Linear Recognition of Molecular Length by Ditopic Coreceptors

2 Linear Recognition of Molecular Length by Ditopic Coreceptors 

The crystal structure 57 of the strong and selective complex formed by the terephthalate dianion with a hexaprotonated macrobicyclic polyamine shows that it is a molecular cryptate 56 with the dianion tightly enclosed in the cavity and held by formation of three hydrogen bonds between each carboxylate and the ammonium groups [4.19]. Both structures 53 and 57 illustrate nicely what supermolecules really are they show two covalently built molecules bound to each other by a set of non-covalent interactions to form a well-defined novel entity of supramolecular nature. Acyclic [4.20a,b] and macrobicyclic [4.20c] hydrogen bonding receptors 

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]. 

Numerous variations in the nature of the binding subunits or of the bridges linking them are conceivable and may be tailored to specific complexation properties (see, for instance, [4,16,4.17, 4.22]). 

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