Cyclodextrin empty

Interplanar Angles and Diameters Characterizing Empty Cyclodextrin... 

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. 

The expulsion of high-energy waters which were found in the empty cyclodextrins (4b, 35). 

Among enzyme modified starch derivatives,cyclo dextrins behave as empty molecular capsules with the ability to entrap guest molecules of appropriate geometry and polarity.The included molecules are protected from surroundings light, heat,oxidation, etc. The flavor cyclodextrin com -plexes show the above advantageous properties while they are in the dry,solld state.On contact with water,cyclodextrin complexes release their flavor content. In Hungary,the spice flavor beta-cyclodextrin complexes have been on the market, since 1982. 

The central cavity of the cylinder-shaped cyclodextrins behaves as an empty capsule It can accommodate so-called guest molecules of appropriate size, shape, and polarity. This "molecular encapsulation" can be utilized for stabilization and for enhancement of solubility of drugs, vitamins, flavors, etc., and utilizing the selectivity of the inclusion complexation, it can be applied for separation of substances, either by non-chromatographic methods, or chromatographic methods. 

Cyclodextrins are torus-shaped molecules with hydrophobic cavities between 0.5 and 0.85 nm. They can store other molecules that fit into these cavities depending on their shape and intermolecular attraction. The empty cavities can be filled with organic molecules from sweat so that their microbiological decomposition and the formation of malodours are prevented. On the other hand, the malodorous decomposition products can also be stored if they can fit into the cavities. These bound molecules can then be removed by normal washing. 

Fig. 7.9. Fluorescence decay of CD-St (16.6 l-iM) in the presence of 0x725 (80.5 pM). A. Experimental decay (points) and best fit (solid line) with a sum of four exponentials with two of them fixed at the values corresponding to the empty cyclodextrins (13.11 and 2.94 ns) the values of the two others are 12 and 90 ps. B. Reconvoluted curve (solid line) corresponding to only uncomplexed cyclodextrins...

Fig. 2 Space-filling representations of (from left to right) empty P-cyclodextrin dimers, a sectioned view showing the empty channel, and sectioned views showing channel occupation by (L)-menthol and (R)-fenoprofen guest molecules in the isostructural clathrates. (Prepared from data in Ref [25].) (View this art in color at www.dekker.com.)...

If Y-cyclodextrin is crystallized as "empty" molecule from water, it arranges in a fishbone type cage structure similar as observed for a- and (5-cyclodextrins (17). If, however, a small guest like n-propa-nol is added, a channel type structure is formed in which the Y-cyclo-dextrin molecules with eightfold symmetry are stacked along a fourfold symmetry axis and therefore exactly linear channels are produced (18), Fig. 3. 

On the basis of the nearly hexagonal symmetry of the a-cyclodextrin molecules in the krypton adducts one should assume that the included krypton atoms occupy at least six statistically disordered positions. Owing to the anisotropic environment of the a-cyclodextrin molecule in this crystal structure only five out of the six possible locations are occupied by krypton atoms, see Figure 9. The location near the most rotated glucose is empty - obviously the slight rotation of this glucose has prevented the krypton atom to fill that position. 

Fig. 9. A composite drawing of the two a-cyclodextrin Kr 5H20 structures depicted in Figures 8a, b. The positions of the krypton atoms are projected into one a-cyclodextrin cavity to show the almost hexagonal distribution of the disordered krypton sites. The diagonal of the small hexagon formed by the krypton atoms is 1.1 A, the site near the glucose 5 which is rotated most towards the viewer is empty owing to steric interactions.

When a-cyclodextrin is dissolved in water in order to allow inclusion formation with an added guest molecule to occur, it exists in the empty state. Investigation of this empty molecule should be of interest in relation to questions concerning the mechanism of inclusion. 

We have seen that the empty a-cyclodextrin molecule in the (H20)2 complex occurs in a strained conformation (Figure 14) and further, that the ring of hydrogen... 

B) The empty a-cyclodextrin molecule gains activation energy, i.e. it transforms into a molecule of almost hexagonal, low energy conformation while the included water molecules pick up energy and become disordered (like the krypton atom in the a-cyclodextrin-Kr complex). The disordered water molecules are replaced by the substrate. 

C) The substrate forms an intermediate complex with the empty a-cyclodextrin and enters the cavity after the complex has gained activation energy. 

Fig. 18. Schematic representation of the a-cyclodextrin - substrate inclusion process. The empty a-cyclodextrin molecule in the upper left hand corner corresponds to the a-cyclodextrin (H20)2 complex found in the crystal structure [12, 13]. Only four of the six 0(2) 0(3) interglucosidic hydrogen bonds are formed and the molecule is in a tense state with high conformational and low hydrogen bonding energy. Upon adduct formation via routes A, B or C it goes into a relaxed , com-plexed state with all 0(2) 0(3) hydrogen bonds formed and with low conformational energy.

Empty column (25 cm x 1 cm I. D.) assembly was purchased from Phenomenex (USA). The columns were packed with perphenyl carbamoylated beta-cyclodextrin bonded onto silica gel using an Alltech pneumatic liquid pump (Alltech, USA) by slurry packing method. The silica gel was supplied by Eka Chemicals AB (Sweden) with particle size of 16 pm (KR100-16-SIL). The eluent (desorbent) used was a binary mixture containing 60% aqueous buffer solution (1% TEAA, pH=4.5) and 40% methanol. The feed solution was prepared by dissolving raeemate propranolol hydrochloride in the desorbent at... 

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