Amidinium benzoate

At pH 7.6, the imprinted polymer accelerated the rate of hydrolysis of ester 18 by more than 100-fold compared to the reaction in solution at the same pH (see equation (c) and Table 4.9). Addition of an equivalent amount of monomeric amidine to the solution only slightly increased the rate. A control polymer prepared from the amidinium-benzoate salt gave a somewhat stronger rate enhancement. 

This shows that all active sites are then occupied and that the reaction becomes independent of substrate concentration. In contrast, in solution, and in solution with the addition of amidine, a much slower linear relationship is observed. The amidinium benzoate shows also some type of Michaelis-Menten kinetics. The benzoate therefore acts as a less effective template. 

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.

Figure 17.15 Absorption spectra of amidinium purpurin 9 (0.63 X 10 M) in protonated amidinium form (black, solid line), deprotonated amidineform (gray, solid line) and with a large excess of benzoate (8.5 x 10 M) (black, dotted) and a large excess of phenyl sulfonate (black, dashed) in CHjClj.

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