beta-Cyclodextrine and water: Semiempirical calculations

AM1 and PM3 calculations were carried out on beta-cyclodextrine (beta-CD) undecahydrate in the experimental conformation at 120 K. The calculated beta-CD/water interaction energies are very small and indicative for each water molecule of an unfavorable condition in respect to that of pure water. The conformationally optimized system was also studied: beta-CD appears highly symmetrical with negligible dipole moment, mainly because of the circular arrangement of the single vectors; Primary hydroxyls can easily rotate, while the secondary ones are stabilized by heteroannular hydrogen bonds and homoannular electrostatic interactions due to the consequent increase of the atomic charges. The beta-CD/water interaction energies in the optimized hydrated system are not significantly different from the experimental ones. This almost hydrophobic character is also shown by MM equilibrated solutions: all water molecules are rejected beyond 2.4 Angstrom; between 2.4 and 2.9 Angstrom highly structured water is present. From a purely enthalpic standpoint the molecule hydration appears highly improbable, thus the formation of beta-CD . 11H(2)O must involve a compensation mechanism.