomer or vice versa (Table 7.2). 

The cross-linker provides up to 90% of the polymerizable groups and thus determines the polymer chemistry of the MIP. Amongst other things it acts as a solvent for the template and 

Most MIP today are prepared via standard free radical polymerization in dispersion. The initiator for the radical polymerization has to be selected with regard to its reliability. There are a few azo-bis compounds like AIBN , which are frequently used in thermo-initiated polymerization, but for UV-initiated processes, ABCHC seems to be more reliable than AIBN. For some special approaches, where molecular imprinting takes place in aqueous media, a water-soluble initiator like ABDV has to be chosen. 

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The principle of molecularly imprinted polymer (MIP) production is schematised in Fig. 4. 

Imprinted polymers have lately shifted into the center of interest, particularly in the quest for sensors for analytes more complex than ions [251]. The principle of molecular imprinting is basically simple a well-optimised mixture of monomers with various functional groups is polymerised in the presence of the analyte (the template ). Hereafter, the analyte is eluted from the polymer matrix, which leaves complementary binding sites in the polymer for the analyte. This polymer can then be used in an analytical system or sensor. Artificial enzymes can also be produced by molecular imprinting, in which a transition state analogue of a chemical reaction is used as the template molecule [252]. 

The principle of molecular imprinting is the assembly of a cross-linked polymer matrix around a template when the template is removed. The principle of molecular imprinting is shown in Figure 2 [26], The template molecule assembles with a cross-linked polymer matrix via non-covalent or covalent interactions. Then the template molecule is removed from the polymers. The processed polymers possess stable capacity of specific recognition and selectivity to the template molecule. 

FIGURE 1.49 Principle of molecular imprinting.169 1 = functional monomers 2 = cross-linking monomer 3 = molecule whose imprint is desired (molecular template). In (A), 1 and 2 form a complex with 3 and hold it in position in (B), polymerization involving 1 2 occurs and the template (imprint molecule) is held in the polymeric structure in (C) and (D) the imprint molecule is removed leaving a cavity complementary to its size and shape into which a target analyte of similar dimensions can fit. (Reproduced with permission from Taylor Francis.)... 

Fig. 1 General principle of molecular imprinting. A molecular template (T) is mixed with functional monomers (M) and a cross-linker (CL) resulting in the formation of a self-assembled complex (1). The polymerization of the resulting system produces a rigid structure bearing imprinted sites (2). Finally removal of the template liberates cavities that can specifically recognize and bind the target molecule (3). Adapted with permission from [3]. Copyright 2003 American Chemical Society...

Ye, L. Mosbach, K. The technique of molecular imprinting—principle, state of the art, and future aspects. J. Incl. Phenom. Macro. Chem. 2001, 41, 107-113. 

Fig. 17.3 The fabrication principle of molecularly imprinted polymers (MIP). The template and the functional monomers are pre-assembled through different kinds of interactions (covalent, hydrogen bonding, etc.). A polymer matrix is then polymerized around the pre-assembled complex. The template is removed. The analyte can then bind specifictdly to the MIP...

Nicholls, I. A. Andersson, H. S., Thermodynamic principles underlying molecularly imprinted polymer formulation and ligand recognition. In Molecularly Imprinted Polymers, Man-Made Mimics of Antibodies and Their Practical Application in Analytical Chemistry, Sellergren, Ed. Elsevier, Amsterdam, The Netherlands 2001, 59-70... 

Finally, note that it has been proposed to modify enzymes using the principles of the molecular imprinting technique to modulate their action [185]. 

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