Cyclodextrin host design

The most difficult problem in the design of the enzyme model clearly lies in the difficulty of specific introduction of the functional groups into the host skeleton. For example, preparation of an enzyme model by use of fi-cyclodextrin often requires the introduction of two or more functional groups at certain positions among 7 primary (C6) and 14 secondary (C2 and C,) reactive positions. Unless one expects an accidental success by the use of any nonspecific functionalization, it is inevitably necessary for the host design to solve these problems. 

We now can prepare, in principle, enzyme models by use of the concept of host design, where artificial enzymes are so designed as multiple recognition hosts schematically shown in Fig. 20. Although unsubstituted cyclodextrins are well known to catalyze some organic reactions such as ester hydrolysis, their catalytic activities are relatively small. Recent progress in cyclodextrin chemistry has shown that it is possible to enhance the catalytic... 

Zwitterionic cyclodextrins were designed and synthesized by Tabushi a long time ago as artificial receptors for amino acids in water [25]. Only a very low enantioselectivity was detected for Trp. Inoue also studied the complexation of two new P-cyclodextrin derivatives bearing m-toluidinyl and [(9-fluorenyl)-amino]alkylamino groups with various D/L-amino acids by fluorescence spectroscopy in buffered (pH=7.2) aqueous solution. An enantioselectivity as high as 33 was found for D/L-leucine and the former host [26]. 

B.V.K.J. Schmidt, M. Hetzer, H. Ritter, and C. Barner-Kowollik, Complex macromolecular architecture design via cyclodextrin host/guest complexes. Prog. Pol ScL, 39 (1), 235-249, 2014. 

As we saw in the previous sections, inclusion compounds have many structural properties which relate them to other systems based on the hierarchy of non-bound interactions, like enzymes or enzyme-substrate complexes. As a matter of fact, most of the so-called artificial enzymes are based on well-known host molecules (e.g. P-cyclodextrin) and are designed to act partly on such bases 108>109). Most of these models, however, take advantage of the inclusion (intra-host encapsulation) phenomena. Construction of proper covalently bound model molecules is a formidable task for the synthetic chemistuo>. Therefore, any kind of advance towards such a goal is welcomed. 

In contrast to cyclodextrins, there is no parent cyclophane structure available in nature. Therefore, the design of the host molecule starts from the... 

By guest design is meant preparation of that framework of the guest molecule most appropriate to mimic a native enzyme reaction for a given reaction and a given host. In this section, we will discuss the scope of guest design by the use of unmodified cyclodextrins. 

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