Boronic acid-containing binding sites

Ester formation occurs with complete stoichiometric conversion at relatively low reaction rates (t./, 100-600 s). In the absence of water no back reaction takes place. There is a very slow ester-ester exchange reaction ty, k 5000 s). The boronic ester moieties can be readily cleaved in water/alcohol. This reaction is, in principle, fast and complete (t./, 100 s), so that the template can be released from an imprinted macroporous polymer in 85-95% yield. 

The re-uptake of template in the imprinted polymer is relatively slow (consider the rates of formation of boronic esters listed above), but more than 90% of the free cavities can be reoccupied. Fortunately, in aqueous alkaline solution or in the presence of certain nitrogen bases (for example NH3 or piperidine) tetragonal boronic esters are formed (equation (b)), which equilibrate extremely rapidly with tetragonal boronic acid and diol [47,55]. In these cases the rate of equilibration 

These equilibria can be even more rapid if the interaction with nitrogen bases occurs intramolecularly (see Table 4.4 entry k) [46,47,56,57]. In those cases in which a basic nitrogen atom is located at a favourable distance from the boronate ester, a rate acceleration of 10 -10 fold compared to the unsubstituted phenylboro-nate can be observed [56,57]. 

This difference is also evidenced by the temperature dependence of the number of theoretical plates for the two enantiomers. The template molecule shows hardly any temperature dependence because of increased two-point binding with slow binding rates at higher temperatures, whereas the number of theoretical plates for the second enantiomer increases rapidly, as expected (see Fig. 4.3a) [21]. The effect of concentration on retention is also very different for two enantiomers (see Fig. 4.3b). The wrong enantiomer does not show much effect, whereas retention of the template molecule is greatly increased at lower concentrations, at which only the most selective cavities are occupied by the template molecule. The selectivity therefore increases sharply [49]. It is interesting to notice that this chromatographic 

Molecular imprinting with covalent or stoichiometric non-covalent interactions a b 

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A. Boronic Acid-Containing Binding Site Functional Monomers... 

Boronic Acids as Binding Sites. Poly(vinylphenylboronic acids) are commercially available and can be used in an alkaline aqueous medium for the purification and chromatography of diol-containing compounds (64-66). In organic solvents they usually form trigonal borondiester bonds (see Equation 1 and 2). 

In fact, one boronic acid can reversibly form a boronate ester with two OH groups (one diol group), and one diboronic acid can immobilize two suitably positioned diol units to form a saccharide-containing macrocycle. Selectivity can be achieved by controlling the relative spatial position of the two boronic acids in relation to the cis-diol moieties on the saccharide. A given monosaccharide possesses at least two binding sites that differ from other monosaccharides. 

In the example given in Scheme 2, boronic acids are used as binding sites. This group is very suitable for covalent binding. With diol-containing templates, relatively stable trigonal boronic esters are formed (see Eq. (a)). 

Sensors containing boronic acids (or derivatives thereof) as binding sites in combination with tandem binding sites have also been developed. Several groups have introduced... 

Heinrichs G, Schellentrager M, Kubik S (2006) An enantioselective fluorescence sensor for glucose based rai a cyclic tetrapeptide containing two boronic acid binding sites. Eur J Org Chem 18 4177-4186... 

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