STRUCTURAL AND DYNAMIC STUDIES OF THE HYDRATE, EXCHANGEABLE HYDROGENS, AND INCLUDED MOLECULES IN BETA-CYCLODEXTRINS AND GAMMA-CYCLODEXTRINS BY POWDER AND SINGLE-CRYSTAL DEUTERIUM MAGNETIC-RESONANCE

To study the degree and time scales of dynamic disorder processes of the hydrate and cavity included molecules in a biomolecule of substantial structural complexity, we have obtained deuterium magnetic resonance spectra of powder and single-crystal samples of beta- and gamma-cyclodextrins crystallized from deuterated water or solutions containing deuterated molecules. With single-crystal samples, results can be compared directly with the structures obtained by X-ray or neutron diffraction. At room temperature and above, we find that the hydrate, sugar hydroxyl groups, and included molecules (benzyl or ethyl alcohol) are dynamically disordered to varying degrees with reorientational rates that are fast on the deuterium NMR time scale, i.e., rate > 10(6) s-1. An unexpected finding is that the 11 lattice waters per beta-cyclodextrin molecule, which are determined by neutron diffraction to occupy 16 positions, 1 give rise to a single exchange averaged doublet in the NMR experiment showing that they freely move among these sites in a time less than 10(-6) s. As a consequence of such exchange averaging, the orientation of the residual quadrupole coupling tensor for the water deuterons is expected to reflect the lattice symmetry (P2(1)) rather than the orientation of the water molecules at individual sites, and this property is experimentally verified. Rapid exchange, which is absent in the less hydrated alpha-cyclodextrin, is also observed for the more hydrated gamma-cyclodextrin. In addition, these results show that included molecules are orientationally disordered to a greater degree than previously thought. These dynamic features are considered in terms of the known esterase activity of cyclodextrins.