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

4 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. 

This cluster of cycles is part of two infinite chains. One runs in b direction, 

Homodromic cycles are indicated by circulator arrows with one head, antidromic cycles by arrows with two heads. Atom designation, 26 means 0(2) of glucose number 6 

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 

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