Cyclomalto-oligosaccharides (cyclodextrins, CDs) are cyclic oligosaccharides composed of at least six (1+4)-linked cY-D-glucopyranosyl residues, which can form inclusion complexes with various types of compounds without any covalent bonds being formed'-'j. Van der Waals forces, hydrogen bonding, and hydrophobic interaction have been proposed as driving forces for the formation of the inclusion com-plexes1v2. The van der Waals interaction includes both dipole-dipole interaction and London dispersion force, and the energetics7q8 and thermodynamics9-l1 of the formation of CD complexes have been discussed. For dipole+lipole interactions to contribute to the formation of the inclusion complexes, the CD molecule must have a sizable permanent or induced dipole moment along the axis of its cavity, but no specific value has been reported. We now report the confirmation by MO calculation that cyclomaltohexaose (cY-cyclodextrin, a-CD) has a large dipole moment and that, for some CD-guest systems, the host-guest dipole+lipole interaction determines the orientation of the guests in the complexes.
The calculations were based on p-nitrophenol (PNP), benzoic acid (BA), and p-hydroxybenzoic acid (PHBA) as the guests, but details are given only for the a+-CD-PNP system. The dipole moments of (r-CD and PNP were calculated separately by the CNDO/Z MO method12, using the geometrical values reported for the a-CD-PNP inclusion-complex determined by X-ray diffraction13. In the crystal, the p-nitrophenyl group of PNP is located in the cavity of a-CD, and the phenolic hydroxyl group protrudes from the side of the secondary hydroxyl groups of a-CD (Fig. 1). The geometry in the crystalline state is the same as that in solu-tion14J5.