Cyclodextrin complexes guest complexation

Ihb = 1, whereas Ihb = 0 when it is inert to hydrogen bonding. Since —AG,° is proportional to log 1/Kd, where Kd is the dissociation constant of a cyclodextrin complex with a guest molecule, we can derive a quantitative structure-reactivity relationship as shown, for example, in Eq. 4 ... 

Matsui and Mochida24) have determined the thermodynamic stabilities (log 1 /Kd) for a- and P-cyclodextrin complexes with a variety of alcohols (Table 2) and analyzed the results in connection with the physicochemical properties of the guest molecules by the multivariate technique. The log 1/Kd values were plotted against log Pe, where Pe is the partition coefficient of alcohol in a diethyl ether-water system. The plots for the a- and P-cyclodextrin complexes with eight 1-alkanols gave approximately straight lines with slopes of around one. 

Nishioka and Fujita 78> have determined the Kd values for a- and P-cyclodextrin complexes with m- and p-substituted phenols at pH 7.0. Taking into account the directionality in inclusion of a guest molecule, they assumed three and two probable orientational isomers for the cyclodextrin complexes with m- and p-substituted phenols respectively (Fig. 6). Then the observed Kd values were divided into two or three terms corresponding to the dissociation of the orientational isomers involved (Eqs. 16, 17) ... 

Auletta T, de Jong MR, Mulder A, van Veggel FCJM, Huskens J, Reinhoudt DN, Zou S, Zapotoczny S, Schonherr H, Vancso GJ, Kuipers L. 3-Cyclodextrin host-guest complexes prohed under thermodynamic equilibrium thermodynamics and AFM force spectroscopy. J Am Chem Soc 2004 126 1577-1584. 

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. 

In addition to 1 1 (host-guest) complexes with different structures, a number of complexes with different host-guest ratios exist in equilibrium when aromatic molecules are included into either a- or p-CD. For example, sodium 1-pyrenebutyrate (75)/a or /J-cyclodextrin complexes in solution form equilibrium mixtures of a 1 1 and 2 1 complexes and form 1 1 and 2 2 complexes with y-CD (Figure 39) [247]. 

Abou-Hamdan, A. Bugnon, P. Saudan, Ch. Lye, P.G. and Merbach, A.E. (2000) High-Pressure Studies as a Novel Approach in Determining Inclusion Mechanisms Thermodynamics and Kinetics of the Host-Guest Interactions for a-Cyclodextrin Complexes, J. Am. Chem. Soc. 122, 592-602. 

Fig. 4.3 Schematic representation of the association-dissolation of the host (cyclodextrin) and guest (p-xylene). The formed guest/host inclusion complex can be isolated as a microcrystalline powder. (From ref. [13])...

Complexes can also be formed using a slurry of cyclodextrin. The guest is added to the slurry and stirred. Since there are no visual indications of formation of the complex,... 

Fig. 13. Schematic representation of complex formation of cyclodextrin with guest molecule...

J. I. Seeman and H. V. Secor, Enantiomeric resolution and chiral recognition of racemic nicotine and nicotine analogues by P-cyclodextrins complexation. Structure-enantiomeric resolution relationship in host guest interaction, Anal. Chem. 60 (1988), 2120. 

Figure 5.8 Schematic representation of the dissolution-dissociation-recrystallisation process of a cyclodextrin complex with a poorly soluble guest. The complex rapidly dissolves, and a metastable oversaturated solution is obtained. The anomalously high level of dissolved guest drops bock but remains higher than the level that can be obtained with noncomplexed drug. Solid curve = complexed drug broken curve = noncomplexed drug.

Of the new Pt catalysts reported since 1990 platinum complexes with new ligands and activators are noteworthy. Cyclodextrin complexes of platinum (as host-guest complexes) have been employed as hydrosilylation catalysts active at elevated temperature after releasing the guest compound [40]. Some other organic compounds have recently been used as activators (ligands) of Pt complexes, e. g., unsaturated secondary and tertiary alcohols and silylated unsaturated alcohols [41], alkadiynes, cyclooctadiene [42], and vinylnorbomene as well as quinones and methylnaphthoquinones [43]. 

Cyclodextrin complexation also depends on a suitable molecular topology that allows the guest molecule to ht within the hydrophobic host cavity. Cyclodextrins form inclusion compounds with hydrophobic guest molecules in... 

K. Hatton, K. Takahashi, M. Uematsu, N. Sakai, Multiple interactions between host cyclodextrin and guest compound assisting asymmetrically selective reduction with NaBH4 in aqueous media, Chem. Lett., 1990, 19, 1463-1466 K. Hattori, K. Takahashi, N. Sakai, Enantioface differentiating reduction of keto-acid in the presence of 6-deoxy-6-amino-j3-cyclodextrin with NaBHi in aqueous media. Bull. Chem. Soc. Jpn., 1992,65,2690-2696 K. Hattori, K. Takahashi, Asymmetric reduction of prochiral inclusion complex in aqueous media, Supramol. Chem., 1993, 2, 209-213. 

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