Hydration of Native and Methylated Cyclodextrins with Positive and Negative Solubility Coefficients in Water: QENS Study on Hydrophobic Interactions

Quasi-elastic neutron scattering (QENS) spectra of heavy water solutions of γ-cyclodextrin (γ-CD), and of methylated derivatives of cyclodextrins, 2,6-di-O-methyl β-CD (DIMEB) and 2,3,6-tri-O-methyl-γ-CD (TRIMEG) were taken in the temperature range from 5 °C to 45 °C. The spectra were compared to a model for the scattering function S(Q, ω) describing translational and rotational dynamics of the solute, explicitly including the contributions due to the solvent D2O molecules. We found that for DIMEB and TRIMEG, in order to account for the QENS intensity in the Q range [0:0.5] Ǻ, introduction of a number of D2O molecules rather tightly bound to the surface of the respective solute molecules is required. Such D2O molecules lead to an increase of the coherent scattering by the solute molecule. This increase manifests itself already when comparing the values of the QENS integrals over energy transfer for both native and methylated cyclodextrins. The number NW of tightly bound water molecules was determined and found to be around 35 per TRIMEG molecule, and around 60 per DIMEB molecule.