Linear ditopic coreceptors

Linear Recognition of Molecular Length by Ditopic Coreceptors... 

In the supramolecular species, [Nj cz (14)-6H ] (15), the ellipsoidal cryptand is built from two protonated tripodal subunits of the tren type N(CH2CH2NH2)3 located at each pole of the molecule and linked by three bridges. The two subunits are situated at a distance such that each may hold one of the two terminal nitrogens of the NJ ion, so that they cooperate in substrate binding. Bis-Tren-6H may be considered as a ditopic coreceptor for linear triatomic substrates of a size compatible with the size of the molecular cavity. 

When two binding subunits are located at the poles of a coreceptor molecule, the complexation of a difunctional substrate will depend on the complementarity between the distance of the two binding sites in the receptor and the distance of the two corresponding functional groups of the substrate. Such linear recognition by ditopic coreceptors has been achieved for both dicationic and dianionic substrates, diammonium and dicarboxylate ions respectively, and corresponds to the binding modes schematically represented by (17) and (18). 

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. 

As in the above case of the binding of diammonium substrates to the macrotricyclic coreceptors, the present chain length selection also describes a linear recognition process based on structural complementarity between the dianionic substrates and the coreceptors in a ditopic binding mode (see (28)). In both cases, the receptor molecule acts as a discriminating sensor of molecular length. 

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