Cyclodextrins structural features

Potentiometric sensing based on type 3 derivatives of a-, P-, and y-cyclodextrins has been reported.We have been interested in the type 1 cyclodextrin derivatives developed by Tagaki, e.g., host 41. This type of cyclodextrin derivative is characteristic in that the primary hydroxyl groups at the C-6 positions are exhaustively substituted with long alkyl chains. At the surface of an oiganic membrane, this structural feature allows the secondary hydroxyl side (wider open end of the cavity) to face the aqueous solution to accommodate a guest molecule. The interfacial receptor functions of these cyclodextrin hosts have been confirmed mainly by rt-A isotherm studies. 

Bouquet -shaped molecules based on either a polyether macrocycle [8.205] or a /3-cyclodextrin (/3-CD) [8.206] core, such as 114 and 115 respectively, have been synthesized and their polycarboxylate forms have been incorporated into vesicle bilayer membranes [8.207]. They present structural features suitable for studies of chundle-type molecular channels (1) the functionalized cyclic annulus possesses substrate selection properties (2) it bears axially oriented bundles of oxygen-containing chains, which provide binding sites for metal cations and are long enough for the molecule to span a typical lipid membrane (the overall length with the chains... 

Redenti, E., L. Szente, and J. Szejtli (2001). Cyclodextrin complexes of salts of acidic drugs. Thermodynamic properties, structural features, and pharmaceutical applicatloFffearm. Sci., 90 979-986. 

The use of chiral stationary phases (CSP) in liquid chromatography continues to grow at an impressive rate. These CSPs contain natural materials such as cellulose and starch as well as totally synthetic materials, utilizing enantioselective and retentive mechanisms ranging from inclusion complexation to Ti-electron interactions. The major structural features found in chiral stationary phases include cellulose, starch, cyclodextrins, synthetic polymers, proteins, crown ethers, metal complexes, and aromatic w-electron systems. 

Numerous examples of artificial channels have been reported since the pioneering work of Tabushi using tetrachained cyclodextrin as a model. Of these examples, the macrocycles, such as crown ethers, calixarenes, cyclodextrins, and cucurbiturils, play an important role. The macrocycles provide not only the platforms for construction of whole channels but also the functional sites necessary to achieve transport selectivity and efficiency. From this viewpoint, pillararenes, new macrocycles with unique structural features, may also act as a platform for building such channels, which will give the channels new functions. This hypothesis has been systematically explored by us. 

The internal hydrophobic cavity is the key structural feature of the cyclodex-trins it provides their ability to complex and hold a wide variety of inclusion molecules. Differences between the cyclomaltodextrins arise from differences in the diameters of their cavities. The organic inclusion molecules have alkyl, aromatic, alcohol, and ester groups. To bind with the cyclomaltodextrin, the inclusion molecule must have a size that fits, at least partially, into the cavity, creating the complex. The inclusion compound, however, does not have to be completely contained in the cavity. Complexes can be formed by the insertion of some specific functional group or part of the molecule to bind in the hydrophobic cavity. This partial insertion of the inclusion compound gives a cap or lid on the cavity. Sometimes more than one cyclodextrin will bind with a single inclusion molecule. 

A set of heptakis-(6-S-alkyl-6-thio)-p-cyclodextrins have been produced in connection with studies of thermotropic liquid crystalsand from monotosyl-P-cycIodextrin a phosphinyl rhodium complex with the structural feature 32 has been produced. 

Since caroviologens are rather fragile compounds, they can be protected from the environment by inclusion into polyanionic derivatives of (J-cyclodextrin in a rotaxane fashion 102 [8.156]. Also, in the design of molecular devices, it may be desirable to introduce some extent of redundancy in order to reduce the risk of device failure. This is the case in the tris-carotenoid macrobicycle 103 that represents a sort of triple-threated molecular cable whose crystal structure 104 has been determined. It forms a dinuclear Cu(i) complex 105 in which the bound ions introduce a positive charge at each of the species, a feature of potential interest for transmembrane inclusion [8.157]. 

There have been several reports which have demonstrated the chromatographic separation of ortho, meta and para structural isomers on 6-cyclodextrin matrices (jj). One common feature of these separations is that the order of retention (greatest to lowest) falls in the order of para > ortho > meta, although the ortho and meta isomers elute very close to one another. This is in contrast to what is obtained, as illustrated above for nitrophenol, with o-cyclodextrin, where the binding affinity for the meta isomer is greater than the ortho. In an attempt to understand this... 

---