Crystal Packing Patterns of Cyclodextrins Are Determined by Hydrogen Bonding

In contrast to the periphery of the cyclodextrins, the internal cavities, with diameters of 5 to 8 A (Ihble 18.1), have hydrophobic character because they are lined by the methylene C-H groups and by the ether-like 0(4) and 0(5) oxygen atoms. The distribution of hydrophilic and hydrophobic surfaces, together with the annular shapes of the cyclodextrins, gives rise to the microheterogeneous environment [555] which is the reason for some of their most interesting properties (Box 18.1). 

2 Crystal Packing Patterns of Cyclodextrins Are Determined by Hydrogen Bonding 

The cyclodextrins crystallize in patterns which form cages and channels illustrated in Fig. 18.2 [558, 5651. These patterns can be classified into the two cages of types A and B, where the cavity of each cyclodextrin is closed on both ends by adjacent molecules, and the channel type C, where cavities of adjacent cyclodextrins merge into infinite tube-like cavities. 

In cage-type A packing motifs, cyclodextrin molecules are arranged cross-wise in a herringbone mode. This packing is found for a-, / -, and y-cyclodextrins if 

2 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. 

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