Neutron Diffraction Analysis of γ-Cyclodextrin: A Hydrogen-Bond Study

Abstract

As part of a long-term project on the nature of the hydrogen bond, we have studied various cyclodextrin complexes. Cyclodex-trins have been studied as model enzyme systems (W. Saenger et al., Bioorg. Chem. 5:187–95, 1976). The hydrogen-bonding systems of both α-cyclodextrin (α-CD; cyclohexaamylose) (B. Klar et al., Acta Crystallogr. B36:1154–65, 1980) and β-cyclodextrin (β-CD; cycloheptaamylose) (W. Saenger et al., Nature 296:581–3, 1982) have been studied by neutron diffraction. The α-CD hydrate structure is crystallized with a well-ordered H-bond network displaying circles of hydrogen bonds. (W. Saenger, Nature 278:343–4, 1979). The β-CD hydrate is a partially disordered system, displaying an unusual “flip-flop” cooperative hydrogen-bond network (op. cit.). γ-Cyclodextrin, with a partially disordered system similar to β-CD, is now being studied to see whether “flip-flop” hydrogen bonding also occurs in this case. Coordinates from the x-ray structure of γ-CD·17H₂O were supplied by J.M. MacLennan and J.J. Stezowski (Biochem. Biophys. Res. Commun. 92:926–32, 1980). Neutron diffraction data were collected at the Oak Ridge High Flux Isotope Reactor (HFIR). The Chemistry Division’s four-circle diffractometer with a monochromatized wavelength of 1.015 Å. was used for the data collection at room temperature. The crystal used had a space group of P2₁, with a. 20.287(10) Å, b = 11.079(7) Å, c = 16.858(12) Å, and β = 105.07(4) Å. An experimental absorption correction was applied. Using just the C, O, and H atoms of γ-CD and excluding all water and hydroxyl hydrogens gave an R-factor of 0.244 for 3509 reflections for which I > σ. A number of additional H-atoms have been added, and the current R-factor is 0.121. (Research sponsored by the U.S. Department of Energy, under contract W-7405-ENG-26 with the Union Carbide Corp.)

supported by GLCA-AMC Oak Ridge Science Semester Program