A-cyclodextrin cavity

These equations show that hydrophobic and steric (van der Waals) interactions are of prime importance in the inclusion processes of cyclodextrin-alcohol systems. The coefficient of Es was positive in sign for an a-cyclodextrin system and negative for a P-cyclodextrin system. These clear-cut differences in sign reflect the fact that a bulky alcohol is subject to van der Waals repulsion by the a-cyclodextrin cavity and to van der Waals attraction by the p-cyclodextrin cavity. 

In these equations, Dmax is the larger of the summed values of STERIMOL parameters, Bj, for the opposite pair 68). It expresses the maximum total width of substituents. The coefficients of the ct° terms in Eqs. 37 to 39 were virtually equal to that in Eq. 40. This means that the a° terms essentially represent the hydrolytic reactivity of an ester itself and are virtually independent of cyclodextrin catalysis. The catalytic effect of cyclodextrin is only involved in the Dmax term. Interestingly, the coefficient of Draax was negative in Eq. 37 and positive in Eq. 38. This fact indicates that bulky substituents at the meta position are favorable, while those at the para position unfavorable, for the rate acceleration in the (S-cyclodextrin catalysis. Similar results have been obtained for a-cyclodextrin catalysis, but not for (S-cyclodextrin catalysis, by Silipo and Hansch described above. Equation 39 suggests the existence of an optimum diameter for the proper fit of m-substituents in the cavity of a-cyclodextrin. The optimum Dmax value was estimated from Eq. 39 as 4.4 A, which is approximately equivalent to the diameter of the a-cyclodextrin cavity. The situation is shown in Fig. 8. A similar parabolic relationship would be obtained for (5-cyclodextrin catalysis, too, if the correlation analysis involved phenyl acetates with such bulky substituents that they cannot be included within the (5-cyclodextrin cavity. 

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

Kasatani K, Kawasaki M, Sato H (1984) Lifetime shortening of the photoisomer of a cyanine dye by inclusion in a cyclodextrin cavity as revealed by transient absorption spectroscopy. J Phys Chem 88 5451-5453... 

Although they are random and not oriented, the motions of a substrate inside a cyclodextrin cavity (see Section 4.5) have molecular ball-bearing flavour. Correlat-... 

FIGURE 19 Schematic representation of solute inclusion in a cyclodextrin cavity. 

The utility of the highly soluble 6-cyclodextrin derivatives (soluble polymer and dimethyl-6-cyclodextrin) in RPTLC is illustrated in the separation of barbiturates. The lipophilicity of a barbiturate or any guest decreases when included in a cyclodextrin-cavity. Therefore its mobility is modified in reversed phase thin layer chromatography. With this simple and rapid method, the stability of a complex can be estimated empirically (Table II). The "b" value of the following equation is characteristic for the complex stability (in water ethanol =4 1 solution, R determined at 5 different cyclodextrin concentrations for 21 barbiturates) ... 

Of the 7.57 water molecules per asymmetric unit, four are fully ordered and one, W(5), shows two positions at 0.64 and 0.36 occupation. These water molecules are located in intermolecular voids, and the remaining 2.57 waters are statistically distributed over four sites in the a-cyclodextrin cavity (see Fig. 18.6 b). Associated with this disorder in the a-cyclodextrin cavity is a round shape of the macrocycle, where in contrast to the other two a-cyclodextrin hydrate forms, all six intramolecular interglucose 0(2) 0(3 ) hydrogen bonds are formed. Because hydrogen atoms attached to the disordered water molecules in the 7.57 hydrate could... 

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. 

The cyclodextrins (CD s) have a proven capability for the formation of complexes with many hydrophobic structures 114.15.16). The a, P, and y cyclodextrins are cyclic oligomers of 6, 7, and 8 glucose (anylose) rings, respectively. The center cavity of the P form can easily include a large portion of a steriod molecule. The a cyclodextrin cavity can capture at best a small portion. Each cyclodextrin sugar unit has three hydroxyl groups, and no other substituents. These materials are not known, by themselves, to have any particular biological functions. 

Inclusion of Short Guests into the a-Cyclodextrin Cavity... 

In the previous reactions the cyclodextrin acted as a reactant, not a catalyst. However, there are some excellent examples in which true catalysis occurs with simple binding into a cyclodextrin cavity. Here we will describe the cases where the cyclodextrin has not been modified, while in later sections we will discuss cases in which additional catalytic groups have been added to the cyclodextrin, and mimics of metaUoenzymes and of enzymes with co-enzymes have been achieved. 

