Cyclodextrins hydrophobic interactions

On the other hand, the values of AH° and AS° for a-cyclodextrin-l-alkanol systems are significantly more negative than those for the corresponding P-cyclOdextrin systems. 1-Alkanols must fit closely into the cavity of a-cyclodextrin, so that the com-plexation is governed by van der Waals interaction rather than by hydrophobic interaction. 

Upon formulating these relationships, phenols with branched alkyl substituents were not included in the data of a-cyclodextrin systems, though they were included in (3-cyclodextrin systems. In all the above equations, the n term was statistically significant at the 99.5 % level of confidence, indicating that the hydrophobic interaction plays a decisive role in the complexation of cyclodextrin with phenols. The Ibrnch term was statistically significant at the 99.5% level of confidence for (3-cyclo-dextrin complexes with m- and p-substituted phenols. The stability of the complexes increases with an increasing number of branches in substituents. This was ascribed to the attractive van der Waals interaction due to the close fitness of the branched substituents to the (3-cyclodextrin cavity. The steric effect of substituents was also observed for a-cyclodextrin complexes with p-substituted phenols (Eq. 22). In this case, the B parameter was used in place of Ibmch, since no phenol with a branched... 

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

The PO mode is a specific elution condition in HPLC enantiomer separation, which has received remarkable popularity especially for macrocyclic antibiotics CSPs and cyclodextrin-based CSPs. It is also applicable and often preferred over RP and NP modes for the separation of chiral acids on the cinchonan carbamate-type CSPs. The beneficial characteristics of the PO mode may arise from (i) the offset of nonspecific hydrophobic interactions, (ii) the faster elution speed, (iii) sometimes enhanced enan-tioselectivities, (iv) favorable peak shapes due to improved diffusive mass transfer in the intraparticulate pores, and last but not least, (v) less stress to the column, which may extend the column lifetime. Hence, it is rational to start separation attempts with such elution conditions. Typical eluents are composed of methanol, acetonitrile (ACN), or methanol-acetonitrile mixtures and to account for the ion-exchange retention mechanism the addition of a competitor acid that acts also as counterion (e.g., 0.5-2% glacial acetic acid or 0.1% formic acid) is required. A good choice for initial tests turned out to be a mobile phase being composed of methanol-glacial acetic acid-ammonium acetate (98 2 0.5 v/v/w). 

Thus, it seems that both the hydrophobic interactions and the van der Waals interactions undoubtedly play a part in inclusion-complex formation, although the relative contribution of each type of interaction may vary with the chemical properties of the guest this would account for the ability of the cyclodextrins to form complexes with a wide variety of guest molecules. The existence of a close spatial fit between the guest and the cyclodextrin cavity is, however, a necessary requirement for the formation of a stable inclusion-complex. 

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. 

An example of the above mentioned cascade complexation of carboxylates by macrocyclic receptors containing metal ionic centers is the inclusion of oxalate by the dien dicobalt complex 9 (Martell, Mitsokaitis) [12]. Similarly, the -cyclodextrin (jS-CD) 10, modified with a zinc cation bound by a triamine side chain, encapsulates anions like 1-adamantylcarboxylate in its cavity, fixing them by combined electrostatic and hydrophobic interactions [13], Zinc s group achieved the enantioselective transport of the potassium salts of N-protected amino acids and dipeptides by making use of the cation affinity of... 

Cyclodextrins (CD) can form inclusion complexes with small molecules by hydrophilic-hydrophobic interactions [33-37], Three CDs are readily available - 20, 21, and 22, denoted a, / , and y-CD, respectively. As the number of saccharides in the cyclic increases from 20 to 22, the cavity becomes larger. The hydroxy groups occupy the outside surface, resulting in hydrophilicity and high solubility in polar solvents. Their interiors consist of hydrocarbon units, making the cavity hydro-phobic. Thus when CD and a hydrophobic chain dissolve in a common polar solvent, the chain tends to occupy the cavity of the CD to achieve a lower energy level relative to its state in the polar medium this is hydrophobic-hydrophilic interaction. 

Diels-Alder catalysis. Water can accelerate the Diels-Alder reaction of cyclopentad iene with methyl vinyl ketone or acrylonitrile by a hydrophobic interaction. / -Cyclodextrin can increase this effect, possibly because the components can fit into the hydrophobic cavity.1... 

Figure 6.3 Examples of two cyclodextrin-based [2]rotaxanes possessing mainly hydrophobic interactions.

Other closely related microheterogeneous environments such as micelles [70] or tailored electron relays capable of micellization upon reduction [71], operate by related hydrophilic-hydrophobic interactions in controlling photosensitized ET processes. Similarly, separation of photoproducts at the molecular level, by means of hydrophobic interactions, has been accomplished by utilizing cyclodextrin receptors [66, 72]. This host component selectively associates one of the photoproducts into the hydrophobic receptor cavity and consequently back ET is retarded. 

An alternative is the solubilization with the help of cyclodextrins because these are soluble in water and can incorporate organic molecules inside their hydrophobic cavity [11-13]. P-cyclodextrin is the most useful regarding the typical size of molecules to be solubilized. Griseofulvin forms inclusion complexes of 1 1 stoichiometry with P-cyclodextrin [14, 15]. One possible problem is the moderate solubility of P-cyclodextrin in water (18.5 g/L) and the even lower solubility of most inclusion complexes. A more dramatic problem is the preparation of inclusion complexes of water-soluble cyclodextrins and organic molecules that are not soluble in water. The complexation takes place by means of hydrophobic interactions inside the cavity, which require the presence of water as a solvent. 

Derivatized cyclodextrins can interact with chiral substances in a number of different ways. If, the positions 2 and 6 are alkylated (pentylated), very dispersive (hydrophobic) centers are introduced that can strongly interact with any alkyl chains contained by the solutes. After pentylation of the 2 and 6 positions has been accomplished, the 3-position hydroxyl group can then be trifluoroacetylated. This stationary phase is widely used and it has been found that the derivatized y-cyclodextrin is more chirally selective than the /3 material. It has been successfully used for the separation of both very small and very large chiral molecules. The cyclodextrin hydroxyl groups can also be made to react with pure S hydroxypropyl groups and then per-... 

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