High static pressure was applied to the inclusion complex of a modified β-cyclodextrin, heptakis(2,6-di-O-methyl)-β-cyclodextrin (DM-β-CD), and the influence of O-methylation on the inclusion equilibrium was investigated. Guest molecules to be included by cyclodextrin were stable nitroxide radicals, α-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-β-CD has been shown to be significantly altered as compared with unmodified β-CD. Thus, we investigated group-inclusion equilibria in DM-β-CD under high static pressure by employing high pressure ESR spectroscopy, and compared the data with that for unmodified β-CD. The reaction volume (δV) of the inclusion equilibrium, a unique parameter which is calculated only from pressure dependence data, showed a drastic shift in some DM-β-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.
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