Determination of microscopic equilibrium constants for the complexation of ditopic guests by cyclodextrins from NMR experiments

The complexation of 3-[(3-cholamidopropyi)dimethylammoniol-1-propanesulfonate (CHAPS) by beta- and gamma-cyclodextrin (beta- and gamma-CD) has been studied by using 1D- and 2D-NMR techniques. For the CHAPS/betaCD system both C-13 and H-1 NMR spectra exhibit split lines consistent with slow exchange rate between one or more complexes and free species. Job's plots suggest that complexes with 1: 1 stoichiometry are formed. However, ROESY experiments suggest that P-CD is complexing the steroid nucleus at two sites, the first one being mainly the side chain of the guest, and the second one being the A/B rings. The conclusion is that two isomeric 1:1 complexes, C-11(Y) and C-11(X) are obtained. The concentrations of free and complexed species were measured from the integration of H-1 NMR signals, allowing an independent determination of the two microscopic equilibrium constants, K-11(Y) and K-11(X), associated with the formation of the two isomeric X Y complexes, and the macroscopic equilibrium constant, K-11 (= K-11(X) + K-11(Y)). A good agreement between the values obtained is observed. The slow exchange rate for the formation and decomposition of the C-11(Y), complex is understood as a barrier penetration problem for the side chain to cross through the cyclodextrin cavity since the methyl groups linked to the charged nitrogen atom of the side chain are bulky enough to force a large distortion of the P-CD during the penetration. However, the origin of the energy barrier for the complexation of the wide end of the steroid nucleus leading to a slow exchange regime is unknown. The travelling of the CHAPS side chain through a larger cyclodextrin cavity should be faster. Therefore, complexation studies between CHAPS and gamma-CD were undertaken. Now, only one type of complex was observed and the fast limited exchange was obeyed. The value for the equilibrium formation constant was obtained from the analysis of the chemical shift displacements of carbon atoms of gamma-CD with increasing CHAPS concentration. ROESY experiments suggest that the side chain is unfolded outside the cyclodextrin cavity forming a rotaxane-type structure.