Host-guest complexes cyclodextrins

P-Cyclodextrin is assumed to form host-guest complexes with diazonium ions (Fukunishi et al., 1982, 1985), but if so, complexation increases the extent of thermal dediazoniation, i. e., it has the contrary effect to that usually seen. 

There are three main types of CDs a-cyclodextrin (a-CD), -cyclodexlrin (p-CD), and y-cyclodextrin (y-CD), which are macrocycles formed by six, seven, and eight sugar ring molecules, respectively. The spatial structure of p-CD is shown on Fig. 3. Review [19] generalizes data on the synthesis, modification, physicochemical and theoretical investigations of CDs, and certain applications particularly for enantio-separation and pharmaceutical applications. CDs are able to form host-guest complexes (pseudorotaxanes) with hydrophobic molecules such as aza-dyes... 

R Kuhn, F Stoeklin, F Emi. Chiral separations by host—guest complexation with cyclodextrin and crown ether in capillary zone electrophoresis. Chromatographia 33 32—36, 1992. 

Very recently a new method was developed that opens the possibility to polymerize even hydrophobic monomers in aqueous solution. This method is based on the finding that hydrophobic monomers can be made water-soluble by incorporation in the cavities of cyclodextrins. It has to be mentioned that no covalent bonds are formed by the interaction of the cyclodextrin host and the water-insoluble guest molecule. Obviously only hydrogen bonds or hydrophobic interactions are responsible for the spontaneous formation and the stability of these host-guest complexes. X-ray diffraction pattern support this hypothesis. Radical polymerization then occurs via these host-guest complexes using water-soluble initiators. Only after a few percent conversion the homogeneous solution becomes turbid and the polymer precipitates. 

Variations of this method are possible in several ways. First of all, cyclodextrin which is available on a large scale by enzymatically catalyzed modification of starch can be tailored by chemical reactions. Furthermore, copolymerizations between different host-guest complexes are possible whereby in some cases the reactivity ratios differ from those reported in literature. 

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. 

However, for the positional isomers of phthalate (56-58), the response selectivity was different for the two types of membranes. Whereas membranes 1 and 2 showed responses in the order of 56 (ortho) > 57 (meta) > 58 (para), membrane 53 interestingly showed a different response order, i.e., 57 (meta) > 58 (para) > 56 (ortho), a selectivity which is quite different from that expected on the basis of simple electrostatic effects. Such a difference in the selectivity is possibly due to host-guest complexation involving not only electrostatic interactions but also inclusion into the P-cyclodextrin cavity, which is capable of recognizing differences in the steric structures of the guests. 

Zhang, X., Gramlich, G., Wang, X. and Nau, W.M. (2002) A joint structural, kinetic, and thermodynamic investigation of substituent effects on host-guest complexation of bicyclic azoalkanes by -cyclodextrin./. Am. Chem. Soc., 124 (2), 254-263. 

The host-guest p-cyclodextrin-C522 complex formation was determined based on fluorescence blue shift as a function of the increasing p-cyclodextrin concentration from 10 6 to 10 2 M. Similar result was observed for coumarin C6 [4] and this blue shift was considered along with anisotropy results as a proof of the host-guest formation. Time-resolved fluorescence spectroscopy was utilized to differentiate between fluorescence dynamics of... 

Time-resolved fluorescence of coumarin C522 was determined in water and in host-guest complex with p-cyclodextrin, representing free aqueous and cavity restricted environments, respectively. Experimental fluorescence clearly showed faster dynamics in a case of water. The time parameters of monoexponential fit for water and p-cyclodextrin at 500 nm and 520 nm were determined to be 1.37 ps and 2.02 ps, and 2.97 ps and 7.14 ps, respectively. Multi-mode Brownian oscillator model, as an attempt to simulate the solvation dynamics, supported these fluorescence dynamics results. 

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]. 

J and Yabe et al. [155J have formed LB films of amphiphilic derivatives of jS-cyclodextrin and have incorporated azobenzene derivatives into them in such a way as to form a host-guest complex. Many workers have used the cis to trans change of structure referred to above and brought about by ultraviolet irradiation to change some measurable physical parameter of LB films formed from azobenzene derivatives [156-62J. 

Interest in biological mimics probably started in the late 1800s with the discovery of cyclodextrin (CD), obtained from the starch digest of a strain of Bacillus. The realization that CDs could form host-guest complexes with a variety of small molecules, and the myriad of uses implied therein, led to extensive study in both academics and industry. 

Figure 5. Enantiodifferentiation of camphor by host-guest complexation with a-cyclodextrin. Left The temperature dependence in light water Right comparison of the differences in the enantiodifferentiation in light versus heavy water.

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