Cyclodextrins mimics

The tense and relaxed forms of a-cyclodextrin mimic the induced-fit model proposed for enzymes. In the inclusion complexes of a-cyclodextrin, the macrocycle adopts a round, relaxed shape with all the 0(2),0(3) hydroxyls engaged in intramolecular 0(2)- (3 ) hydrogen bonds (Fig. 18.8) [577]. The average O - - O distance of around 3.0 A is consistent with an H 0 separation of about 2.0 A, indicating relatively weak interactions. This form of a-cyclodextrin persists even when an included guest molecule, such as krypton (3.8 A diameter) is too small to fully occupy the 5.0 A cavity and is disordered over several sites [578], analogous to the gas hydrate structures discussed in Part IV, Chapter 21. 

This allylation-oxidation reaction has been extended to hydroquinone derivatives to give compounds related to a- and -tocopherylquinones and to ubiquinone. Again /3-cyclodextrin mimics allylquinone synthetase. ... 

Some of the analogs of coenzymes are introduced to cyclodextrin, where cyclodextrin mimics an apoenzyme. 

The cyclodextrin may encircle a linear pol5mier chain so that the chain assumes the proper conformation and avoids coagulation. So, a cyclodextrin mimics the strategy that a living system uses to form polymers. It is believed that such a system could provide an environmentally friendly route to produce biodegradable functional polymers (67). 

The chemically catalyzed oxidation of carotenoids by metalloporphyrins has also been described in the literature. In 2000, French et al. described a central cleavage mimic system (ruthenium porphyrin linked to cyclodextrins) that exhibited a 15,1 S -regiosclectivity of about 40% in the oxidative cleavage of [3-carotene by tert-butyl hydroperoxide in a biphasic system (French et al. 2000). 

To mimic this reaction our first approach aimed to construct 33, a tube-like bis-P-cyclodextrin linked to a ruthenium porph5rrin which... 

Cyclodextrins, products of the degradation of starch by an amylase of Bacillus macerans(1), have been studied in terms of chemical modifications, mainly for the purpose of developing efficient enzyme mimics(2). Not only their unique cyclic structures, but also their ability to form Inclusion complexes with suitable organic molecules, led us to Investigate the total synthesis of this class of molecules(3) We describe here an approach to a total synthesis of alpha(l), gamma(2), and "iso-alpha" cyclodextrin (3). 

S-cyclodextrin condensation, 630 horseradish peroxidase mimic, 628-9, 630 hydrogen peroxide determination, 639 hydroperoxide determination, 681, 708 Hemoglobin see Haemoglobin Henry s law constants... 

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. 

Regio- or stereoselectivity of enzymes action is much easier to mimic than the enormous yields of reactions they catalyze (accelerations factors of 106-1012). A simple example of this kind is provided by chlorination ofanisol 180 that produces only p-chloroanisol 181 in the presence of a-cyclodextrin 13 while both 181 and 182 are formed without the latter factor [109]. By using suitably... 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

Of all the cavity-containing compounds, it is perhaps the cyclodextrins that provide the best basis for the construction of small(ish) molecule enzyme mimics. See Section 6.3 for an introduction to... 

We saw in Section 12.3.1 the use of the cyclodextrins as mimics for transacylases, a well-understood class of enzymes that perform the task of transferring an acyl group from one substrate to another (e.g. from an ester to water). Transacylase chemistry has also been addressed by Cram,5 who used chiral corands, such as 12.11, related to 3.106 bearing thiolate nucleophiles situated above and below the plane of the macrocycle. An acyclic analogue 12.12 was also prepared for comparison. The salient features of 12.11 are shown diagrammatically in Figure 12.4. 

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