Molecularly imprinted polymers three-dimensional imprinting

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... 

One direct approach to the separation of chiral compounds is called molecular imprint polymers (MIPs) that involves the formation of a three-dimensional cavity with the shape and electronic features that are complementary to the imprinted or target molecule. 

Fig. 3 Application of the Doehlert experimental design to optimize a MIP for propranolol with respect to the type of cross-linker (EDMA or TRIM) and the degree of cross-linking, (a) Three-dimensional representation of response surfaces for the percentage of bound [3H]propanolol to the molecularly imprinted polymer (MIP) and the corresponding non-imprinted control polymer (NIP), (b) Contour plot of the function describing binding of [3H]propanolol to MIPs relative to the degree and the kind (bi or trifunctional) cross-linking. The values were corrected for non-specific binding to the non-imprinted control polymer. Adapted from [31] with kind permission from Springer Science + Business Media...

It should be noted that the presence of cross-links results in the partial or complete loss of control over the size of the polymer molecules, even if the living character of the polymerization can sometimes be preserved. Incidently, one of the characteristics of MIPs is that they are cross-linked polymers. This cross-linking is necessary in order to maintain the conformation of the three-dimensional binding sites obtained through the molecular imprinting process, and thus the ability of the polymer to recognize specifically and selectively its target molecule. Nevertheless, even with cross-linked polymers, the use of CRP methods may be beneficial, as it can, up to a certain point, improve the structure of the polymer matrix. Indeed, all of the above CRP methods have been applied to MIPs. 

Molecular imprinting allows the generation of specific three-dimensional cavities in polymer matrices by using a template molecule around which functional monomers and cross-linker are self-assembled in a pre-polymerisation state. Following polymerisation and template removal, the polymer matrix is left with the free three-dimensional cavities capable of rebinding the molecule, or others structurally very similar, used for the imprinting. 

Molecular imprinting in synthetic polymers was reported for the first time in 1972 [1--4]. The initial idea was to obtain in the polymer highly specific binding clefts with a predetermined size, shape and three-dimensional arrangement of functional groups. Later on, further experiments demonstrated that such functionalised cavities could be tailored to mimic the active sites of enzymes ( enzyme analogue built polymer ). 

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. 

Nolte et al 46) produced an artificial enzyme based on the T4 replisome and applied it to the epoxidation of double bonds in synthetic polymers. Smith et al 51) reported that horseradish peroxidase catalyzes the oxidative polymerization of glucuronic acid. In recent literature, many biomimetic macromolecules with enzyme-like structures or functions have been reported including those that are dendrimers 64-66), those that have specified three-dimensional structures or recognition elements created by molecular imprinting 67), and other enzyme mimics 68). 

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