Rigid truncated cone structure

For many years, CDs were considered to have rigid truncated-cone structures although this concept was incompatible with the ease of their complex formation and several experimental findings (mainly obtained by NMR technique). Recent experimental results demonstrate that the complexes, held together by weak nonbonded interactions, must be flexible not only in solution but also in the solid state. The truncated-cone structure of HP-)3-CDs became more flexible since the intramolecular hydrogen bond is broken by the hydroxylpropyl substituent. This flexibility preferably explains the improvement of complex forming ability and dynamic character of the complexes of HP- -CDs [22]. 

Fig. 37.1 Schematic representation of a macrocyclic amphiphilic molecule (a) and chemical structures of the native caiix[4]arene (b) and resorcin[4]arene (c). The caiix[4]arene macrocycle, composed of four phenolic units, forms a fairly rigid truncated cone-shaped molecule when adequately modified to lock the structure to avoid conformational changes to partial cone conformations. The aromatic alcohol functions are exposed at the narrow rim, and the para phenolic positions at the wide rim. Applying different chemistries, the macrocycle can be selectively modified either on one rim or on the other. The regio-selectivity of the chemical modifications is not restricted to the selection of the modified rim the number and the position within a rim could also be controlled...

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