Thermodynamics of inclusion complexes of natural and modified cyclodextrins with propranolol in aqueous solution at 298 K

The association constant, standard Gibbs energy, enthalpy and entropy for formation of inclusion complexes of propranolol, a β-blocker, with various natural and modified cyclodextrins have been determined by calorimetry at 298 K. Both natural and methyl-modified α-cyclodextrins do not form complexes, while β- and γ-cyclodextrins do. Complexing ability of 2-hydroxypropyl-β-cyclodextrin depends on the average substitution degree. For γ-cyclodextrin, hydrophobic interactions play the major role in binding the guest. The association of natural and modified β-cyclodextrins is ruled by van der Waals interactions and hydrogen bonding because of the tighter fit of the guest into the cavity. Decreasing pH determines increasingly negative values of the association enthalpies.

Cyclodextrins, cyclic oligosaccharides, present a prevailingly hydrophobic cavity. They can form inclusion complexes having 1:1 stoichiometry. The length of the alkyl chain of the guest and the size of the macrocycle mostly determine the kind of forces ruling the association process.Figure optionsDownload as PowerPoint slide