Cyclodextrin cavity, water

Water plays a crucial role in the inclusion process. Although cyclodextrin does form inclusion complexes in such nonaqueous solvents as dimethyl sulfoxide, the binding is very weak compared with that in water 13 Recently, it has been shown that the thermodynamic stabilities of some inclusion complexes in aqueous solutions decrease markedly with the addition of dimethyl sulfoxide to the solutions 14,15>. Kinetic parameters determined for inclusion reactions also revealed that the rate-determining step of the reactions is the breakdown of the water structure around a substrate molecule and/or within the cyclodextrin cavity 16,17). 

Dendritic hosts can be used in aqueous solution to encapsulate water-soluble fluorescent probes. Changes in the photophysical properties of these encapsulated probes are useful to understand the properties of the microenvironment created by the dendritic interior. For example, adamantyl-terminated poly(pro-pylene amine) dendrimers from the first to the fifth generation (36 represents the third generation) can be dissolved in water at pH<7 in the presence of -cyclodextrin because of encapsulation of the hydrophobic adamantyl residue inside the /1-cyclodextrin cavity and the presence of protonated tertiary amine units inside the dendrimer [72]. Under these experimental conditions, 8-anifi-... 

In addition, cyclodextrins, because of their hydrophobic cavity, are capable of hydrophobic binding of the diene and/or dienophile into the cyclodextrin cavity in water. Therefore, cyclodextrins with the... 

Rhodopsin is a seven ot-helix trans-membrane protein and visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In this photoreceptor, retinal is the chromophore bound by opsin protein, covalently linked to Lys296 by a Schiff base linkage. Kpega et al.64 have studied NMR spectra of Schiff bases being derivatives of all-frans retinal and amino-p-cyclodextrins as a model of rhodopsin, where p-cyclodextrin plays a role of a binding pocket. On the basis of analysis of the chemical shift differences for the model compound in the presence and in the absence of adamantane carboxylate, it has been shown that the derivative of 3-amino-p-cyclodextrin forms dimer in water and retinoid is inserted into p-cyclodextrin cavity [31]. 

Empty ft- (11) and y-cyclodextrins (12) also take normal torus shapes, as revealed by X-ray crystallography, and no significant deformations are observed for these two cyclodextrins when they bind guest molecules. It is noteworthy that all empty cyclodextrins include water molecules in their cavities as shown in Table IV and Fig. 4. Since no water molecules were observed in inclusion complexes of cyclodextrins with organic guest molecules, it is evident that the expulsion of these water molecules in the cyclodextrin cavities is one of the important factors for formation of the inclusion complexes. 

Fig. 4. Schematic representation of water molecules in cyclodextrin cavities (a) a-cyclo-dextrin (Form I) (b) /i-cyclodextrin.

Fluorescence dynamics of coumarin C522 in water and in cyclodextrin cavity... 

Unlike lipid bilayer membranes and proteins in which the ESPT of aromatic hydroxy compounds have been extensively used for probe purposes, a diverse variety of ESPT molecules have been studied as inclusion complexes in cyclodextrin cavities. The well-defined CD cavities often accommodate molecules in distinctly different but well-defined orientations. This is reflected in their ESPT behavior. For the conventional range of ESPT molecules, the size of (3-CD seem to be more appropriate and a fairly large number of studies are reported on it a-CD appears too small and 7-CD too large for forming suitable inclusion complexes. The effects of local polarity and water accessibility in ESIPT have been topics of active interest recently. These aspects are discussed next. 

In an aqueous solution, the slightly apolar cyclodextrin cavity is occupied by water molecules that are energetically unfavored (polar-apolar interaction), and therefore can be readily substituted by appropriate guest molecules , which are less polar than water (Fig. 4.3). The dissolved cyclodextrin is the host molecule, and part of the driving force of the complex formation is the substitution of the high-enthalpy water molecules by an appropriate guest molecule. One, two or three CD molecules. 

We examined the ability of our bis-imidazole cyclodextrin artificial enzymes to perform other bifunctionally-catalyzed reactions, where again the availability of the A,B and A,C and A.D isomers let us learn mechanistic details. As an important example, we examined three isomeric catalysts ability to promote the enolization of substrate 48, which binds into the cyclodextrin cavity in water [138]. Here there was again a strong preference among the isomers, but it was the A,D isomer 49 that was the effective catalyst It was also more effective than a cyclodextrin mono-imidazole that cannot use the bifunctional mechanism. 

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