The cyclodextrins as hosts

It is of interest to compare the cyclodextrins with the spherands, since each consists of cyclic oligomers composed of rigid structural units such that enforced cavities result. However, they differ in that the spherands 

It should be noted that molecular complexes of the cyclodextrins may be isolated as crystalline solids for example, a crystalline complex is obtained with iodine (which resembles the well known blue complex between iodine and starch) as well as with a large number of other inorganic and organic guests. 

Cyclodextrins as catalysts and enzyme models It has long been known that cyclodextrins may act as elementary models for the catalytic behaviour of enzymes (Breslow, 1971). These hosts, with the assistance of their hydroxyl functions, may exhibit guest specificity, competitive inhibition, and Michaelis-Menten-type kinetics. All these are characteristics of enzyme-catalyzed reactions. 

An additional example is the observed moderate acceleration in the cleavage of particular phenyl esters in the presence of a cyclodextrin. In such cases, the bound ester is attacked by an hydroxyl group on the cyclodextrin to yield a new ester. There was found to be a significant enhancement of phenol release from meta-substituted phenyl acetate on interaction with cyclodextrin (relative to other esters which do not fit the cavity so well) (Van Etten, Clowes, Sebastian Bender, 1967). During the reaction, the acyl moiety transfers to an hydroxyl group on the 

Structurally developed cyclodextrins. Effective procedures for the selective functionalization of peripheral hydroxyl groups on the cyclodextrins have been developed. A motivation for these studies has been to produce suitably functionalized hosts which will induce enhanced reaction rates 

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Buschmann HJ, Schollmeyer E (1997) Cucurbituril and P-cyclodextrin as hosts for the complexation of organic dyes. J Incl Phenom Macrocycl Chem 29 167-174... 

When fast atom bombardment mass spectrometry (FARMS) and tandem mass spectrometry (MS/MS) were applied using P-cyclodextrin as host and thioglycerol as matrix for compound 56, the main peaks were the molecular peak [56 +host + matrix+ H]+ (100%) and [56 +host + matrix - H2O +H]+ (6%) <1997JMP807>. 

For example, Kaifer et al. decorated the periphery of a POPAM dendrimer with ferrocene and cobaltocenium units. Such a fourth-generation dendrimer (Fig. 4.4) proved to be a suitable guest system - with numerous docking sites -for /9-cyclodextrin as host compound (see Section 8.3.6) [8]. 

This particular property allows an application of the cyclodextrins as chiral material for chromatographic racemate resolution or as substrates for the asymmetric induction leading to bond closure or bond cleavage (enzyme modelling). Apart from the topological (static) conditions, the dynamic processes of complex formation and complex dissociation here also play a part in the host/guest interactions and are therefore important for the efficiency of the separation effect. 

Inclusion compounds using hydroxypropyl-/3-cyclodextrin as host molecules have been used to electropolymerize bithiophene for the first time in aqueous medium (51). Thin films of polybiothiophene were produced by electrooxidation of an aqueous solution of bithiophene, cyclodextrin, and L1C104 at platinum electrodes, using cyclic voltametry and galvanostatic method. 

Influence of Methylated /3-Cyclodextrin as Host Molecule on the Kinetics of the Free-Radical Copolymerization... 

Interest in cyclomalto-oligosaccharides (Schardinger dextrins, cyclodextrins) as hosts and as potential catalysts has continued unabated. A book on the subject has appeared and there have been several reviews on inclusion compound formation with cyclomalto-oligosaccharides improving pharmaceutical formulations/ affecting the action of pesticides/ and being used in industry. ... 

This value of AG is an overall measurement of the driving forces (both enthalpic and entropic) for the inclusion process, and can include contributions from van der Waals attractions, electrostatic attractions, and hydrogen bonding between the host and guest, as well as other factors including release of solvent molecules from the host cavity, conformational strain release, and the hydro-phobic effect. The relative importance of these various driving forces for the specific case of cyclodextrins as hosts will be discussed in Section 1.2. 

A different non-classical approach to the resolution of sulphoxides was reported by Mikolajczyk and Drabowicz269-281. It is based on the fact that sulphinyl compounds very easily form inclusion complexes with /1-cyclodextrin. Since /1-cyclodextrin as the host molecule is chiral, its inclusion complexes with racemic guest substances used in an excess are mixtures of diastereoisomers that should be formed in unequal amounts. In this way a series of alkyl phenyl, alkyl p-tolyl and alkyl benzyl sulphoxides has been resolved. However, the optical purities of the partially resolved sulphoxides do not exceed 22% after... 

The theory and development of a solvent-extraction scheme for polynuclear aromatic hydrocarbons (PAHs) is described. The use of y-cyclodextrin (CDx) as an aqueous phase modifier makes this scheme unique since it allows for the extraction of PAHs from ether to the aqueous phase. Generally, the extraction of PAHS into water is not feasible due to the low solubility of these compounds in aqueous media. Water-soluble cyclodextrins, which act as hosts in the formation of inclusion complexes, promote this type of extraction by partitioning PAHs into the aqueous phase through the formation of complexes. The stereoselective nature of CDx inclusion-complex formation enhances the separation of different sized PAH molecules present in a mixture. For example, perylene is extracted into the aqueous phase from an organic phase anthracene-perylene mixture in the presence of CDx modifier. Extraction results for a variety of PAHs are presented, and the potential of this method for separation of more complex mixtures is discussed. 

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. 

The hydrolysis of esters by the nickel derivative (271) provided an early example of the use of a metal-capped cyclodextrin as a catalyst (shown here as its p-nitrophenyl acetate inclusion complex) (Breslow Overman, 1970 Breslow, 1971). The synthesis of this host involves the following steps (i) covalent binding of the pyridine dicarboxylic acid moiety to cyclodextrin, (ii) coordination of Ni(n) to this species, and (iii)... 

A recent study,209 in which previous results on the complexation of a series of non-centrosymmetrical guests with CDs were re-evaluated, suggested that the two observed relaxation processes could possibly be interpreted as a mechanism involving two parallel reactions inclusion of the guest through either the wide or narrow rim of the cyclodextrin. This mechanism was shown to lead to the same dependence of observed rate constants on concentration of cyclodextrin as the consecutive mechanism. This study showed that even for seemingly simple host systems the mechanistic details for complexation can be quite complex and still controversial. 

Cyclodextrins (Table 12.2) are cyclic molecules made up of glucose monomers coupled to form a rigid, hollow, tapering torus with a hydro-phobic interior cavity (Figure 12.2). Because of the presence of the cavity, cyclodextrins are able to act as hosts, binding with small guest molecules held within the internal cavity. 

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