Receptor-substrate supermolecule

According to these basic concepts, molecular recognition implies complementary lock-and-key type fit between molecules. The lock is the molecular receptor and the key is the substrate that is recognised and selected to give a defined receptor—substrate complex, a coordination compound or a supermolecule. Hence molecular recognition is one of the three main pillars, fixation, coordination, and recognition, that lay foundation of what is now called supramolecular chemistry (8—11). 

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. 

Porphyrin and a, tt -bipyridine (bipy) groups have been introduced as metal-ion binding units into macropoly cyclic coreceptors (see also [4.38]) containing in addition macrocyclic sites for anchoring NH3+ groups [4.66a]. These receptors yield mixed-substrate supermolecules by simultaneously binding metal ions and diammonium cations as shown in 70. The bis-porphyrin receptor present in 70 binds to... 

Chemical systems may store information either in an analog fashion, in the structural features (size, shape, nature and disposition of interaction sites, etc. [1.27]) of a molecule or a supermolecule, or in a digital fashion, in the various states or connectivities of a chemical entity. Information theory has been applied to the description of the features of molecular machines [10.2]. The evaluation of the information content of a recognition process based on structural sensing in receptor-substrate pairs... 

C-NMR determinations of the relaxation and correlation times of supermolecules (25) have indicated that complementary receptor-substrate pairs display similar molecular motions for both partners [19]. This dynamic fit may be considered to result from the combination of steric fit with dihapto binding. Thus, complementarity between components of a supramolecular species expresses itself in both its structural and dynamic properties. 

Molecular receptors are defined as organic structures held by covalent bonds, that are able to bind selectively ionic or molecular substrates (or both) by means of various intermolecular interactions, leading to an assembly of two or more species, a supermolecule. 

Supramolecular reactivity and catalysis thus involve two main steps binding, which selects the substrate, and transformation of the bound species into products within the supermolecule formed. Both steps take part in the molecular recognition of the productive substrate and require the correct molecular information in the reactive receptor. Compared to molecular reactivity, a binding step is involved that precedes the reaction itself. Catalysis additionally comprises a third step, the release of the substrate. 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

This is the case for the macrotricycles (21-24) which yield supramolecular species [ H3N-(CH2) -NH3 cz Receptor] of type (25), where the diammonium substrate is located inside the receptor cavity and held by anchoring of each terminal -NH group to an [18J-N204 macrocycle via hydrogen bonds [16]. This was confirmed by the crystal structure of [ NH3-(CH2)5-NH3 c (22)] which also provided a clear picture of the coreceptor-substrate relationship in the supermolecule [17]. 

The design of the compounds capable of molecular recognition of cations, anions, and neutral molecules is one of the main achievements of the chemistry of molecular ensembles and intermolecular bonds. When such compounds are used as catalysts, the conversion of the reacting particles into products is preceded by the formation of supermolecules, due to the selective binding of the substrate molecule by the receptor molecule (Fig. 4.1) [23,24], This approach enables the development of catalysts with the activity and selectivity determined by intermolecular interactions between the substrate and the catalyst. 

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