Molecularly imprinted cavities

Abstract Recent progresses in molecularly imprinted metal-complex catalysts are highlighted in this chapter. Molecular imprinting is a technique to produce a cavity with a similar shape to a particular molecule (template), and the molecularly imprinted cavity acts as shape-selective reaction space for the particular reactant. The application of the molecular-imprinting technique to heterogeneous metal-complex catalysts is focused in the viewpoint of a novel approach in the design of shape-selective catalysis mimicking enzymatic catalysis. 

The first attempt to imprint a metal complex with a reaction intermediate coordinated to the metal center was reported by Mosbach and coworkers [51], A Co monomer coordinated with dibenzoylmethane, which is as an intermediate for the aldol condensation of acetophenone and benzaldehyde, was tethered to a styrene-DVB copolymer matrix. After, the template, dibenzoylmethane was removed from the polymer, the resulting molecularly imprinted cavity had a shape similar to the template due to the interaction of the template with the polymerized styrene-DVB monomers through n-n stacking and van der Waals interactions. The rate of aldol condensation of adamantyl methyl ketone and 9-acetylanthracene was lower than the rate of condensation with acetophenone, indicating some degree of increased substrate selectivity. This is the first known formation of a C-C bond using a molecularly imprinted catalytic material. 

The preparation of polymers and other materials with molecularly imprinted cavities has now reached a high degree of sophistication. The application of these materials is becoming more and more interesting. First industrial applications of imprinted materials are envisaged, especially as stationary phases in chromatography, e.g., for the resolution of racemates. Other interesting applications can be seen in membranes and in sensors. 

The explanation offered by the authors is that the MIP was cross-reactive for naphthalene sulfonates. However, at pH 2.3, only the naphthalene sulfonates were anionic (pfCa 0.5-0.6). It is quite plausible that in this example, molecular recognition was predominantly an anion-exchange mechanism, and that this overshadowed the influence of any molecularly imprinted cavities. In this way, the MeOH wash step could be seen to have disrupted non-covalent, non-ionic interactions, leaving behind the anionic naphthalene sulfonates, which were retained much more strongly via ionic interactions with the protonated pyridyl moieties. Unfortunately, this is always a poten-... 

Koh, J.H. Larsen, A.O. White, P.S. Gagne, M.R. Disparate roles of chiral ligands and molecularly imprinted cavities in as mmetric catalysis and chiral poisoning. Organo-metallics 2001, 21, 7-9. 

Recently, molecularly imprinted polymers (MIPs) have gained attention as new, selective sorbents for chromatography and SPE. The cavities in the polymer... 

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.)... 

To prepare artificial enzymatic systems possessing molecular recognition ability for particular molecules, molecular imprinting methods that create template-shaped cavities with the memory of the template molecules in polymer matrices, have been developed [22, 30-35] and established in receptor, chromatographical separations, fine-chemical sensing, etc. in the past decade. The molecular... 

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... 

In addition to imprinted acid-base catalysts [49-55], attempts to imprint metal complexes have been reported and constitute the current state of the art [46, 47]. In most cases of metal-complex imprinting, ligands of the complexes are used as template molecules, which aims to create a cavity near the metal site. Molecular imprinting of metal complexes exhibits several notable features (i) attachment of metal complex on robust supports (ii) surrounding of the metal complex by polymer matrix and (iii) production of a shape selective cavity on the metal site. Metal complexes thus imprinted have been appHed to molecular recognition [56, 57], reactive complex stabilization [58, 59], Hgand exchange reaction [60] and catalysis [61-70]. 

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. 

This chapter will introduce the field of sensors based on molecular imprinted polymers (MIPs). MIPs are highly cross-finked polymers that are formed with the presence of a template molecule (Haupt and Mosbach 2000 Wulff 2002). The removal of the template molecule from the polymer matrix creates a binding cavity that is complementary in size and shape to the template molecule and is fined with appropriately positioned recognition groups (Scheme 15.1). 

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...

The transport properties across an MIP membrane are controlled by both a sieving effect due to the membrane pore structure and a selective absorption effect due to the imprinted cavities [199, 200]. Therefore, different selective transport mechanisms across MIP membranes could be distinguished according to the porous structure of the polymeric material. Meso- and microporous imprinted membranes facilitate template transport through the membrane, in that preferential absorption of the template promotes its diffusion, whereas macroporous membranes act rather as membrane absorbers, in which selective template binding causes a diffusion delay. As a consequence, the separation performance depends not only on the efficiency of molecular recognition but also on the membrane morphology, especially on the barrier pore size and the thickness of the membrane. 

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. 

In 2008 Resmini et al. [76] presented their work on the synthesis of novel molecularly imprinted nanogels with Aldolase type I activity in the cross-aldol reaction between 4-nitrobenzaldehyde and acetone. A polymerisable proline derivative was used as the functional monomer to mimic the enamine-based mechanism of aldolase type I enzymes. A 1,3-diketone template, used to create the cavity, was... 

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