Cyclodextrins cavity size

Macaudiere et al. first reported the enantiomeric separation of racemic phosphine oxides and amides on native cyclodextrin-based CSPs under subcritical conditions [53]. The separations obtained were indicative of inclusion complexation. When the CO,-methanol eluent used in SFC was replaced with hexane-ethanol in LC, reduced selectivity was observed. The authors proposed that the smaller size of the CO, molecule made it less likely than hexane to compete with the analyte for the cyclodextrin cavity. 

In this equation, AG°CS is taken to be negligible for p- and y-cyclodextrin systems and to be constant, if there is any, for the a-cyclodextrin system. The AG W term is virtually independent of the kind of guest molecules, though it is dependent on the size of the cyclodextrin cavity. The AG dw term is divided into two terms, AG°,ec and AGs°ter, which correspond to polar (dipole-dipole or dipole-induced dipole) interactions and London dispersion forces, respectively. The former is mainly governed by the electronic factor, the latter by the steric factor, of a guest molecule. Thus, Eq. 2 is converted to Eq. 3 for the complexation of a particular cyclodextrin with a homogeneous series of guest molecules ... 

An interesting equilibrium study was performed by Kasatani and coworkers on the interaction of a series of cyanine dyes with beta and gamma cyclodextrin, using u.v.-visible spectrophotometric measurements. The results were consistent with the enhancement of dimer formation by inclusion of the dye molecules within the cyclodextrin. As expected from the work of others, such geometrical factors " as the size of the aromatic groups and the lengths of the central methine chains strongly influence the occurrence of dimer formation within the cyclodextrin cavity. 

Cyclophanes has several great advantages in use as enzyme models. First, the design of the preparation of substituted cyclophanes is well established. Second, they are very stable, even more by stable comparison with corresponding cyclodextrins. Moreover, a cyclophane of certain cavity size is readily available. Because of these advantages, cyclophanes attract increasing attention from chemists. 

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. 

When a-cyclodextrin is free in aqueous solution, it adopts the tense form found in the two hexahydrate crystal structures (Fig. 18.6 a) one glucose unit is rotated inwards to reduce the size of the a-cyclodextrin cavity so that the enclosed water molecules are held more tightly. The two intramolecular 0(2) 0(3 ) hydrogen bonds to this glucose unit are broken, the other four remain intact. 

Calixarenes were developed later than crown ethers and cyclodextrins but have stillbeen extensively researched. Macrocycles of calix[n]arenes are constructed by linking a number of phenol residues via methylene moieties (Fig. 2.16). Like crown ethers, the name calixarene reflects the structures of these molecules, since a calix is a chalice. Calixarenes with various cavity sizes have been designed, each of which has conformation isomers, and their phenolic hydroxyl groups are often modified. These structural characteristics allow us to create calixarene derivatives with various structural modifications. 

To achieve chiral separations on CD CSPs, a part or all of the solute molecules must enter the cyclodextrin cavity. In most cases, the solutes that are successfully resolved contain an aromatic moiety at or adjacent to the stereogenic center, and it is the aromatic portion of the molecule that inserts itself into the chiral cavity of the cyclodextrin molecule to form the inclusion complex (66,67). The size of the aromatic moiety and cyclodextrin cavity determine which CD CSP will form the best inclusion complex, and single aromatic rings fit best in the ot-CD, naphthylrings in the p-CD and aromatic system larger than naphthyl in the -y-CD (68), In addition to the... 

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