Molecular specificity of cyclodextrin

This interesting result strongly suggests that even for the substituted cyclodextrins the capacity of inclusion formation is much the same as for the parent cyclodextrin. Therefore, we may extend the basic concept of the structure of cyclodextrin inclusion to molecular design for the preparation of artificial enzymes having satisfactory substrate specificities and catalytic activities. 

As mentioned above, some naturally occurring cyclic hosts that possess molecular recognition capabilities were known before crown ethers (the first artificial host molecules) were discovered. For example, the cyclic oligopeptide valinomycin and the cyclic oligosaccharide cyclodextrin were found to bind to specific guest molecules. The chemical modification of cyclodextrin was particularly well-researched, and artificially modified cyclodextrins became one of the most important compoimds used in host-guest chemistry. 

Cyclodextrins (CDs) are commonly used to improve the stability of flavor via the encapsulation of certain specific ingredients that naturally exist in food materials. The method is often called molecular encapsulation because the flavor ingredients are encapsulated in the molecular cavity of CDs. CDs form inclusion complexes with a variety of molecules including flavors, fats, and colors. Most natnral and artificial flavors are volatile oils or liquids, and the complexation with CDs provides a promising alternative to the conventional encapsulation technologies for flavor protection. 

At first, the substituent replaced one end of the CD hydroxyl. Then, through multipoint connection of hydroxyl sites or stereochemical structure of the substituent itself, capped CDs, i.e. CDs with a cover of the substituent, could be formed. Capped CDs can be used to enhance the inclusion ability of the CD and improve molecular recognition of the specificity guest. A typical example is the basket CD which can be obtained by the reaction between biphenyl-4, 4 -di chloride and fi-CD at A, D-position, resulting in sulfonic esters. Then the reaction with potassium iodide to form the di-iodo-/3-cyclodextrin. di-iodo-j8-CD can be precipitated by using H2O / (CH2CH2)Cl4 (20 1) at 0°C. After reactingwith 10-fold excess of aza-crown ether in dry DMF under the protection of the chlorine reaction at 55 C for 24 h, the solvents are removed by evaporation. Purification can be achieved by sephadex LH-20 column with DMF/H2O (1 1). 

It is remarkable how much new chemistry emerges when a new molecular template is discovered. This is especially true of the discovery and development of a new molecular turn. Sauvage s turn and Stoddart s turn have been exceptionally productive in leading to new molecular architectures. New turns will continue to create new opportunities. For example, there is no turn specific for cyclodextrins. although it is reasonably clear how to proceed in attempts to design such a potent addition to the toolbox of parts for interlocked molecules. 

Enantioresolution in capillary electrophoresis (CE) is typically achieved with the help of chiral additives dissolved in the background electrolyte. A number of low as well as high molecular weight compounds such as proteins, antibiotics, crown ethers, and cyclodextrins have already been tested and optimized. Since the mechanism of retention and resolution remains ambiguous, the selection of an additive best suited for the specific separation relies on the one-at-a-time testing of each individual compound, a tedious process at best. Obviously, the use of a mixed library of chiral additives combined with an efficient deconvolution strategy has the potential to accelerate this selection. 

The distribution of a-, P- and y-CDs is highly dependent on the origin of biocatalyst used (See section on Cyclodextrin Glucanotransferases). Product distributions maybe altered by the addition of specific precipitants, such as aromatics and long chain alcohols 9, 20), Depending on molecular size, these precipitants preferentially complex with specific CD species and are removed from solution. 

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