Binding site monomers

The imprinting procedure can be exemplary described by considering the polymerisation of the template monomer 1, which was used for the optimisation of the method [5,17-19]. In this case, phenyl-a-o-mannopyranoside la acts as the template molecule. Two molecules of 4-vinylbenzeneboronic acid (the binding site monomer) are covalently bound by diester linkages to the template. The template monomer... 

The usefulness of these novel amidine binding site monomers in molecular imprinting was demonstrated by their application in the preparation of polymers that were imprinted with optically active A -(4-carboxybenzoyl)-phenylglycine (15) the resulting materials could discriminate between the enantiomers of the template molecule with a values of up to 2.8 [12]. 

Scheme 4.V. Schematic representation of the polymerisation of template 19 in the presence of two equivalents of binding site monomer 13 (A), removal of 19 (B), and catalysis causing alkaline hydrolysis of 18 through a tetrahedral transition state [11].

Schematically shown in Fig. 5 is the preparation of an enzyme mimic for the hydrolysis of ester 6 by molecular imprinting. Phosphonate 5 is an analog of the transition state for the alkaline hydrolysis of Ester 4. It was used as a template for polymerization with two equivalents of the binding-site monomer iVJV -diethyl-4-vinyl-benzamidine. Amidinium groups were chosen, because they can interact electrostatically with the side carboxyl-ate group as well as with the anionic transition state of the alkaline hydrolysis, thus achieving substrate recognition and transition-state stabilization. Polymerization of the preassembled binding-site monomer with the template (Fig. 5A) followed by template removal (Fig. 5B) leaves a cavity that acts as transition-state receptor for the ester substrate (Fig. 5C). The imprinted polymer accelerates the hydrolysis of 6 more than 100-fold compared to the reaction at the same pH in buffer solution without the polymer. The reaction kinetics is of the Michaelis-Menten type. A polymer obtained with amidinium benzoate as a control, with a statistical distribution of amidinium groups, is ca. one order of magnitude less active in the hydrolysis of 6.

For the preparation of polymeric structures resembling the structure of natural receptors or enzymes, it is therefore preferable to use synthetic polymers. As will be discussed later in some detail, strong interaction between binding site monomers and template during imprinting will control the arrangement of the functional groups... 

As already outlined in Section IV, stoichiometric noncovalent interactions combine the advantages of covalency and noncovalency without suffering from their disadvantages. Stoichiometric interaction in our sense means, if a 1 1 equimolar mixture of template and binding site monomer is used, more than 90% (or even better 95%) of the template should be bound to the binding site monomer. 

It is, therefore, not necessary to use any excess of binding site monomers in order to saturate the template nearly completely. In order to reach in equimolar concentrations (0.1 molar) during imprinting 90% or better 95% degree of association, association constants of 900 (or better 3800) are necessary. Usual noncovalent interactions show much lower values. Due to this stoichiometric interaction, we have called this type stoichiometric noncovalent interaction [18,26,27]. 

This type of interaction has now been used to prepare a variety of imprinted polymers where A-toluoyl-glycyl-L-valine methyl ester acted as chiral template and 3-trifluoroacetylamino-5-(4-styryl)pyrazole 23f as the binding site monomer. In batch rebinding experiments, some of them showed a pronounced enantioselectivity towards rebinding of template (up to a = 2.7). The re-uptake of substrate to the polymers was generally only low. 

L-Phenylalanine as template was used by Nicholls and coworkers and a diacryloyl-P-cyclodextrin as the binding site monomer. In addition, a polymerizable sulfonic acid was used in the conventional noncovalent fashion. An enantioselectiv-ity, though relatively low, was observed independent of the inherent enantio selectivity of the cyclodextrin moiety [65]. 

With ampiciiiin 24 possessing both an amino group and a carboxyl group, functional monomers are needed which interact with both groups of the template but not with each other. This problem was solved by introducing two different electroneutral binding site monomers for molecular imprinting. 

With those two new binding site monomers, imprinted polymers have been prepared for ampicillin 24 as the template using all three complex partners in stoichiometric amounts. 

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