Neutron diffraction of alpha, beta and gamma cyclodextrins: hydrogen bonding patterns

Cyclodextrins (CD's) have proved useful as model systems for the study of hydrogen bonding. They are torus-shaped molecules composed of six(alpha), seven(beta) or eight(gamma) (1----4) linked glucoses. Because of their particular geometry, they are able to act as a "host" to form inclusion complexes with "guest" molecules very much like enzymes. Cyclodextrins have been shown to exert catalytic activity on suitable included-substrate molecules; they catalyze the hydrolysis of phenylacetates, of organic pyrophosphates and of penicillin derivatives. They also accelerate aromatic chlorinations and diazo coupling by means of their primary and/or secondary hydroxyl groups, so that the rates of hydrolysis are enhanced by up to a factor of 400. In order to understand the hydrogen bonding in these enzyme models, neutron diffraction data were collected to unambiguously determine the hydrogen atom positions, which could not be done from the x-ray diffraction data. alpha-CD has been shown to have two different structures with well-defined hydrogen bonds, one "tense" and the other "relaxed". An "induced-fit"-like mechanism for alpha-CD complex formation has been proposed. Circular hydrogen bond networks have also been found for alpha-CD due to the energetically favored cooperative effect. beta-CD with a disordered water structure possesses an unusual flip-flop hydrogen bonding system of the type O-H...H-O representing an equilibrium between two states: O-H...O in equilibrium O...H-O. gamma-CD with a disordered water structure similar to beta-CD also possesses the flip-flop hydrogen bond. This study demonstrates that hydrogen bonds are operative in disordered systems and display dynamics even in the solid state.