Monomer-template complex

Metalloporphyrins have been used for the development of MIP for detection of a 9-ethyladenine nucleobase derivative [35]. With the increase of the template concentration, the bulk polymer exhibited a red shift in the absorbance spectra. This shift allowed for the quantitative detection of the template, which showed formation of a 1 1 monomer-template complex. Moreover, cinchonidine was imprinted using metalloporphyrin and MAA as the functional monomers [40]. The resulting MIP provided high selectivity against a diastereomer of the... 

These template polymerizations suffer from three fundamental problems (i) In most cases the binding of the polymer to the template is stronger than the binding of the monomer due to the cooperativity of the interaction between the polymers. As a consequence the newly formed macromolecules are not released from the template and multiple replication is not possible without multiple separation steps, (ii) We lack the possibility to start the polymerisation reaction at the terminal group of the monomer-template complex, (iii) While a weak interaction between the template and the monomer is favourable to allow easy separation of the template and the newly formed macromolecule, it leads to incomplete complexation of the template and interraption of the polymerisation along the chain. A solution of these problems would require a relatively strong complexation of the monomers in combination with sufficient anticooperativity in the complexation of the polymer. The latter however would inevitably impede the polymerisation reaction and require therefore a living polymerisation mechanism which does not suffer from a slowed down rate of polymerisation. 

The solvent should be capable of fully solubilizing the monomers and template in one pot. For monomer-template interactions stabilized by polar forces, non-protic solvents of low polarity should be chosen, since they are less likely to compete with the monomers for the template. The functional monomer-template complexes are often based on hydrogen bond interactions. If the solvent is a good hydrogen bond donor or acceptor, it will compete with the monomers and destabilize the complexes. For monomer-template systems stabilized by solvatophoic forces, more polar solvents and higher temperatures may be favorable. 

Since the interactions between the monomers and the templates are of weak nature, several combinations of monomer-template complexes exist in the prepolymerization mixture. In addition, the monomers are often present in excess, which results in randomly distributed non-specific binding points. After polymerization, this plurality will exist also in the binding sites of the polymer the sites are heterogeneous. The heterogeneity of the recognition sites is reflected in a distribution in affinity for the template. 

Figure 2.2 Imprinting of phenyl a-D-mannopyranoside using (4-vinylphenyl)boronic acid. Formation of monomer-template complex (1) polymerization (2) cleavage and extraction of template (3) and rebinding (4).

Svenson J, Ning Z, Fohrman U, Nicholls IA (2005) The role of functional monomer-template complexation on the performance of atrazine molecularly imprinted polymers. Anal Lett 38(l) 57-69... 

Covalent interactions (Fig. 7) this approach involves the formation of an easily cleavable functionalised monomer-template complex [61, 18]. The first example of an imprinted polymer network was prepared by Wulff s group and used the reversible formation of an ester bond between a diol and 4-vinylphenylboronic acid [15]. The reaction rates to reach equilibrium between the ester and boronic acid are comparable to those obtained with... 

It should be noted that spectroscopic titration usually shows the formation of a 1 1 adduct between template and functional monomer, even when several functional monomers interact with several parts of template. In other words, these interactions are almost independent of each other in solutions (n 1 monomer/template complexes are hardly formed therein because of the unfavorable entropy term). Even in these cases, the monomers which are interacting with the template at different positions are covalently bound to each other in the polymerization. These steps should proceed in a stepwise manner. Thus, the polymerization occurs around a site of the template. Independently, polymers are also formed at other sites of the template. Finally, these polymer do-... 

Based on the occurring interactions during the functional monomer-template complex formation and template rebinding, three approaches regarding molecular imprinting are reported ... 

A primary indication on how well the monomers have been chosen is to simply see whether they are capable of assisting solubilization of the template in the prepolymerization mixture. A small-scale solubility test may thus be a good way to initially screen the monomers for strong monomer-template interactions. Weak interactions may be revealed by complexation induced spectral changes (in NMR,UVor fluorescence spectra). The complexation induced shifts of the characteristic H-NMR signals of the template upon increasing monomer concentrations are often used to estimate the monomer-template association constants. Prior to this, however, knowledge about the stoichiometry of the monomer-template complexation and the tendency of the monomer and template to self-associate are required.The former can be obtained by means of a so-called Job s plot whereas the latter by a dilution experiment. 

The modeling results clearly indicate that the formation of monomer-template complexes is dependent upon the temperature. It was anticipated that polymers synthesized at higher temperatures (> 283 K or 10°C) would be inferior to those prepared at lower temperatures due to the lack of template binding to the second monomer. In order to prove this hypothesis, a series of polymers were synthesized at different temperatures ( 30°C, 20°C, 10°C, 0°C, 20°C, and 80°C) and tested in HPLC studies. The details of this work are published elsewhere [76]. The chromatographic evaluations were performed at temperatures varying from 10°C to 55°C. The results of this evaluation, expressed in terms of temperature dependence of separation factors for ephedrine enantiomers, are presented in Fig. 13. As expected, a clear decrease in separation factors (a) from a = 4.04 (for polymers made at 30°C... 

Table 5 Influence of the Temperature on Distance Between Hydroxyls in Monomers-Template Complex...

Once the imprinting system has been devised to yield favorable monomer-template complexation and the necessary porosity, the preparation of monolithic polymer rods for HPLC is relatively simple. The general protocol detailed below uses an in situ polymerization method developed by Frechet and Svec [4]. This technique was used by Matsui in the preparation of MIP monolith rods for HPLC separation of antimalarial cinchona alkaloids, ( ) cinchonidine and (+) cinchonine, as well as the structural analogues quinidine and quinine [45]. 

The principal idea behind MIP synthesis is the generation of solution state complexes between the template ligand in hand and appropriate functional monomers, followed by subsequent freezing of these complexes by copolymerization of the above with an excess of a cross-linking monomer. These monomer-template complexes are stabilized by non-covalent interactions, reversible covalent interactions, or metal ion-mediated interactions. The types of interactions that are usually exploited in molecular imprinting are 1) cleavable covalent bonds 2) tt-tt interactions 3) hydrogen bonds 4) hydrophobic van der Waals interactions 5) crown-ether/cyclodextrin type interactions 6) metal-Ugand... 

In this connection, it should be underlined that the best MAA/L-Phe-An ratio found in column chromatography was 4 1, using a mixture of 10% acetic acid in acetonitrile as eluent, while no separation of optical antipodes was observed for 2 1 ratio [44]. In the latter case, a monomer/template complex (1 1) was obtained, while in the former case a 2 1 complex was observed, which was able to give selective binding sites by ionic bonding formation with the free primary amine and hydrogen bonds with amide (see Figure 4.6) [45]. 

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