Circular hydrogen bonds

Abstract

CIRCULAR hydrogen bonds present a new, experimentally demonstrated principle showing how hydration water molecules and hydroxyl groups of macromolecules can cooperate to form a network-like pattern. Quantum chemical calculations show that chain-like H-bonds in the crystal lattice¹ are energetically favoured above individual ones^2,3. This is due to the cooperative effect, which leads to increased H-bonding activity of an OH-group if it is already accepting or donating an H-bond. These linear structures can close up to form circular arrangements comprising four and more OH-groups^4–6. Such circles have actually been described for crystal structures of ice⁷ and of ice clathrates⁸, but in these cases the water molecules within the circles are related by crystallographic symmetry elements and are therefore not independent of each other. One should expect to find lattice-independent circular H-bonds in crystal structures of large O—H-rich molecules which co-crystallise with water of hydration, conditions which are satisfied by the cyclodextrin family. The smallest member, α-cyclodextrin (α-CD; cyclohexaamylose), consists of six α(1, 4)-linked glucose molecules and contains six primary and 12 secondary hydroxyl groups ((C₆H₁₀O₅)₆, molecular weight 973). From aqueous solution, α-CD crystallises as hexahydrate, α-CD·6H₂O, and this complex, with a total of 120 hydroxyl groups (4 × 18 from α-CD and 4 × 12 from the six H₂O) in one unit cell has been studied by X-ray and neutron diffraction methods (ref. 9, and Klar, Hingerty and W.S., unpublished). Two other complexes, a second modification of the hexahydrate (K. Lindner and W.S., unpublished) and α-CD·methanol·4H₂O (ref. 10) have been investigated by X rays. As refinement in all cases is around R = 4% for data extending to 0.89 Å resolution, hydrogen atom positions could be assigned. I discuss here results obtained from the X-ray/neutron study of α-CD·6H₂O.