Cationic recognition

Shi, X. Fettinger, J. C. Cai, M. Davis, J. T. Enantiomeric self-recognition Cation-templated formation of homochiral isoguanosine pentamers. Angew. Chem. Int. Ed. 2000, 39, 3124-3127. 

Another example of vims clearance is for IgM human antibodies derived from human B lymphocyte cell lines where the steps are precipitation, size exclusion using nucleases, and anion-exchange chromatography (24). A second sequence consists of cation-exchange, hydroxylapatite, and immunoaffinity chromatographies. Each three-step sequence utilizes steps based on different properties. The first sequence employs solubiUty, size, and anion selectivity the second sequence is based on cation selectivity, adsorption, and selective recognition based on an anti-u chain IgG (24). 

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). 

The macrocychc hexaimine stmcture of Figure 19a forms a homodinuclear cryptate with Cu(I) (122), whereas crown ether boron receptors (Fig. 19b) have been appHed for the simultaneous and selective recognition of complementary cation—anion species such as potassium and fluoride (123) or ammonium and alkoxide ions (124) to yield a heterodinuclear complex (120). 

Fig. 19. Multiple and multisite substrate recognition (a) a homo dinuclear (dicationic) and (b) a heterodinuclear (cation and anion) iaclusion complex (c) a...

Given in Table 4.5 in addition to the Hammett equation are ct and substituent constant sets which reflect a recognition that the extent of resonance participation can vary for different reactions. The values are used for reactions in which there is direct resonance interaction between an electron-donor substituent and a cationic reaction center, hereas the a set pertains to reactions in which there is a direct resonance interaction between the substitutent and an electron-rich reaction site. These are cases in which the resonance conqionent of the... 

Cholanic acid also possesses the ability of transporting cations across a lipophilic membrane but the selectivity is not observed because it contains no recognition sites for specific cations. In the basic region, monensin forms a lipophilic complex with Na+, which is the counter ion of the carboxylate, by taking a pseudo-cyclic structure based on the effective coordination of the polyether moiety. The lipophilic complex taken up in the liquid membrane is transferred to the active region by diffusion. In the acidic region, the sodium cation is released by the neutralization reaction. The cycle is completed by the reverse transport of the free carboxylic ionophore. 

For cations 74-75 (Fig. 27), low temperature NMR experiments were necessary to reveal stereodynamical behaviors and allow the observation of split signals for the enantiomers [38,144]. Stereoselective recognition between the chiral cations and anions was observed in essentially all cases as integration of the split signals revealed the preferential occurrence of one diastereomeric salt over the other. 

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