Gamma-cyclodextrin forms a highly compressible complex with 1-adamantanecarboxylic acid.

We report temperature-dependent ultrasonic velocimetric and densimetric data on changes in volume, expansibility, and adiabatic compressibility associated with the binding of 1-adamantanecarboxylic acid (AD) to gamma-cyclodextrin (gamma-CD). We compare these results with our previous data on the binding of AD to beta-cyclodextrin (beta-CD) [Taulier, N.; Chalikian, T.V.J. Phys. Chem. B 2006, 110, 12222-12224]. The comparison reveals that, in contrast to the tight AD-beta-CD complex with little void space left inside the cavity, AD forms a loose complex with gamma-CD with approximately 30 A3 of void space between the guest molecule and the inner walls of the cavity. The presence of the void renders the AD-gamma-CD complex highly compressible; the intrinsic coefficient of compressibility of the AD-gamma-CD complex is 37x10(-6) bar(-1) at 18 degrees C and decreases to 23x10(-6) bar(-1) at 55 degrees C. Such large compressibility is suggestive of only weak contacts between the interacting AD and gamma-CD atomic groups in the cavity. Our results are consistent with the notion that the AD-gamma-CD complex is predominantly stabilized by the hydrophobic effect with only modest contribution from intermolecular van der Waals interactions. This notion is in contrast to the AD-beta-CD complex which is stabilized by strong host-guest van der Waals interactions in addition to the hydrophobic effect.