Receptor-guest interactions

FIGURE 6.13 Receptor-Guest Complex Cgg 24 60- Molecular structure drawing created using OF data. 

---

The suitability of fluorescent PET systems for use in the emerging field of molecular switching devices was pointed out in Sect. 4 due to their natural on-off action induced by ion input. Discussions in Sect. 6 have also illustrated the value of PET sensor ideas in the design of reagents and also of reporters on receptor-guest interactions. Such versatility of the fluorescent PET sensor logic makes this research worthwhile. 

The frontier molecular orbital model for donor-acceptor complexes provides a convenient framework for a discussion of hydrogen bonding and receptor-guest interactions. 

Figure 1 Receptor-guest interaction versus intramolecular site-site or intermolecular receptor-receptor interactions.

Scheme 37 The equilibrium established in a mixture of receptors (Rn) can be modified by the presence of a good template (guest for the receptor, T). This shifts the equilibrium amplifying the formation of the receptor that interacts more strongly with the template...

Receptor models 71a and 71b for dinucleotides (dinucleoside monophosphates) have been developed out of 69 by replacing the naphthoyl residue, which is not involved in any host-guest interactions in the complexes, by a second carbazolyl cleft. The adenine selectivity of the two pincers was reflected in single extraction experiments dissolved in CH2CI2,71a removes one full equivalent of both adenylyl(3 -+ 5 )adenosine (ApA) and 2 -deoxyadenylyl(3 ->5 )-2 -deoxyadenosine [d(ApA)] 72 out of aqueous solutions of guest (8-fold excess) but only half the amount of d(ApG) [103]. 

Studies on molecular recognition by artificial receptors are thus one of the most important approaches to such characterization in relation to supramolecular chemistry [4]. Functional simulation of intracellular receptors in aqueous media has been actively carried out with attention to various noncovalent host-guest interactions, such as hydrophobic, electrostatic, hydrogen-bonding, charge-transfer, and van der Waals modes [5-10]. On the other hand, molecular recognition by artificial cell-surface receptors embedded in supramolecular assemblies has been scarcely studied up to the present time, except for channel-linked receptors [11-13]. 

Figure 2. Host-guest interaction of p-benzoquinone (1) and a synthetic receptor 2.

This crystal structure shows that the iodide forms hydrogen bonds to all six NH of the two peptide moieties in the complex. It also demonstrates how effectively the anion is embedded between the cyclopeptides. Complex formation thus shields the guest from surrounding solvent molecules, an effect that strengthens receptor-substrate interactions this might be one reason for the anion affinity of 5 in aqueous solution. 

Furthermore, host-guest interaction is also favoured if the interacting atoms of each entity have the same degree of softness or, respectively, hardness. If the receptor features soft bases as donor atoms (i.e. S, P) they will preferentially interact with soft Lewis acids (e.g. 

Surface imprinting has recently been used in our laboratory to prepare phosphate and phosphonate receptors in silica xerogels [38]. In an effort to obtain faster on/off binding kinetics and gain thermodynamic control of the host-guest interaction, we positioned the receptor sites as close as possible to the gel surface. This resulted in observed equilibration binding times of less than a minute. 

Host-guest interaction of a poiymer-suppotted receptor with the barbituric acid tag... 

---