An ESR study of pressure effects on the inclusion-complex formation of cyclodextrins with Di-tert-butyl nitroxide

The volume change upon the inclusion-complex formation of cyclodextrins (β,γ-CD's) with di-tert-butyl nitroxide (DTBN) has been studied by a high-pressure ESR technique. Anisotropic motion of DTBN in γ-CD suggested a loose inclusion of DTBN in the γ-CD cavity. The equilibrium constants (K's) for the inclusion-complex formation of CD's with DTBN decreased with increasing external pressure. From the pressure dependence of K, the reaction volumes were estimated to be 5 ± 1 cm³ mol^-1 for the DTBN/β-CD system and 20 ± 2 cm³ mol^-1 for the DTBN/γ-CD system, respectively. The difference has been ascribed to the difference in the number of water molecules excluded from the CD cavity upon inclusion.