Receptors for Zwitterions

We saw in Section 4.6.1 the use of zwitterionic hosts for anions in which the counter-cation is non-speciflcally bound on the outside of the molecule. We now turn to zwitterionic guests where the guest has both cationic and anionic parts. We have already described in Section 5.1.1 the zwitterionic structure of amino acids and amino acid binding has been a major part of the interest in the construction of hosts for zwitterions. An excellent example is compound 4.30 (Section 4.5) which is 

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The same variety of structures that is found in the construction of receptors for cations or anions can be found in the case of the receptors for zwitterionic species. Nevertheless, this is a field that has been much less studied, not only because of the above-mentioned design difficulties but, in particular, also because of the relatively reduced number of zwitterionic species that have been explored. The most obvious examples of such species can be easily found in the biological world. Most likely, amino acids along with some related species including some peptides represent a very significant percentage of the total species considered up to now in this area. 

Many different structural fragments have been described for the interaction with anions, but one of the simplest ones is an ammonium group. Figure 5 contains some examples of receptors for zwitterionic species based on a molecule having a crown-ether fragment and an ammonium group. 

Compound 7, developed by Barboiu as a receptor for zwitterionic a-amino acids, contains a dibenzo 18-crown-6 unit as the receptor site for the ammonium group, while a simple primary ammonium group, separated from the crown subunit by a long aliphatic spacer, is the site for the interaction with the carboxylate group of the amino acid. According to the protonated nature of the ammonium group, the behavior of this receptor is pH sensitive and care needs to be taken to guarantee its full protonation at... 

Figure 8 Miscellaneous receptors for zwitterionic species containing a crown-ether fragment.

Figure 30 Some examples of miscellaneous receptors for zwitterionic species.

Trivalent rare earth cations form complexes with a series of hydroxycarboj lic acids and amino acids in aqueous solutions, some of which are listed in fig. 8. Earlier studies of Katzin (1968, 1969) revealed that these biological substrates effectively act as bidentate ligands toward the rare earth cations. Rare earth complexes with multidentate chelators were recently applied as receptors for zwitterionic amino acids of biological and artificial interests. Aime et al. (2001a) characterized the complexation behaviors of zwitterionic amino acids with oc-... 

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