The extent of conformational rigidity determines hydration in nonaromatic hexacyclic systems.

We conducted an ultrasonic study of the hydration number for hexacyclic systems. We find from these experiments that cyclohexane-based molecules such as cyclohexanol and myo-inositol show a very small increase in hydration number despite the large difference in the number of hydroxyl groups present in each of the molecules. There is however a dramatic increase in hydration number when shifting from molecules with a cyclohexane frame to molecules with a cyclopyranose frame particularly glucose. An analysis of classical and quantum molecular dynamics simulation trajectories reveal that the hydration number is strongly linked to the conformational flexibility within the molecule. Cyclopyranose is a more rigid ring system compared with cyclohexane and so its ring fluctuates in a smaller range and frequency. The effect of the ring rigidity is that the hydroxyls tethered to the cyclopyranose ring undergo less positional diffusion compared with those attached to the cyclohexane ring. This allows for long time intermolecular hydrogen bonds between the hydroxyls bonded to cyclopyranose rings and the surrounding waters, which leads to an increase in the hydration numbers of carbohydrates compared with those of hydroxylated cyclohexanes.
(© 2011 American Chemical Society)