Solid-state cyclodextrin hydrates

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. ... 

Buvari-Barcza and co-workers [77] showed, by comparing several com-plexing agents, that y-CD is the best host for solubilising C6q fullerene in a water environment. The interaction of C6q and y-CD forms a C6o-(y-CD)2 inclusion complex. The analytical data and solid state NMR indicated that the essentially 1 2 complex exists as two different forms. In the violet colored form, unhydrated C60 is included, while in the brownish one, C q is also hydrated. The inner diameter of the y-cyclodextrin is only 0.95 nm while the diameter of C6q is estimated to be 1.0 nm. Because of this dimensional difference, complete inclusion is inconceivable, but the secondary hydroxyls of the y-CD rims can be connected by hydrogen bonds and possibly mediated by water molecules (Fig. 29). 

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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