Cyclodextrin hosts

Figure 11. Amphiphilic cyclodextrins (Host molecules) and naphthalene derivatives (Guests).

Host-guest inclusion complexes, 262—263 antibiotic hosts, 231—233 cahxarene hosts, 228—231 chiral crown ether hosts, 213—218 cyclic oligosaccharide hosts, 218—222 cyclodextrin host selectivities, 223/ host molecular size, 221 hnear ohgosaccharide hosts, 222—228 ir- TT stacking interactions, 217 proteic hosts, 231 Human 15-hpoxygenase, 52/... 

B Chankvetadze, G Endresz, G Schulte, D Bergenthal, G Blaschke. Capillary electrophoresis and H NMR studies on chiral recognition of atropisomeric binaphthyl derivatives by cyclodextrin hosts. J Chromatogr A 732 143-150,... 

Very recently a new method was developed that opens the possibility to polymerize even hydrophobic monomers in aqueous solution. This method is based on the finding that hydrophobic monomers can be made water-soluble by incorporation in the cavities of cyclodextrins. It has to be mentioned that no covalent bonds are formed by the interaction of the cyclodextrin host and the water-insoluble guest molecule. Obviously only hydrogen bonds or hydrophobic interactions are responsible for the spontaneous formation and the stability of these host-guest complexes. X-ray diffraction pattern support this hypothesis. Radical polymerization then occurs via these host-guest complexes using water-soluble initiators. Only after a few percent conversion the homogeneous solution becomes turbid and the polymer precipitates. 

Cyclodextrin can also be used in order to stabilize monomers that would otherwise oxidize upon contact with air. The cyclodextrin host shields the monomer from oxygen in the air. Monomers such as pyrrole can be stored as a cyclodextrin complex for months without any noticeable degradation. The complex is a colorless powder that does not change color over time unlike pure pyrrole, which would oxidize and therefore turn black via yellow. Another advantage of the method is the fact, that the complex is odorless whereas pyrrole itself has an unpleasant smell. 

Auletta T, de Jong MR, Mulder A, van Veggel FCJM, Huskens J, Reinhoudt DN, Zou S, Zapotoczny S, Schonherr H, Vancso GJ, Kuipers L. 3-Cyclodextrin host-guest complexes prohed under thermodynamic equilibrium thermodynamics and AFM force spectroscopy. J Am Chem Soc 2004 126 1577-1584. 

Ramamoorthy et al. (444) found that a-phenyl-A-(4-methylphenyl)nitrone can be the guest molecule in inclusion complexes with a p-cyclodextrin host in 1 1 and 1 2 ratios (guest/host), and that the latter undergoes a 1,3-dipolar cycloaddition reaction with electron-deficient alkenes. In more recent work, they have formed 1 1 inclusion complexes of the bowl-shaped p-cyclodextrin 383 with (3-nitrostyrene 381 or 1-nitrocyclohexene 382, which leave the alkene moiety exposed (Fig. 1.9) (445). Complexes 381 and 382 undergo cycloaddition reaction with ot-phenyl-A-(4-methylphenyl)nitrone in the solid state after thorough homogenization (60 °C, 3 h) to give the 4-substituted products exclusively in 80 and 85% yield, respectively. 

Potentiometric sensing based on type 3 derivatives of a-, P-, and y-cyclodextrins has been reported.We have been interested in the type 1 cyclodextrin derivatives developed by Tagaki, e.g., host 41. This type of cyclodextrin derivative is characteristic in that the primary hydroxyl groups at the C-6 positions are exhaustively substituted with long alkyl chains. At the surface of an oiganic membrane, this structural feature allows the secondary hydroxyl side (wider open end of the cavity) to face the aqueous solution to accommodate a guest molecule. The interfacial receptor functions of these cyclodextrin hosts have been confirmed mainly by rt-A isotherm studies. 

Keywords to-amino carboxylic acid, cyclodextrins, host-guest chemistry, condensation, polyamides... 

Figure 7-19. Three examples of supramo-lecules formed from the assembly of two molecular species. In 7.33, the 1,4-butane-dioic acid is specifically hydrogen bonded to the bisamide. In 7.34, rc-stacking interactions between the cationic cyclophane and the host 1,4-dimethoxybenzene stabilise the structure, whilst in 7.35 the cavity of the ct-cyclodextrin host is the correct size for the benzaldehyde guest. In each case, hydrogen atoms have been omitted for clarity carbon atoms are grey, nitrogen atoms white and oxygen atoms black.

