Supramolecular interconverting

Oligomerization of nucleobases can be advantageous to reinforce the H-bonding supramolecular motifs when supramacromolecular polymers are desired. Moreover the different interconverting outputs that may form by oligomerization define a dynamic polyfunctional diversity which may be extracted selectively under the intrinsic stability of the system or by interaction with external factors by polymerization in the solid state. 

Fig. 10.10 Diverse sets of interconverting supramolecular and macromolecular polymers which may be generated by using only two adenine and uracil nucleobases (the gray formulas are stereochemically unstable).

Dynamic combinatorial libraries (DCLs) are continuously interconverting libraries that evenmally evolve to an equilibrium distribution [61-65]. This approach has been used successfully in the discovery of stable supramolecular assemblies from mixtures. Due to the nearly endless possible peptide sequences that can potentially be synthesised, the DCL approach is attractive for the identification of supramolecular peptide interactions. Indeed, disulfide exchange between cysteine residues has been explored for this purpose [66, 67] as has peptide-metal binding [68]. We have recently demonstrated protease-catalysed amide exchange in this context, which allows for the evolution of the self-assembled peptide structures, and will therefore allow exploration of peptide sequence space for biomaterials design. 

Affinity and selectivity in directional supramolecular systems are composite responses of frequently counteracting influences. The analysis of structure in many instances may then be a dull tool in their study because of the averaging between rapidly interconverting species. Here, an example from abiotic host-guest complexation demonstrates the utility of an energetic analysis how to avoid the pitfall of false reasoning that may follow from all too simple complementarity concepts. 

Much of the attraction that supramolecular chemists have for calixarenes is due to their conformational mobility. The calix[4]arenes are particularly widely used as their conformers, illustrated in Fig. 1.10, interconvert until large groups are attached... 

A set of interconverting supramolecular or molecular entities represents a real or virtual [35, 42] constitutional dynamic library (CDL). The full ensemble of all constituents, i.e., of all possible combinations of the components, may be termed the combinome of the system under consideration. The set of components may be considered as the genotype of the dynamic system and the sets of constituents generated from these components in given conditions its phenotypes. 

Networks of DYNAMICALLY INTERCONVERTING CONSTITUENTS connected either structurally molecular and supramolecular arrays) or reactionally sets of connected reactions or interactions)... 

The supramolecular systems discussed in this section so far composed of a chiral host and an achiral guest, while the ICD approach is also effective for achiral host and chiral guest systems. The achiral 2,2 -bisphenol 11 (Figure 9) takes two energetically equivalent axially chiral conformations that rapidly interconvert to each other. The complex of the CD spectrum of 11 and chiral /rans-l,2-cyclohexadiamine guest 12a, however, showed a broad exciton-type ICD at around 350 and 300 nm due to... 

Dynamic combinatorial chemistry (DCC) has proven extremely useful in creating complex mixtures of interchanging compounds termed dynamic combinatorial libraries (DCL). Key to the formation of such DCLs is a reversible chemical process that allows the library members to interconvert. The formation of imines from aldehydes and amines is a prominent example for the creation of a DCL. Since the overall distribution of compounds is under thermodynamic control, external stimuli can be used to bias the DCL toward a specific member of the library. This approach has been exploited successfully in the search of potent receptors for molecules of pharmacological interest, the creation of supramolecular assemblies, and ligands for biomacromolecules. ... 

For each case in Figure 6.14, we have stereoisomers—structures with the same connectivities but differing arrangements of the atoms in space. They are not enantiomers, so they must be diastereomers. The novelty lies in the fact that these stereoisomers interconvert by a translation or reorientation of one component relative to the other. In some ways these structures resemble conformers or atropisomers, which involve stereoisomers that interconvert by rotation about a bond. For the supramolecular stereoisomers, however, interconversion involves rotation or translation of an entire molecular unit, rather than rotation around a bond. Note that for none of the situations of Figure 6.14 do we have topological stereoisomers. In each case we can interconvert stereoisomers without breaking and reforming bonds. 

The self-assembly of interconverting multinuclear arrays has been demonstrated recently with the supramolecular products being studied using both NOESY methods and X-ray diffraction. Treatment of a pentatopic decadentate... 

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