Enantioselective ester hydrolysis catalyzed by imprinted polymers. 2

Highly cross-linked network polymers prepared by molecular imprinting catal yzed enantioselectively the hydrolysis of N-tert-butoxycarbonyl phenylalani ne-p-nitrophenyl ester (BOCPheONP). The templates were designed to allow in corporation of the key catalytic elements, found in the proteolytic enzyme chymotrypsin, into the polymer active sites. Three model systems were evalu ated. These were constructed from a chiral phosphonate analogue of phenylal anine (series A, C) or L-phenylalanine (series B) attached by a labile este r Linkage to an imidazole-containing vinyl monomer. Free radical copolymeri zation of the template with methacrylic acid (MAA) and ethylene glycol dime thacrylate (EDMA) gave a highly cross-linked network polymer. The templates could be liberated from the polymers by hydrolysis, giving catalytically a ctive sites envisaged to contain an enantioselective binding site, a site c omplementary to a transition state like structure (series A, C), and a hydr oxyl, imidazole, and carboxylic acid group at hydrogen bond distance. As pr edicted, the enantiomer of BOCPheONP complementary to the configuration of the template was preferentially hydrolyzed with D-selectivity for the serie s A polymers (kD/kL = 1.9) and L-selectivity for the series B polymers (kL/ kD = 1.2). The maximum rate enhancement, when compared with a control polym er, prepared using a benzoyl-substituted imidazole monomer as template, was 2.5, and comparing with the imidazole monomer in solution, a maximum rate enhancement of 10 was observed. The catalytic activity was higher for polym ers subjected to the nucleophilic treatment. This was explained by a higher site density and flexibility of the polymer matrix caused by this treatmen t. In a comparison of template rebinding to polymers imprinted with a templ ate containing either a carboxylate (planar ground state structure) or a-ph osphonate (tetrahedral transition state like structure) functionality, it w as observed that imprinted polymers are able to discriminate between a tran sition state like and a ground state structure for transesterification. How ever the influence of transition state stabilization on the observed rate e nhancements remains obscure. Only at acidic pH's was catalysis observed, wh ereas at basic pH's the polymers inhibit the reaction. At a later stage, th e catalytic activity of the polymers for nonactivated D- and L-phenylalanin e ethyl esters was investigated. A rate enhancement of up to 3 was observed when compared to the blank. Mast important, however, the polymers imprinte d with a D template preferentially hydrolyzed the D-ethyl ester and exhibit ed saturation kinetics.