Polymers imprinted with templates

In a different approach, Lin et al. have used particles derived from a ground MI-bulk polymer and mixed with a polyacryl amide gel for chiral separation. Using a polymer imprinted with L-phenylalanine, D-phenylalanine could be separated from the template with a separation factor of 1.45 [35]. Although the combination of MIP with capillary electrochromatography is still not widely used, the ability to separate enantiomers in nanoliter samples promises interesting developments for the future. 

Comparable results were presented when the number of Mi-polymers was extended to other templates of similar steroid libraries. The use of a polymer imprinted with 11-deoxycortisol in a separation of 11-a-hydroxyprogesterone, progesterone and 11-deoxycortisol resulted in a chromatogram where the 11-de-oxycortisol eluted last (Fig. 17, bottom). When an 11-a-hydroxyprogesterone imprinted polymer was used, on the other hand, the 11 -a-hydroxyprogesterone was found to elute last (Fig. 17, top) [80]. 

The first reported attempt of using MIPs to control the stereochemical course of a reaction dates back to 1980, when the two research groups of Neckers and Shea published, simultaneously, examples of bulk polymers able to control the formation of the product by using a chiral template. Shea et al. reported that bulk polymers imprinted with stereochemically pure ( )-/ra/w-l,2,cyclobutane-dicarboxyilic acid (6) were able to keep a molecular memory of the asymmetry of the template [8]. In fact, this was transferred to an achiral substrate, such as fumaric acid (7), inducing a diastereoselective methylation, which led to trans-1,2,cyclopropane-dicarboxyilic... 

Mosbach and collaborators, in 2001, described an innovative approach to drug discovery using polymers imprinted with a biologically active template [31]. The approach, called anti-idiotypic for the similarity with anti-idiotypic antibodies in the immune-response, can be used to create synthetic receptors able to generate inhibitors or receptor antagonists by exploiting the complementarity with the cavity. The imprinted cavity promotes preferentially the formation of compounds with high affinity, which can later be evaluated for the inhibitory activity and the more active selected for further analysis. 

The authors reported the preparation of polymers imprinted with kallikrein (47) as a template, a known tissue inhibitor, which, after template removal, were used to allow direct coupling between a di-chloro-triazine (48) and a series of aromatic amines. The first substrate was used to resynthesise the compound that was used as template and the yield of this was four times higher than with the corresponding control polymer. Moreover, the same reaction performed in free solution in the same conditions gave no product. The polymer was then tested against the other substrates to verify whether it was possible to synthesise compounds with slightly different properties. This experiment resulted in compounds with lower yields, 21 % for one and 34% for another, whilst using a bulkier substrate it was not possible to obtain any product. 

One of the most investigated type of reaction in the field of catalytic imprinted polymers, as indicated by the large number of publications available, is certainly ester hydrolysis. In particular, a great deal of work has been carried out on systems inspired by hydrolytic enzymes since 1987. In 2000, Shea et al. [37] reported the preparation of enantioselective imprinted polymers for the hydrolysis of N-tert-butoxycarbonyl phenylalanine-p-nitrophenyl ester (55), using a system already developed by the same group in 1994 [19]. The system was inspired by the natural hydrolytic enzyme chymotrypsin and polymerisable imidazole units (27) were used as functional monomers coupled via ester linkages to a chiral phosphonate (56), analogue of (d)- or (L)-phenyl-alanine. After template removal, the imprinted polymers showed selectivity towards the hydrolysis of the enantiomer with which they were imprinted. The ratio of the rate constants, k /k, was 1.9 for the polymer imprinted with the D-enantiomer and kjku was 1.2 for that imprinted with the L-enantiomer. Moreover, the imprinted polymer showed a 2.5-fold increase in the rate of the reaction when compared with the control polymer, imprinted with a... 

Both imprinted polymers showed an enhancement in the catalytic activity that was about 50-fold higher than the control polymer (P0) and turnover of the catalytic cavities was also demonstrated. However, when comparison was made with a polymer containing Co(II) but which was not imprinted with the template (PI), the rate acceleration dropped to about fourfold. In addition, the control of the enantioselectivity of the reaction was very low. In fact, the polymer, imprinted with the diketone derived from the / -camphor, was able to catalyse the reaction, between the 5-camphor and benzaldehyde, with an acceleration rate almost identical to that obtained with the polymer imprinted with the opposite enantiomer. The rate enhancement between the two polymers was in fact equal to 1.04. 

Wulff G. Molecular recognition in polymers prepared by imprinting with templates. ACS Symposium Series 1986, 308, 186-230. 

G. WullF, Molecular Recognition in Polymers Prepared by Imprinting with Templates, in Polymeric Reagents and Catalysts (ed. W. T. Ford), American Chemical Society Symposium Series (ACS), 308, Washington DC, 1986, p. 186. 

Mosbach prepared imprinted polymers with not only a more specific substrate binding but with a true catalytic turnover [148]. Certain N-protected amino acid derivatives formed pseudotetrahedral complexes in solution in the presence of Co and two molecules of 5-vinylimidazole. These were polymerised in the presence of divinylbenzene and the metal-ion and amino acids eluted together afterwards. Hydrolysis of the corresponding 4-nitrophenylester of the amino acid template showed an increase in catalytic activity (factor 2-4) compared with a blank prepared without the template. The polymer also exhibits a clear preference for the activated ester of the template, which suggests that hydrolysis indeed takes place inside the cavities. Two years later the scope was further broadened by the preparation of the first polymer imprinted with a transition state analogue. 

Fig. 4,3. Differences in the effect of temperature on the number of theoretical plates and of the amount of chromatographed substance on the retention of template molecules and their enantiomers, (a) Temperature dependence of the number of theoretical plate Wh) in the resolution of D-la and L-la on a polymer imprinted with 1 [21]. (b) Dependence of the capacity factor k on the amount of chromatographed substance in the resolution of l- and of D-phenylalanine anilide on a polymer imprinted with L-phenylalanine anilide [49],...

Fig. 4.7. Template re-uptake of a polymer imprinted with a monomer consisting of 15 and two moles of 13 in dependence to the offered amount of template 15.

In work similar to that described in Section 20.2.5.10., Haupt and co-workers have found yet another use for their 2,4-D-imprinted polymer [51]. Whereas in the work described above the competing analyte was CMMC (see Fig. 20.14), in these experiments 2,4-dichlorophenoI (DCP) was used as the competitor. This compound is an enhancer of the peroxidase-catalysed chemiluminescence of the well known reaction of luminol and H2O2. Thus, in a certain concentration range, the amount of DCP present in solution is an indicator of the amount of template bound to the MIP. Competitive radioimmunoassays were used to determine the affinity of DCP for the 2,4-D-imprinted polymer and it was found that, while 500 pg of MIP were required to bind 50% of the CMMC, only 250 pg of polymer were required to bind 50% of the DCP. In order to verify this unexpected result, competitive binding assays were performed in the presence of C-2,4-D and it was found that the relative affinities of the polymer for CMMC and DCP were approximately 6 and 10% that of 2,4-D. However, while CMMC bound poorly to the non-imprinted polymer, DCP bound equally as well as to the MIP. This indicates that, in the latter case, binding is almost entirely non-specific. Thus DCP was a poor probe for this system. Fluorescein was also examined as a probe for polymers imprinted with... 

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