Cyclodextrins hydrophobic complexation

Polymerization in P-cyclodextrin (CD) complexes with monomer offers a route to polymerization, as well as other organic reactions, in water without the need for organic solvents [Ritter and Tabatabai, 2002]. P-Cyclodextrins are toms-shaped, cyclic oligosaccharides obtained by degradation of starch. The hydroxyl groups of the glucose repeat unit of CD are located on the outer surface. This makes the outer surface hydrophilic, whereas the inner surface and cavity are hydrophobic. Water-insoluble monomers become solubilized in water when mixed with CD or CD derivatives because the monomers are absorbed into the cavity. This allows polymerization in aqueous, not organic media, with water-soluble initiators. 

Fig. 1. Topographies of cyclodextrin inclusion complexes a-CD - nitromethane (75) (top), p-CD - adamantane-carboxylate (76, 77) (center), and y-CD - 12-crown-4 (76, 78). The striking correspondence of hydrophobic surface regions of guest and CD-host at their interfaces may be viewed in color on the Internet (79).

Figure 5 Preliminary schematic representation of a ternary iron-cyclodextrin-pollutant complex believed to be involved in improving the efficiency and selectivity of Fenton degradation of hydrophobic organic pollutants.

Many plant secondary metabolites produced by cell cultures and substrates used in culture media are often hydrophobic and have low solubility in aqueous medium. Improving the production of metabolites in the plant cell system has also been attempted by enhancing the solubilities of metabolites by using the cyclodextrin-guest complex formation. There are two major approaches, one involves the improvement of substrate solubility and the other improves metabolite. 

Concerning the significance of the hydrophobic polymer environment some examples are shown, such as hydrolyses of PNPA and 8-acetoxyquinoline-5-sulfonate by the acylated cyclodextrin—Ni complex (262), cyclodextrin-Cu(II) complex (262) and bis-salicylimine of gramicidin-Cu(II) complex (163). 

Hingerty BE, Saenger W (1976) Ibpography of cyclodextrin inclusion complexes. 8. Crystal and molecular structure of the a-cyclodextrin-methanol-pentahydrate complex. Disorder in a hydrophobic cage. J Am Chem Soc 98 3357-3365... 

Cyclodextrins are water-soluble hosts for hydrophobic guests. The formation of inclusion complexes is controlled by the dimensional compatibility of the guest and host cavity. Commercially available dextrins are, in fact, inclusion complexes of cyclodextrins with two water molecules closing the entrance to the cavity. The formation of cyclodextrin inclusion complexes is reversible and, therefore, is governed by concentration of guests competing for a place inside the cavity. 

In catalysis the excess of a phosphine ligand is often necessary because it preserves the active species in the medium [2a]. However, it retards to some extent the co-ordination of the alkene to the metal center. Recent studies, performed by Monflier and coworkers, have shown that the water-soluble TPPTS ligand could reduce the rate of the reaction by another effect. Indeed, TPPTS can be included partially in the cyclodextrin hydrophobic cavity [53,54] NMR measurements, observation by UV-visible spectroscopy and circular dichroism, as well as scanning tunneling microscopy are consistent with a 1 1 inclusion complex in which the phosphorus atom would be incorporated into the torus of the /S-CD. NMR investigations carried out on (m-sulfonatophenyl)diphenylphosphine have shown that a phenyl group is incorporated [55]. Thus, the phosphorus ligand could modify the association constant of the alkene with the cyclodextrin so that the mass transfer between the two phases could be decreased. 

The low selectivity of the formation of cyclodextrin inclusion complexes with guests of similar volume but of different shape or charge is explained by the existence of several binding types hydrophobic and van der Waals interactions as well as hydrogen bonds. 

R. Breslow, R Campbell, Selective aromatic substitution within a cyclodextrin mixed complex, J. Am. Chem. Soc., 1969, 91, 3085 R. Breslow, P. Campbell, Selective aromatic substitution by hydrophobic binding of a substrate to a simple cyclodextrin catalyst, Bioorg. Chem., 1971,... 

On a laboratory scale, the complexes are prepared by the coprecipitation method the guest is added to an aqueous solution of the hydrophilic cyclodextrin. The complex is usually less soluble, precipitates and can be isolated by filtration or centrifugation. For water-insoluble guests, diethyl ether can be used to facilitate the phase transfer. The ether then can be blown away by bubbling argon through the solution. The main driving forces for the complex formation are hydrophobic and van-der-Waals interactions. These are the... 

The transfer of acyl groups is commonly encountered in biology, and catalyzed by the acyltransferase family of enzymes. For example, acetylcholinesterase hydrolase hydrolyzes excess acetylcholine after nerve impulses. Many chemical models built to mimic the enzymatic function of acyltransferases have been developed, classically employing cyclodextrins which form hydrophobic complexation intermediates. ... 

Dihydroquercetin (taxifolin, DHQ) is a natural flavonoid, which possesses antioxidant activity and other pharmacological properties (anti-inflammatory, anti-atherosclerotic, etc.). Dihydroquercetin is hydrophobic compound, that s why it can t be administered intravenously, also its oral bioavailability is reduced. Recently, many new dihydroquercetin derivatives were synthesized, including water-soluble forms (cyclodextrin inclusion complexes with dihydroquercetin derivatives). In addition to the protective effect of antioxidants against lipid peroxidation, increasing attention is paid to the possibilities of antioxidants including dihydroquercetin to prevent an oxidation of proteins. Fibrinogen is more susceptible to oxidation than most other plasma proteins. 

Dihydroquercetin is hydrophobic compound, that s why it can t be administered intravenously, also its oral bioavailability is reduced. Recently many new dihydroquercetin derivatives were synthesized including water-soluble forms (cyclodextrin inclusion complexes with dihydroquercetin derivatives). 

In its primary structure, the AGU are existing in the conformation c (chair conformation). The valence angles between the AGU are favoring a helical conformation, formed by 6-8 AGU, as the energetically most suitable state. The normal state in solution is that of a disturbed helix. An ideal stable helix conformation is formed and stabilized when hydrophobic molecules (iodine, aliphatic alcohols and acids) are allowed to penetrate into the molecular channel. The formation of such inclusion complexes is a typical property of a. and may be compared best with the inclusion behavior of - cyclodextrin. Insoluble complexes with organic solvents are used to precipitate amylose from starch solutions during fractionation. 

The major driving forces for the cyclodextrin inclusion complexation are believed to be van der Waals and hydrophobic interactions [5]. It is also believed that hydrogen bonding and release of strain energy of CD upon complexation have certain roles to the stability of the complexes. In the complexation of ionic guests with ionic group-modified CD, the electrostatic interaction also affects the stability of the complexes... 

Cyclodextrins form complexes not only with hydrophilic polymers, but also hydrophobic polymers, such as oligoethylene and polyisobutylene. Table 6 shows the yields of the complexes between cyclodextrins and some hydro-phobic polymers. a-CD forms complexes with oligoethylenes, although neither p- nor 7-CD form complexes with oligoethylenes under the same conditions. On the other hand, p- and 7-CD form complexes with polyisobutylene (PIB), although a-CD does not form complexes with PIB. 

Fig. 8. A hydrophobic inclusion complex between a chiral analyte and a cyclodextrin.

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