Encapsulation of Small Ionic Molecules within alpha-Cyclodextrins

JAVIER RODRIGUEZ; M. DOLORES ELOLA

JOURNAL OF PHYSICAL CHEMISTRY B

AMER CHEMICAL SOC

Lugar: NW, WASHINGTON, DC 20036 USA; Año: 2009 vol. 113 p. 1423 - 1428

1089-5647

Results from  molecular dynamics experiments pertaining to the encapsulation of ClO$_4^-$ within the hydrophobic cavity of an aqueous $alpha-$cyclodextrin ($alpha-$CD) are presented.  Using a biased sampling procedure, we constructed the Gibbs free energy profile associated with the complexation process.  The profile presents a global minimum at the vicinity of the primary hydroxyl groups, where the ion remains tightly coordinated to four water molecules via hydrogen bonds.  Our estimate for the global free energy of encapsulation yields $Delta G_mathrm{enc}sim -2.5$~$k_BT$. The decomposition of the average forces acting on the trapped ion reveals that the encapsulation is controlled by Coulomb interactions between the ion and OH groups in the CD, with a much smaller contribution from the solvent molecules.  Changes in  the previous results, arising from the partialmethylation of the host CD and modifications in the charge distribution of the guest molecule are also discussed.  The global picture that emerges from our results suggests that the stability of the $mathrm{ClO_4^-}$ encapsulation not only involves the individual ion, but its first solvation shell as well.