Receptor neutral substrate

Receptor and substrate are terms describing the species involved in complex formation. Throughout the chapter the receptor will refer tp the macrocyclic ligand, the substrate to other interacting species. Substrates may be metal or molecular catibns, neutral molecules, or atomic or molecular anions. The terms receptor and substrate imply that the complex formed has the well-defined structural and chemical properties of a supermolecule, as in biological receptor-substrate associations. They exclude species formed only in the solid state (clathrates). They are also easily converted and understood in many languages. 

Macrocyclic receptors have also been found to complex neutral substrates,21 and complexes of both variable and exact stoichiometries have been prepared. The latter category includes acidic CH— and NH— or polar neutral molecules, such as acetonitrile, nitromethane, benzyl chloride and dimethylmalonitrile. X-Ray data indicate complex formation to be mainly a result of hydrogen bonding and dipole-dipole interactions. Section 21.3.8 contains a more detailed treatment of these complexes. 

The spherically shaped cryptophanes are of much interest in particular for their ability to bind derivatives of methane, achieving for instance chiral discrimination of CHFClBr they allow the study of recognition between neutral receptors and substrates, namely the effect of molecular shape and volume complementarity on selectivity [4.39]. The efficient protection of included molecules by the carcerands [4.40] makes possible the generation of highly reactive species such as cyclobutadiene [4.41a] or orthoquinones [4.41b] inside the cavity. Numerous container molecules [A.38] capable of including a variety of guests have been described. A few representative examples of these various types of compounds are shown in structures 59 (cyclophane) 60 (cubic azacyclophane [4.34]), 61a, 61b ([4]- and [6]-calixa-renes), 62 (cavitand), 63 (cryptophane), 64 (carcerand). 

In summary, our approach of using cyclopeptides with natural amino acids and 3-aminobenzoic acid subunits for the development of macrocydic receptors has afforded remarkably efficient hosts. The cation affinity of 4b, for example, exceeds that of many calixarene derivatives. Even more interesting is the high anion affinity of 5 in aqueous solution. By introdudng additional functional groups such as car-boxylates to the periphery of the cavity, we recently also obtained cydopeptides that interact with neutral substrates, for example, carbohydrates [25]. Our peptides therefore represent a versatile dass of artificial receptor that should prove useful in supramolecular and bioorganic chemistry. 

Early in the development of the field, compounds were often prepared to define the limits of both the synthetic methods and the stable products that could be formed. To date, many thousands of heteromacrocycles have been prepared. The dominant application of the vast family of host or receptor molecules has been to bind or complex a guest structure. The guests can be metal cations, organic cations, neutral substrates, anions, or complementary molecules. The complexation process can be understood from the simple example of 18-crown-6 complexing K+Cl- in solution. Ignoring structural and solvation/desolvation issues, the process can be described simply as... 

A further fascinating feature of the above crystal structure is that two molecules of methanol were found to be tethered to the core NH groups of the macrocycle via hydrogen bonds. This unexpected result led Sessler and coworkers to propose that this system could function as a potential receptor for neutral substrates. In accord with this notion, these workers found that macrocycle 9.26 forms 1 1 complexes in CD2CI2 solution not only with methanol, but also with other neutral molecules such as ethanol, trifluoroethanol, phenol, and catechol. Interestingly, this latter substrate was found to be bound with an affinity constant that exceeds lO M . By contrast, neither this substrate nor any of the other neutral species tested was found to be bound appreciably by the smaller 2 + 2 analog 9.28. ... 

Peptides composed of various coded and noncoded amino acid residues self-assemble to form various types of supramolecular architectures, including supramolecular helices and sheets, nanotubes, nanorods, nanovesicles, and nanofibers. The higher-order self-assembly of supramolecular (3-sheets or supramolecular helices composed of short synthetic acyclic peptides leads to the formation of amyloid-like fibrils. Synthetic cyclic peptides were used in supramolecular chemistry as molecular scaffolding for artificial receptors, so as to host various chiral and achiral ions and other small neutral substrates. Cyclic peptides also self-assemble like their acyclic counterparts to form supramolecular structures, including hollow nanotubes. Self-assembling cyclic peptides can be served as artificial ion channels, and some of them exhibit potential antimicrobial activities against drug-resistant bacteria. 

Sessler. J.L. Gale. P.A. Calixpyrroles Novel Anion and 31. Neutral Substrate Receptors. In The Porphyrin Handbook-,... 

Calixpyrroles Novel Anion and Neutral Substrate Receptors... 

EXPANDED PORPHYRINS. RECEPTORS FOR CATIONIC, ANIONIC, AND NEUTRAL SUBSTRATES... 

Expanded porphyrins. Receptors for cationic, anionic and neutral substrates... 

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