Ditopic recognition

Obviously, high binding affinity is expected via ditopic recognition when the salt is bound to the receptor as a contact ion pair. For this purpose, besides having crown ether moieties, aromatic pendant arms can also be tagged to the receptor to achieve ditopic recognition of ion pairs or salt. Consequently, in this case, the recognition of cations involves cation- n interactions due to the aromatic pendant arms. For example, uranyl receptors, 15 and 16, endowed with aromatic pendant arms, are used as ditopic receptors of contact ion pairs/salts such as alkali metal halides and quaternary ammonium haUdes. ... 

Besides face-binding mode of ditopic recognition, anion complexation by azamacrocycles involves other different modes of ditopic binding such as bipyramidal... 

Boronic acid-based fluorescent PET sensors developed for the recognition of simple monosaccharides have been extended to include ditopic recognition sites and so introduce selectivity for a diverse range of guest species. 

Polytopic macrocyclic receptors 1, 2 (Figure 10.1) are able to complex the zwitterionic form of the amino acids by a double non-covalent charge interaction [28,29]. The unsymmetrical benzocrown sulfonamide derivative, 2 which contains benzo-18-crown-6 and benzo-15-crown-5 moieties was used as a ditopic receptor for multiple molecular recognition of the amino acids, by combining two non-covalent interactions ammonium-crown hydrogen bonding and carboxylate- complexed Na+-benzo-15-crown-5 charge interactions [28,33]. 

Figure 2.4. Ditopic receptors for the recognition of a,co-alkane diammonium ions.

The stability of the polypyridyl rhenium(I) compounds mentioned above stimulated applications of this coordination chemistry. Thus, new heterotopic bis(calix[4]arene)rhenium(I) bipyridyl receptor molecules have been prepared and shown to bind a variety of anions at the upper rim and alkali metal cations at the lower rim. A cyclodextrin dimer, which was obtained by connecting two permethylated /3-cyclodextrins with a bipy ligand, was used for the preparation of a luminescent rhenium(I) complex. The system is discussed as a model conipound to study the energy transfer between active metal centers and a bound ditopic substrate. The fluorescence behavior of rhenium(I) complexes containing functionalized bipy ligands has been applied for the recognition of glucose. ... 

Development of ditopic receptors 58 for both an anion and a cation is a logical extension [19b]. It should be stressed that the recognition by cyclodextrins, calixarenes, hemicarcerands, and many other systems does not involve specific recognition sites, whereas tetrahedral recognition by 53 and 54, the adenine 56 selective binding, and multiple recognition like the one shown in formula 59 [20] require the presence of specific sites in receptor capable of directional interactions. 

Linear Recognition of Molecular Length by Ditopic Coreceptors... 

Thus, for both the terminal diammonium and dicarboxylate substrates, selective binding by the appropriate receptors describes a linear recognition process based on length complementarity in a ditopic binding mode. Important biological species, such as polyamines, amino acid and peptide diamines, and dicarboxylates [4.18] may also be bound selectively. Recognition is achieved by multiple coordination to metal ions in dinuclear bis-macrocyclic coreceptors that complex selectively complementary bis-imidazole substrates of compatible length [4.21]. 

Hosseini, M. W., Lehn, J. M., Anion coreceptor molecules - linear molecular recognition in the selective binding of dicar-boxylate substrates by ditopic polyammonium macrocycles. Helv. Chlm. Acta 1986, 69, 587-603. 

A related anthracene-based ditopic sensor (11.28) has been constructed, which can recognise diammonium cations along the same lines as 3.103 (Section 3.12.3). Use of the anthracene-derived group as a spacer as in 11.27 gives fluorescent recognition of 11 N1 (Cl I NI I3+ guests as a function of spacer length.20... 

Aoki, S. and Kimura, E. (2000) Highly selective recognition of thymidine mono- and diphosphate nucleotides in aqueous solution by ditopic receptors zinc(II)-bis(cyclen) complexes (cyclen = 1, 4, 7, 10-tetraazacyclododecane), J. Am. Chem. Soc. 122, 4542-4548. 

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