Hydrogen bonds/bonding molecular recognition processes

The interactions between solutes and solvents are noncovalent in nature (barring the occurrence of chemical reactions), and therefore fall into the same category as those that govern molecular recognition processes, the formation and properties of liquids and solids, physical adsorption, etc. Hydrogen bonding, in its many manifestations, is a particularly prominent and important example. 

There are several types of natural and synthetic molecular hosts, such as cyclodextrin and cyclophane, that are shaped to accommodate neutral and charged organic molecules in the three-dimensional cavity. The inclusion complexation by molecular hosts is driven by various weak forces like van der Waals, hydrophobic, hydrogen bonding, ion-dipole, and dipole-dipole interactions, and therefore the molecular recognition process seems much more complicated. In expanding the scope of the present theory, it is intriguing and inevitable to perform the extrather-... 

It is reasonable to assume that the complementary units form the expected triply hydrogen bonded pairs, so that the entirely different behaviour of the pure compounds and of the 1 1 mixtures may be attributed to the spontaneous association of the complementary components into a polymolecular entity based on hydrogen bonding. The overall process may then be described as the self-assembly of a supramolecular liquid-crystalline polymer based on molecular recognition (Figure 40). The resulting species (TP2, TU2) is represented schematically by structure 174. 

The bis-porphyrin supramolecular box 26 has also been assembled using a molecular recognition process involving hydrogen bonding between pendant-arm... 

Triply hydrogen-bonded 1 1 liquid-crystalline complexes 23 are formed by molecular-recognition processes of uracyl and 2,6-diaminopyridine derivatives [27], The complex with R - CH3, k = 1 = 10, m = 11, and n = 16 exhibits a metastable columnar hexagonal phase below 72 °C, In this case, the complex contains four long aUcyl chains. An X-ray study has shown that two complex molecules form one column side-by-side. 

Multiple-interaction self-assembly involves the formation of more than one type of interaction. Illustrated in Fig. 3b is a multiple-interaction process in which two types of coordinate bonds are formed (i.e., double subroutine self-assembly). Depicted in Fig. 3c is a multiple-interaction process in which one type of coordinate bond and one type of hydrogen bond are formed (i.e., a multiple molecular recognition process). Processes like that depicted in Fig. 3 b involve mediation by one class of interaction only, and can therefore be referred to as unmediated.Processes of the type depicted in Fig. 3c involve mediation by two different classes of interaction and are therefore refeiTed to as multimediated. ° ... 

In the previous sections of this chapter, the molecular recognition process was described that involved n-n, hydrophobic, and possible subtle H-bonding effects. Now, I wish to discuss a new molecular recognition process based primarily on selective hydrogen bonding interactions between the host, tranx-[Cp Rh-r -N3)-l-methylcytosine)( x-OH))2(OTf)2,2, and several examples of aromatic amino acid guests, L-tryptophan and L-phenylalanine, L-Trp and L-Phe, in water at pH 7.0 (2). 

HB is undoubtedly the most frequently utilized noncovalent interaction in molecular-recognition processes. However, halogen bonding (XB) is a noncovalent interaction that is in some ways analogous to HB, and it may therefore be used as a practical tool for cocrystal synthesis. In HB, a hydrogen atom is shared between an atom, group, or molecule that donates and another that accepts it. In XB, it is a halogen atom X that is shared between a donor atom D and an acceptor A. Thus, the two types of interaction can be described as in Scheme 13. 

Kato, T., Nakano, M., Moteki, T, Uryu, T, Ujiie, S. (1995), Supramolecular liquid-cry stalline side-chain polymers built through a molecular recognition process by double hydrogen bonds. Macromolecules, 28,8875-6. 

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