Recognition cavities

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

It should be noted that benzyl methyl sulfoxide was included without the recognition of their chirality (entry 7 in Table 4).26 By single crystal X-ray analyses of this inclusion compound, it was elucidated that the molecules of 1 form the layer structure similar to the one of the above alkyl phenyl sulfoxide-inclusion crystals, and the sulfoxides are included between these layers. There are two different recognition cavities on the upper side for its S enantiomer and on the lower side for its K enantiomer. The upper side cavity can be illustrated by motif A, and the lower side one by motif B in Figure 16. 

Table 1 shows the results of competitive adsorption of 2-phenylpropanal and 3- phenylpropanal on both 2-phenylpropanal-imprinted and blank polymers. The blank polymer showed nearly equal adsorption capacity for both 2-phenylproanal and 3-phenylpropanal (48% via. 52%) while the imprinted polymer favored the template (i.e. 2-phenylproanal) adsorption (61% for the template). The difference in the adsorption selectivity between the blank and imprinted polymers indicated that a certain nnmber of the molecular recognition cavities were obtained for 2-phenylpropanal (template) on the surface of the... 

They used the conformational maps of mimetics to determine the geometry of recognition cavities on the BK receptor,... 

The generation of molecular imprints at surfaces is an obvious solution to the problems of mass transfer and accessibility, and potentially affords control over more subtle parameters such as binding site orientation and local solvation state. While a standard general protocol does not as yet exist for surface imprinting, a number of strategies that create the required recognition cavities at a surface or interface have now been developed and selected examples of these are considered in this chapter. 

Cyclodextrins are macrocyclic compounds comprised of D-glucose bonded through 1,4-a-linkages and produced enzymatically from starch. The greek letter which proceeds the name indicates the number of glucose units incorporated in the CD (eg, a = 6, /5 = 7, 7 = 8, etc). Cyclodextrins are toroidal shaped molecules with a relatively hydrophobic internal cavity (Fig. 6). The exterior is relatively hydrophilic because of the presence of the primary and secondary hydroxyls. The primary C-6 hydroxyls are free to rotate and can partially block the CD cavity from one end. The mouth of the opposite end of the CD cavity is encircled by the C-2 and C-3 secondary hydroxyls. The restricted conformational freedom and orientation of these secondary hydroxyls is thought to be responsible for the chiral recognition inherent in these molecules (77). 

Most effective differentiation of the receptor between substrates will occur when multiple interactions are involved in the recognition process. The more binding regions (contact area) present, the stronger and more selective will be the recognition (17). This is the case for receptor molecules that contain intramolecular cavities, clefts or pockets into which the substrate may fit (Fig. 1). 

In a word, all these receptors are more or less able to discriminate against cations that are either smaller or larger than thek cavity (44). However, in a strict sense, discrimination of metal-ion spheres does not concern with molecular recognition but selection of the carbon ball C q certainly does. In fact, the fuUerene C q has been included into the cavity of octa-/ f2 butylcalix[8]arene (Fig. 8c) shutting out C q and making a very convenient and efficient C q purification possible without any expensive apparatus (45). 

However, all the receptors hitherto discussed are monomolecular species which possess a monomolecular cavity, pocket, cleft, groove or combination of it including the recognition sites to yield a molecular receptor—substrate complex. They can be assembled and preserved ia solution although there are dependences (see below). By way of contrast, molecular recognition demonstrated ia the foUowiag comes from multimolecular assembly and organization of a nonsolution phase such as polymer materials and crystals. 

S-2.2.2 Neutral Carrier Electrodes hi addition to charged liquid ion exchangers, liquid-membrane electrodes often rely on the use of complex-forming neutral carriers. Much effort has been devoted to the isolation or synthesis of compounds containing cavities of molecular dimensions. Such use of chemical recognition principles has made an enormous impact upon widespread acceptance of ISEs. The resulting neutral carriers can be natural macrocyclic molecules or synthetic crown... 

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