Host-guest inclusion complexes can be precipitated from aqueous solutions of pCD and BT or 3T (Scheme 1). Elementary analysis shows that stable complexes with 1 1 stoichiometry are obtained. In order to confirm the complex formation, fluorescence spectroscopy was used. The inclusion of a guest inside a cyclodextrin cavity produces a change in the electronic environm t of die guest. As a result, differences in the fluorescence spectra of free and CD-complexed monomer are expected. Fluoresc ce spectroscopy provides generally important information on die stoichiometry and equilibrium constants of inclusion... 

CPK structural model shows that the dibromide and even smaller dichloride molecules can not enter nor be fitted fully into a-cyclodextrin cavity, but the cavity of 3-cyclodextrin is able to include these dihalides of the ester. 

Fig. 10. a Structural representation of a-cyclodextrin, b re-gioselective chlorination of anisole encapsulated in a cyclodextrin cavity [48]... 

Figure 26 Mechanism of ester hydrolysis by (a) a Zn + complex appended to a cyclodextrin cavity with subsequent inhibition by the product (left) (b) Breslow s system consisting of two cyclodextrins appended to a mononuclear Cu + complex. ...

Fig. 5. (a) The herring bone pattern cage type structure in a schematic (bottom) and in a more realistic (top) presentation, the latter taken from the a-cyclodextrin-(H20)-4H20 structure [13]. In the schematic drawing, the a-cyclodextrin cavities are indicated by hatching. 

The purpose of the study of this complex was to look at a guest molecule of dimensions smaller than the a-cyclodextrin cavity of 5.0 A. The a-cyclodextrin Kr 5H2O complex was crystallized from aqueous solution under krypton pressures of 43 PSI and 200 PSI and the crystals obtained with both krypton pressures were analyzed. 

It is obvious that the adduct obtained at lower krypton pressure contains less krypton but more water within its cavity (in brackets, Table VII) than the adduct obtained at higher krypton pressure. In both cases, however, the ratio krypton/a-cyclodextrin is less than one. A remarkable feature of the a-cyclodextrin krypton adduct is the disorder of the krypton atom within the a-cyclodextrin cavity, with the occupancies given in the legend to Figures 8a, b. Further, not only krypton but also at least one water molecule enters the cavity and, at higher krypton pressure, is partially replaced by krypton. 

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.

The iodine atoms occupy the linear channel provided by the stacked a-cyclodextrin cavities. The iodine atom between the planes through atoms 0(2), 0(3) is twofold statistically disordered with 70% and 30% occupancy the other iodine atoms are at well defined positions, Figure 17. From the I--I distances it follows that if and I2 units alternate, with the disorder producing either if, I2 or l2 lf sequences. 

This scheme for an inclusion mechanism can explain why a-cyclodextrin forms complexes with such a diversity of substrate molecules The driving force is independent of the nature of the substrate. The only requirement is that the substrate molecule should be small enough to fit into the a-cyclodextrin cavity. Of course the other a-cyclodextrin-substrate interactions mentioned above will also play a role, depending on the nature of the substrate. The main and general energy contribution, however, will come from the a-cyclodextrin itself. In agreement with this statement are also the kinetic data presented in Table VIII [21]. 

Fig. 19. A scheme representing the a-cyclodextrin w-propanol complex formation. The hydration shell around the a-cyclodextrin molecule is indicated by an outer contour. From geometrical considerations it is to assume that the substrate molecule enters the a-cyclodextrin cavity from the more open 0(2), 0(3) side rather than from the 0(6) side which is closed by the two C(6)—0(6) bonds in gauche, trans conformation (19a). The -propanol molecule enters the cavity with its hydrophilic O—H group head on, the included H2O molecules are released, the a-cyclodextrin torus transforms from the tense to the relaxed state (19b). Hydrogen bonds between -propanol and the a-cyclodextrin 0(6) groups form, the hydration shell is reconstituted around the complex (19c).

Breakdown of the water structure around that part of the substrate molecule which is going to be included into the a-cyclodextrin cavity and transport of some water molecules into the solution ... 

In the adduct structures described above three different types of substrate molecules were bonded into the a-cyclodextrin cavity ... 

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