The Concepts of Molecular Imprinting

Molecular imprinting, sometimes referred to as template polymerization, is a technique for preparing synthetic polymers of predetermined selectivity. Receptor-like binding sites are tailor-made in situ by the copolymerization of cross-linkers and functional monomers, which are interacting covalently or non-covalently with print molecules (or templates). After polymerization, the print molecules are removed from the polymer, either by extraction or by chemical cleavage, leaving recognition sites complementary to the print molecules in the shape and positioning of functional groups. The polymer is subsequently able to rebind the print molecules. The noncovalent approach of molecular imprinting is exemplified in Figure 1.

The association/dissociation kinetics of non-covalent MIPs is in general faster than is observed with polymers prepared by the covalent approach. For this reason, the former polymers are more attract ive as stationary phases for chromatography. Even if several examples of covalently imprinted stationary phases have been reported, it was not until the development of the noncovalent approach that the technique became competitive for the preparation of CSPs. This review will therefore focus on noncovalent molecular imprinting. The covalent approach has been covered in several excellent reviews by Wulff.

Even if the general understanding has been that the selectivity of MIPs is due to the formation of specific recognition sites complementary to the print molecules, early critics of the technique argued that the recognition could come from binding to print molecules entrapped in the highly cross-linked polymers. A small percentage of the print molecules are inaccessible and remain in the polymer network after extraction (normally less than 1%). However, this was ruled out as the cause of the selectivity by experiments showing that a polymer containing covalently bound print molecules did not exhibit any selective binding of the print molecule.

Figure 1 Schematic representation of the concept of noncovalent molecular imprinting. Functional monomers, in this case meth-acrylic acid, interact with the print molecule, Z-Glu-OH. Cross-linker is added and the polymerization is initiated. The interactions are maintained in the resulting polymer. The print molecule is removed from the polymer by extraction, leaving a specific recognition site complementary to the print molecule in shape and positioning of the functional groups. The polymer has attained a memory of the print molecule and is able selectively to rebind it.

Figure 1 Schematic representation of the concept of noncovalent molecular imprinting. Functional monomers, in this case meth-acrylic acid, interact with the print molecule, Z-Glu-OH. Cross-linker is added and the polymerization is initiated. The interactions are maintained in the resulting polymer. The print molecule is removed from the polymer by extraction, leaving a specific recognition site complementary to the print molecule in shape and positioning of the functional groups. The polymer has attained a memory of the print molecule and is able selectively to rebind it.

Molecular imprinting produce recognition sites with a distribution of binding strengths; the sites are heterogeneous. Some sites have a highly selective affinity for the template, whereas others are less selective. When used for chromatographic applications, the heterogeneity is reflected in band broadening and asymmetric peaks.

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