Atom-atom potential analysis of the complexing characteristics of cyclodextrins (host) with benzene and p-dihalobenzenes (guest)

AM1 calculations on inclusion complexes of cyclomaltoheptaose (beta-cyclodextrin) with 1,7-dioxaspiro[5.5]undecane and nonanal, and comparison with experimental results

Semiempirical calculations on cyclomaltoheptaose (beta CD), 1,7-dioxaspiro[5.5]undecane (1), nonanal (2), and the inclusion complexes of beta CD with 1 and 2 were carried out using the AM1 method. The structure of beta CD after complete geometry optimization was in very good agreement with crystallographic structures. Moreover, the calculated dipole moment of beta CD was found to depend strongly upon the orientation of the primary hydroxyl groups. Different possible positions of the guest molecules in the beta CD cavity were examined, a few of them resulting in a gain of energy. These corresponded to partial inclusion of 1 from the secondary side but total inclusion of 2. Conclusions regarding the geometries of the complexes were in satisfactory agreement with the dominant structures in aqueous solutions as derived from NMR experiments. Thermodynamic data (delta H(O), delta S(O)) in aqueous solutions were obtained from van't Hoff plots using 1H NMR, and were compared with the computed heats of formation. The forces responsible for host-guest association are discussed in the light of the above results.

Electrospray ionization mass spectrometric study of encapsulation of amino acids by cyclodextrins

Electrospray ionization (ESI) mass spectrometry has been used to study inclusion (host-guest) complexes of cyclodextrins (CDs) with amino acids. Host-guest complexes formed in solution are stable for characterization by ESI mass spectrometry: The relative abundances and the stoichiometry of the complexes formed in solution can, thus, be determined in the gas phase. The studies verified that β- and γ-cyclodextrin better accommodate protonated amino acids than α-cyclodextrin, and that chemically modified cyclodextrins such as heptakis(2,6-di-O-methyl)-β-cyclodextrin (DM-β-CD) may show profound improvement in complexation. The preferential formation of DM-β-CD-aromatic amino acid over DM-β-CD-aliphatic amino acid complexes is confirmed by the experiments, and the relative gas-phase stabilities determined by repeller-collimator collision-induced dissociation show an identical trend to the complexation in solution. Although molecular mechanics studies also may predict the encapsulation preference of protonated amino acids by cyclodextrins, only small differences in the total complexation energies are obtained because of the inability of the calculations to consider hydrophobic interactions. An experimental approach based on ESI mass spectrometry is, therefore, more reliable in predicting host-guest interactions that involve cyclodextrins and amino acids than the theoretical calculations that employ molecular mechanics models.