Cyclodextrins conformational changes

The results just discussed suggest that it is only alpha cyclodextrin that shows a conformational change on complex-formation, due to the rotation of one of its D-glucosyl units. Whether this conformational change contributes to the stability of the complex, as proposed by Saenger, is, however, debatable. Bergeron and Meeley " showed that methylation of the... 

Further support for the presence of a conformational change has come from the work of Orstan and Wojcik, who also studied azo dye-alpha cyclodextrin complexes by using the temperature-jump technique. The dyes that they investigated showed three different types of behavior. Certain dyes showed a limiting value of 1/t at high alpha cyclodextrin concentrations, others showed a linear dependence of 1/t on alpha cyclo-... 

Up to now, evidence for the presence of a conformational change during the inclusion process has been presented only for alpha cyclodextrin, and then only in the case of certain guest molecules. Whether the two-step mechanism is generally applicable is not known, because, owing to the nature of kinetic studies, the results are frequently open to more than one interpretation. Nevertheless, sufficient support for the two-step mechanism has appeared to necessitate its consideration in any future kinetic studies. 

There is evidence that the a-cyclodextrin macrocycle can adjust its cavity diameter, within certain limits, to accommodate either two hydrate water molecules in the collapsed or tense form, or a nonaqueous guest molecule in the round or relaxed form. This is the concept of the induced fit [576] which has been applied to comparable conformational changes observed with several enzymes upon substrate binding. 

Figure 7 Sketch of the guest-induced conformational change of chromophore-modified cyclodextrins that causes the fluorescence...

Solvent-dependent conformational changes in a mono-functionalized 3-cyclodextrine, mono-(3-Tyr-3-deoxy- / ro)- P-CD, have been followed by... 

Capping of one side of the torus with an apolar moiety enhances the binding ability of a cyclodextrin. This effect is attributable to the increase in the apolar nature of the cavity due to the capping by the apolar groups. For example, capped )8-cyclodextrin (la) binds sodium l-anilino-8-naphthalenesulfonate 24 times better than parent -cyclodextrin [5]. The binding ability of another capped j8-cyclodextrin (lb) changes on photoirradiation, since the conformation of /8-cyclodextrin is varied due to the cis-trans photoisomerization of the azobenzene moiety [6]. 

Similarly mono-(o-hydroxyphenyl)triphenylporphyrin (65) was covalently attached to a cyclodextrin unit, in the hope of preparing a water-soluble oxygen carrying model While 123 did not display the desired properties, guest inclusion in the cyclodextrin moiety appears to induce novel conformational changes in aqueous solutions. 

More recent studies show, however, that CICS may be caused not only by an inclusion in the cyclodextrin cavity but also by conformational changes even in a part of a molecule... 

---