Formation of inclusion complexes between dimers of (R)-3-hydroxybutanoic acid and β-cyclodextrin: thermodynamic study of the complexation and conformational analysis of the complexed dimers

The formation of inclusion complexes of di[(R)-3-hydroxybutanoate] (Dimer 1) and the methyl ester of di[(R)-3-hydroxybutanoate] (Dimer 2) with β-cyclodextrin (β-CD) was studied by NMR spectroscopy. The stoichiometry of the complexation was 1 ∶ 1 (host ∶ guest). Thermodynamic analysis revealed that the complex formation is enthalpically favorable, but entropically unfavorable. Dimer 1 forms hydrogen bonds with β-CD more frequently than Dimer 2 because 1 has two hydroxy groups. Conformational analysis of the 3HB (3-hydroxybutanoate) dimers in the complexes indicates that they have extended (trans) forms. In contrast, in solution without β-CD, the end of both Dimers 1 and 2 takes a sickle (gauche) shape due to formation of intermolecular hydrogen bonds.