Recognition medium effects

It should be emphasized that best design (each application corresponding to a particular design), proper choice, and correct use of fluorescent probes require a thorough knowledge of the basic phenomena involved in ion recognition medium effect on complexation equilibrium, fundamental photophysical processes, and possible changes from other causes than complexation. 

While the previous receptors are typically used in organic solvents, except for the cyclodextrins, there are special cases of cyclophane receptors supphed with peripheral charges (ammonium units) (107—12) or ionizable groups (carboxylate functions) (113,114) (Fig. 17) to allow substrate recognition, as in nature, in an aqueous medium, profiting from the solvophobic effects of water (115). 

With amino-acid salts the effect of the medium is considerably larger. Peacock and Cram (1976) reported that the degree of chiral recognition of DL-phenylglycine perchlorate by crown ether [285] depends on the ratio of acetonitrile and chloroform. The observed EDC values vary from 6 to 52, which corresponds to a difference in free energy of —1.15 kcal mol-1 (Table 72). The optimum is very sharply defined (23.1% of acetonitrile) and is... 

Effect of the polarity of the medium on the degree of chiral recognition of DL-phenylglycine perchlorate by [285] at 0°C ... 

Finally, Sanfilippo and coworkers describe the enzymatic kinetic resolution of atropisomeric ( )-3,3 -bis(hydroxymethyl)-2,2 -bipyridine N,N-dioxide by enantioselective esterifcation in an unusual medium of 2-propanol/vinyl acetate (20 80) [139]. Lipase from Mucor miehei (immobilized lipase preparation, Lipozyme ) was found to give good enantioselectivity with an (aS)-enantiopreference in the axial recognition and allowed efficient preparation of both enantioforms with > 98%. Despite the fact that the propanol reacts with the acyl donor, this did not diminish its positive effects on the solubility of the bipyridyl substrate. 

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