Group-Inclusion Complex and Its External High Pressure Effect in O-Methylated beta-Cyclodextrin as Compared with Unmodified beta-Cyclodextrin

摘要

High static pressure was applied to the inclusion complex of a modified beta-cyclodextrin, heptakis(2,6-di-O-mefhyl)-beta-cyclodextrin (DM-beta-CD), and the influence of O-methylation on the inclusion equilibrium was investigated. Guest molecules to be included by cyclodextrin were stable nitroxide radicals, alpha-substituted 2,4,6-trimethoxybenzyl(t-butyl)-nitroxides. Each guest molecule had more than one bulky group that can form ESR-spectroscopically separable isomeric inclusion complexes. Previously, using selected nitroxide probes, group-inclusion equilibrium in DM-beta-CD has been shown to be significantly altered as compared with unmodified beta-CD. Thus, we investigated group-inclusion equilibria in DM-beta-CD under high static pressure by employing high pressure ESR spectroscopy, and compared the data with that for unmodified beta-CD. The reaction volume (DELTA V) of the inclusion equilibrium, a unique parameter which is calculated only from pressure dependence data, showed a drastic shift in some DM-beta-CD group-in complexes, such as cyclohexyl-and s-butyl-in complex. We concluded that the O-methylation mediated changes in the hydrophobic void space and the number of residual water molecules in the CD cavity are causal factors for the difference.