Cooperativity and steric hindrance: Important factors in the binding of α-cyclodextrin with para-substituted aryl alkyl sulfides, sulfoxides and sulfones

Binding constants for 22 para-substituted aryl alkyl sulfides, sulfoxides and sulfones have been determined spectrophotometrically. It was found that the presence of sulfur containing substituents generally results in destabilisation of complex formation, and it is postulated that the angle of the sulfur bond in these compounds results in steric hindrance with the 5-H protons at the rear of the cyclodextrin cavity, causing it to be displaced from its optimal position or orientation. Additionally, the observation, for several sulfides, of cooperativity in the binding of a second molecule of cyclodextrin is discussed in terms of binding induced changes in electrical potential within the cyclodextrin cavity.