Covalent rebinding

The cavity left behind after cleaving off the covalently bound template moiety is often too small to accommodate the target molecule during non-covalent rebinding. This problem was addressed by Whitcombe in 1994 when the sacrificial spacer approach was introduced [72]. In this approach, the template and the monomer is joined by a spacer, which is cleaved off when the template is cleaved from the polymer (Figure 2.3). The sacrificial spacer approach has been applied to the imprinting of cholesterol [73], DDT [74] and heterocyclic aromatic compounds [75]. 

Fig. 1. Preparation of configurational biomimetic imprinted networks for molecular recognition of biological substrates. A Solution mixture of template, functional monomer(s) (triangles and circles), crosslinking monomer, solvent, and initiator (I). B The prepolymerization complex is formed via covalent or noncovalent chemistry. C The formation of the network. D Wash step where original template is removed. E Rebinding of template. F In less crosslinked systems, movement of the macromolecular chains will produce areas of differing affinity and specificity (filled molecule is isomer of template).

There are two general classes of imprinted polymers covalent and noncovalent MlPs. These two categories refer to the types of interactions between the functional monomer and the template in the prepolymerization complex. There are also hybrid MlPs that utilize a combination of covalent and noncovalent interactions in the preparation and rebinding events (Klein et al. 1999). Covalent MlPs utilize reversible covalent interactions to bind the template to the functional monomers. In contrast, noncovalent MlPs rely on weaker noncovalent functional monomer-template interactions. Each type has specific advantages and disadvantages with respect to sensing applications that will be addressed in subsequent sections. 

Preformed polymers can also be employed to prepare imprinted core-shell particles [143]. The group of Chang recently prepared a poly(amic acid) bearing oestrone as a template molecule covalently bound to the polymer through a urethane linker (see Fig. 2). A layer of this polymer was subsequently deposited on silica particles (10 pm diameter) prefunctionalised with amino groups at their surface. Thermal imidisation of the polymer yielded finally a polyimide shell (thickness about 100 nm) on the silica particles. Subsequent template removal yielded the imprinted cavities, which exhibited selective rebinding of oestrone in HPLC experiments. 

The special advantage of amidine-based stoichiometric non-covalent interactions is illustrated in Fig. 4.7. The templates 15 were removed from an imprinted polymer and then the polymer was re-offered different amounts of template 15 in methanol for rebinding. Figure 4.7 shows a nearly stoichiometric uptake with a reloading yield of about 99%. This is in marked contrast to non-stoichiometric non-covalent interactions in which only around 15% of the cavities can be reloaded. 

ACIDIC FUNCTIONAL MONOMERS GIVING ENHANCED REBINDING SELECTIVITY IN NON-COVALENT MOLECULAR IMPRINTING... 

Molecular imprinting technique was recently used to prepare highly selective tailor-made synthetic affinity media used mainly in chromatographic resolution of racemates or artiftcial antibodies [130-133]. A complex between the template molecule and the functional monomer is first formed in solution by covalent or non-covalent interactions (Figure 3.10). Subsequently, the three-dimensional architecture of these complexes is confined by polymerization with a high concentration of cross-linker. The template molecules are then extracted from the polymer leaving behind complementary sites (both in shape and functionahty) to the imprinted molecules. These sites can further rebind other print molecules. 

Rebinding of the template to the binding site is achieved either by the reestablishment of the original bonds or by non-covalent interactions at the precisely positioned functional groups of the imprinted site [15]. The pioneering work in this area utilised the rapid and reversible nature of the boro-nic acid/diol reaction [15] (Figure 6.4). 

Figure 2.3 Semi-covalent molecular imprinting of cholesterol using the sacrificial spacer approach. Polymerization (1) cleavage and extraction (2) rebinding (association) (3) and dissociation (4).

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