NMR spectroscopic studies on complex formation between dimeric (R)-3-hydroxybutanoic acid and β-cyclodextrin

Dimeric (R)-3-hydroxybutanoic acid (3HB) was found to form an inclusion complex selectively with beta-cyclodextrin (beta-CD) in aqueous solution. The complex formation was confirmed by the changes of H-1 and C-13 NMR spectra of both dimeric 3HB and beta-CD in D2O solution. The continuous variation experiments showed the formation of a 1:1 inclusion complex. The dissociation constants at various temperatures between 15 and 47 degrees C were determined and the thermodynamic analysis of the complex formation was performed. The results indicated that the complex formation is enthalpically favorable but entropically unfavorable. We propose that the 3HB unit with carboxy end-group is located in the narrow side of the beta-CD cavity.