6-cyclodextrin-water-inclusion complex, crystal

ABSTRACT. Smooth and stable cyclodextrin membranes are successfully prepared from the oriented-cyclodextrin-polymers, by drying layers of the polymers. The polymers are obtained by the reaction of the crystal of 3-cyclodextrin-water inclusion complex with hexamethylene diisocyanate as cross-linking agent in anisole. The cyclodextrin membranes exhibit selective permeation of various materials in water. 

Fujiwara T, Yamazaki M, Tbmiza Y, Ibkuoka R, Tomita K-I, Matsuo T, Suga H, Saenger W (1983) The crystal structure of a new form of yS-cyclodextrin water inclusion compound and thermal properties of /teyclodextrin inclusion complexes. Nippon Kagaku Kaishi 2 181-187... 

An important advantage of the inclusion complexes of the cyclodextrins over those of other host compounds, particularly in regard to their use as models of enzyme-substrate complexes, is their ability to be formed in aqueous solution. In the case of clathrates, gas hydrates, and the inclusion complexes of such hosts as urea and deoxycholic acid, the cavity in which the guest molecule is situated is formed by the crystal lattice of the host. Thus, these inclusion complexes disintegrate when the crystal is dissolved. The cavity of the cyclodextrins, however, is a property of the size and shape of the molecule and hence it persists in solution. In fact, there is evidence that suggests that the ability of the cyclodextrins to form inclusion complexes is dependent on the presence of water. Once an inclusion complex has formed in solution, it can be crystallized however, in the solid state, additional cavities appear in the lattice, as in the case of the hosts previously mentioned, which enable the inclusion of further guest molecules. ... 

Chacko KK, Saenger W (1981) Tbpography of cyclodextrin inclusion complexes, 15. Crystal and molecular structure of the cydohexaamylose 7.57 water complex. Form III. Four and six-membered circular hydrogen bonds. J Am Chem Soc 103 1708-1715... 

A SCSC [2+2] photodimerziation involving 7-hydroxy-4-methylcoumarin has been shown to occur in the cavity of (3-cyclodextrin, as reported by Stezowski and co-workers (Fig. 2.3.6) [53]. The SCSC reaction was facilitated by the inclusion of both reactants within a single cyclodextrin cavity. In that way, the photodimerization produced very small changes in the overall shape of the inclusion complex. In addition, water molecules of crystallization were shown to afford an environment favorable for the SCSC transformation by providing a way to relieve the strain induced by the reaction. 

The crystallinity of the PHB block in the copolymers increases as compared with that of the pure PHB precursor, presumably being caused by the presence of the soft PEG block. In contrast, the crystallinity of the PEG block in the copolymers decreases as compared with that of the PEG precursor because of the presence of a hard PHB block, which restricted the crystallization of the PEG blocks. Triblock copolymers with lower PHB contents such as EHE(20-05-20), EHE(50-08-50), and EHE(50-38-50) are water-soluble (the numbers in parentheses show the indicative molecular mass of the respective block in 100 gmoL ). Amphiphilic triblock copolymers can self-assemble to form micelles in an aqueous medium, and these micelles contain dense cores of the insoluble blocks, surrounded by diffuse outer shells formed by the soluble blocks. Li et al. (2005a, b) prepared a series of water-soluble EHE triblock copolymers and studied the micelle formation of the copolymers. Inclusion complexes of biodegradable amphiphilic EHE triblock copolymers with a-cyclodextrin or y-cyclodextrin were prepared from an aqueous medium. The formation of inclusion complexes led to an increase in the thermal stability of both cyclodextrins and the triblock copolymers (Li et al. 2003b). 

In this paragraph, X-ray crystallographic results on a-cyclodextrin inclusion complexes will be presented. In order to obtain crystals suitable for X-ray investigations, a-cyclodextrin and the guest compounds were dissolved in hot water near saturation in the ratio 1 5 to 1 10 and the solutions allowed to cool down slowly [10, 11]. In case of water-insoluble guest compounds, biphasic systems were used with a-cyclodextrin dissolved in water and the guest compounds dissolved in ether or some other solvent. 

Cyclodextrins (CDs) have recently found use as stationary phases in gas-solid chromatography (GSC) [1-8, 12-14] and in gas-liquid chromatography (GLC) [8-11], because of their selective separation capability. Their application to separations of stereoisomers (alkenes, pinenes) and positional isomers of aromatics (xylenes, trimethylbenzenes) has been found to be very advantageous. The inclusion process, which underlies selective separations, is, with cyclodextrins, also affected by the presence of water. It is well known that cyclodextrins form crystal hydrates and that the water of crystallization participates in the formation of inclusion complexes [15]. On the formation of an inclusion complex, the water molecules included in the CD cavity are liberated preferentially. This liberation is further enhanced under the dynamic conditions of gas chromatography. It can thus be assumed that water also plays an important role in the equilibrium processes between CD and a guest (sor-bate) in the gaseous state. 

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