A molecular orbital study of cyclodextrin (cyclomalto-oligosaccharide) inclusion complexes. III, dipole moments of cyclodextrins in various types of inclusion complex

Abstract

Large values (10–20 D) of the dipole moments were calculated for cyclomalto-hexaose, -heptaose, and -octaose (αCD, βCD, and γCD) and also two types of methylated CD by means of the CNDO/2 MO method, using the X-ray structures of various inclusion complexes. The relative strengths of the dipole moments were αCD < βCD < γCD and TM-αCD < αCD. These properties are explained in terms of integration of the partial dipole moment attributed to each glucosyl residue and are a typical example of the so-called “macrocyclic effects”. The dipole moment of each CD was directed from the secondary to the primary hydroxyl side. In the inclusion complexes of αCD with some aromatic guest molecules, the guest-dipole moment is antiparallel to that of the host. The dipole moment of the CD strongly influences the orientation of polar guest molecules, and binding of guest molecules distorts the CD and causes large variations in its dipole moment. The overall electrical field of CDs is expressed significantly within the cavity.