Cyclodextrins hydrates

The structure of the included water of the crystalline cyclodextrin hydrates has been investigated by Saenger and coworkers " " using X-ray... 

Figure X-ray powder diagrams of crystalline beta-cyclodextrin-hydrate and the cinnamon oil beta-cyclodextrin complex.

Cyclodextrin hydrate was purchased from Aldrich Chemical Company, Inc. and used without further purification. 

P-Cyclodextrin hydrate p-Cyclodextrin, hydrate (10) (68168-23-0) p-Toluenesulfonyl chloride (8) Benzenesulfonyl chloride, 4-methyl- (9) (98-59-9) N,N-Dimethylformamide cancer suspect agent Formamide, N,N-dimethyl-(8,9) (68-12-2)... 

Cyclodextrin hydrate was purchased from Acros Organics, Fisher Scientific Company, and was used without further purification or drying (Rf = 0.41, 2-PrOH/H20/EtOAc/concd NH4OH 5 3 1 1). p-Cyclodextrin and the title compound were dissolved in water, spotted on silica gel 60 F254 aluminum-backed plates (EM Separations Technology), and dried on a hot plate prior to development in the solvent systems indicated. 

Figure 12. Structures of terpenoic hydrocarbons a-pinene, /3-pinene, cis-pinane, trans-pinane and 2 carene resolved on a-cyclodextrin hydrate (Koscielski et al., 1986).

The presence of water was subsequently identified as being essential for the performance of the above set-up and for maintaining high enantiose-lectivity (Lindstrom et al., 1990). The preparative-scale enantiomeric separation of racemic camphene with a separation factor as high as a — 3.7 (measured at 30°C on an analytical column) was subsequently achieved at 45°C on a 2.1 m x 4 mm (i.d.) column containing a-cyclodextrin hydrate in formamide (1 5, w/w) coated on Chromosorb W (AW, 45-60 mesh) by saturating the carrier gas (helium) with water vapour (Lindstrom et al., 1990). 

Intersecting chains form four-connected nets, observed in carbohydrate hydrates, cyclodextrin hydrates, some hydrated nucleosides, also in some hydrochlorides ... 

Cycles observed in ices, clathrate hydrates, layer hydrates, and cyclodextrin hydrates u — O-H ... 

In the cyclic systems, more commonly observed in the cyclodextrin hydrates, all three alternatives, 1, 2, 3, are expanded to correspond to the configurations 5, 6, and 7, for which the descriptors homodromic, 5, antidromic, 6, and heterodromic, 7, have been applied3 [109], which can also be used for the description of linear... 

In all instances where this disorder was actually observed by X-ray or neutron diffraction methods, X and A are hydroxyl oxygen atoms, as in the ices Ih and Ic, in certain of the high pressure ices, and in the cyclodextrin hydrates, see Parts III and IV. In ice Ih the disorder gives rise to the well-known residual entropy of 0.82 0.05 cal deg-1 [114 to 117]. 

Because in the cyclodextrin hydrates the cavity is occupied by water molecules, they can be considered as the inverse of the clathrate hydrates discussed in Part IV, Chapter 21. In these, the water molecules form the host structure and the organic molecule is the guest. 

In only a few of the many publications on X-ray studies of the cyclodextrin inclusion compounds and of the cyclodextrin hydrates have the hydrogen atoms of... 

Cooperative, Homodromic, and Antidromic Hydrogen-Bonding Patterns in the a-Cyclodextrin Hydrates... 

The cooperative, infinite chains and cycles formed by O-H 0 hydrogen bonds in the a-cyclodextrin hydrates are a characteristic structural motif [109]. As with the simpler carbohydrate crystal structures described in Part II, Chapter 13, the hydrogen bonds can be traced from donor to acceptor in the cyclodextrin hydrate crystal structures. Networks of O-H 0-H 0-H interactions are observed in which the distribution of hydrogen bonds follows patterns with two characteristic motifs. One are the "infinite chains which run through the whole crystal lattice, and the others are the loops or cyclically closed patterns (a special case of the "infinite chains). As in the small molecule hydrates, such as a-maltose monohydrate, the chains and cycles are interconnected at the water molecules to form the complex three-dimensional networks illustrated schematically in Fig. 18.5, with some sections shown in more detail in Fig. 18.7 a, b, c. 

Fig. 18.5a-c. Hydrogen-bonding structures in a-cyclodextrin hydrates, a a-Cyclodextrin 6H20, form I b a-cyclodextrin 6 0, form II c a-cyclodextrin 7.57 H20 , intramolecular. Data in a are from neutron and in b, c from X-ray analyses. In c covalent O-H bonds are normalized to 0.97 A. 

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... 

In the crystal structure of / -cyclodextrin 11H20, the hydrogen bonding is even more complicated than in the a-cyclodextrin hydrates [454]. There are not only more hydroxyl groups and water molecules in the crystal asymmetric unit, but they also display considerable positional disorder giving rise to more complex hydrogen bonding patterns. 

Three other rubredoxins have been studied in which all, or almost all, of the water corresponds to discrete peaks and appears to be ordered. However, the complexity of the hydrogen bonding linking these solvent peaks is such that, hitherto, it has not been possible to deduce a rational bonding scheme analogous to that described in Fig. 18.5 for the cyclodextrin hydrates [835 a]. 

These studies, in combination with recent crystallographic data from cyclodex-trins, proteins, and nucleic acids (see next chapter), clearly demonstrate that water of hydration around solute molecules is organized in certain recurrent patterns. The most favored motif is the pentagon, because the internal angle of 108 0 is close to the H-6-H angle in water, 105 °. The four- and six-membered rings found in clathrates and cyclodextrin hydrates, are also expected to play a role in the hydration of biological macromolecules. 

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