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Receptors for Water Molecules

Even though there are a number of examples of foldamers that bind to molecules of water in the solid state [13-15], there are currently only a few cases in which

Fig. 7.2 (a) Structure of the quinoline/pyridine oligoamide foldamer. The dashed lines indicate hydrogen bonds that contribute to the stabilization of the helical conformation; (b) X-Ray structure of the quinoline/pyridine oligoamide foldamer (light gray) with a molecule of water trapped in the cavity (dark gray). Isobutyl and benzyl chains have been omitted for clarity (Reproduced with permission from the authors).

Fig. 7.2 (a) Structure of the quinoline/pyridine oligoamide foldamer. The dashed lines indicate hydrogen bonds that contribute to the stabilization of the helical conformation; (b) X-Ray structure of the quinoline/pyridine oligoamide foldamer (light gray) with a molecule of water trapped in the cavity (dark gray). Isobutyl and benzyl chains have been omitted for clarity (Reproduced with permission from the authors).

the binding to water molecules has been demonstrated in solution [16]. Pyridine oligoamides have been shown to adopt a helical conformation in the solid state and co-crystallize with water molecules in the helical cavity [15]. The restricted rotation about the aryl amide bonds and an extensive network of intramolecular hydrogen bonds are responsible for folding into a rigid structure. It has been shown that water-soluble derivatives of this foldamer maintain the folded structure even in solvents competing for hydrogen bonds such as water. Huc and coworkers transformed this helical scaffold into a capsule by adding two dimeric fragments of quinoline monomers flanking a central trimeric moiety of pyridine monomers as shown in Fig. 7.2a [16].

The X-ray structure obtained from crystals grown in organic solvents showed that the foldamer adopts a capped helical conformation with a water molecule trapped in the inner cavity (Fig. 7.2b). These results validated the design of the foldamer as a receptor for water in the solid state. However, the study of this system in solution proved to be more laborious. Since the molecule of water was completely encapsulated in the crystal structure, the binding event must involve a partial unwinding of the foldamer to allow the entrance of the guest water molecule. To test this hypothesis, Huc et al. performed variable temperature JH NMR studies in dry and wet organic solvents. When the experiments were carried out at low temperature, two different water peaks were observed which were assigned to free water and encapsulated water. This was supported by the observation that the encapsulated water showed NOEs with some of the amide NH hydrogens in the inner cavity.

198 | 7 Foldamer-based Molecular Recognition 7.2.2

Receptors for Ammonium Cations

Over the last few years there have been numerous reports of positively charged molecules that bind in the polar interior of circular or helical foldamers. For example, Li et al. designed and synthesized a series of hydrogen bonded aryl oli-goamide foldamers [15] and studied their binding to ammonium salts in chloroform (Fig. 7.3) [17]. The researchers were able to crystallize a fragment of the foldamers' basic repeating unit which revealed a planar rigid conformation locked by a bifurcated hydrogen bond between the amide N-H and the aryl methoxy groups (see Chapter 1). Inspection of the JH NMR peaks showed significant de-shielding of the amide protons indicative of strong hydrogen bonding in solution. NOE experiments showed correlation of the amide NHs with the methoxy hydrogens but not with the aromatic protons. Taken together, the NMR experiments suggest that the scaffolds adopt a planar rigid conformation in solution stabilized by hydrogen bonding. Infrared spectroscopy further corroborated the presence of

Fig. 7.3 Structures of aryl oligoamide foldamers and the ammonium salts used as guest molecules.

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