Examples of these reactions are the reduction of the non-hydrated form of formaldehyde or that of acetic acid in aqueous solution at a mercury electrode [9] as well as the reduction of many inorganic ions in their complexed forms [49], Among organic species there are also many examples of this reaction scheme like the reduction of benzoic acid at room temperature ionic liquids [50] or that of the oxidation of ferrocenecarboxylate in the presence of a p-cyclodextrin host (see Fig. 3.21) [51]. 

Fig. 3.21 CE mechanism for the oxidation of ferrocenecarboxylate in the presence of a p-cyclodextrin host. Reproduced with permission of reference [51]...

In an elegant self-assembling system based on the use of cyclodextrin hosts, DeCola and coworkers noncovalently linked an alkylcarboxyan-thracene to an a-cyclodextrin bearing Ru(II) complex that also had an Os(II) bipyridyl derivative attached via noncovalent interaction with a -cyclo-dextrin substituent on the Ru complex (Fig. 14). The system yields an energy transfer cascade from the photoexcited anthracene to the Ru to bpy 3 MLCT state, which in turn transfers energy to the Os(II) complex 3MLCT state. All rate constants are > 108 s-1 [88]. 

It was found that formation of the 6-cyclodextrin inclusion complex improved the compound solubility from less than 0.5 mg/mL to 160 mg/mL, eliminating the formulation problems associated with solubility concerns [65]. Although retinoic acid does exhibit intrinsic CD when dissolved in dimethyl sulfoxide, the CD spectrum is drastically upon formation of the cyclodextrin inclusion complex (see Figure 10). That CD can be measured in the absorption bands located at 350 and 400 nm can be taken as evidence that it is the conjugated sidechain of the retinoic acid solute which becomes included in the cavity of the cyclodextrin host system. 

The rigid structure of the cyclodextrin host results in well defined but different inclusion and interaction patterns for any potential guest molecule. Treating a mixture of compounds with a dissolved or solid, immobilized CD, leads to the formation of inclusion complexes of different stability and solubility. Consequently separations can be based either on strongly modified solubility in water of the CD-complex of a certain component, or on the... 

In electrokinetic chromatography (EKC, electroosmosis, electrophoresis) the highly soluble ionic cyclodextrin-derivatives (e.g. carboxymethyl-6-cyclodextrin) are employed (47). A non-charged guest will migrate in the electric field, because it is transported by the charged cyclodextrin host. 

Saenger W (1985) Nature and size of included guest molecule determines architecture of crystalline cyclodextrin host matrix. Israel J Chem 25 43-50... 

FIGURE 13.6, Scattering curves for the host and guest molecules in a / -cyclodextrin (host) - 2iV-acetylphenylalanine methyl ester (guest) complex. Data for the host (J cyclodextrin) are indicated by solid lines, and for the guest (the methyl ester) by broken lines. The guest is somewhat disordered in the channel provided by the more ordered structure of the crystallized host (Ref. 24). 

The most important property of cyclodextrins is their great ability to form stereoselective inclusion complexes with a variety of compounds. The inclusion complex is formed when the guest molecule, or at least its part, enters the cavity of cyclodextrin-host and covalent bondings are not formed. The main factors responsible for the stability of such a complex are the geometrical fitting of the guest molecule to the cyclodextrin cavity, the polarity of the guest, the temperature and the solvent. 

Jeon WS, Moon K, Park SH, Chun H, Ko YH, Lee JY, Lee ES, Samal S, Selvapalam N, Rekharsky MV, Sindelar V, Sobransingh D, Inoue Y, Kaifer AE, Kim K (2005) Complexation of ferrocene derivatives by the cucurbit[7]uril host a comparative study of the cucurbitmil and cyclodextrin host families. J Am Chem Soc 127 12984-12989... 

J Ramirez, F He, CB Lebrilla. Gas-phase chiral differentiation of amino acid guests in cyclodextrin hosts. J Am Chem Soc 120 7387—7388, 1998. 

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