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

Area of Science:

• Supramolecular chemistry
• Physical chemistry
• Chemical thermodynamics

Background:

• Cyclodextrins (CDs) are cyclic oligosaccharides widely used as host molecules in supramolecular chemistry.
• The binding of guest molecules to cyclodextrins influences their physicochemical properties.
• Understanding host-guest interactions is crucial for designing novel materials and drug delivery systems.

Purpose of the Study:

• To investigate the binding interactions between 1-adamantanecarboxylic acid (AD) and gamma-cyclodextrin (gamma-CD) using ultrasonic velocimetry and densimetry.
• To compare the binding characteristics of AD with gamma-CD to those previously reported for beta-cyclodextrin (beta-CD).
• To elucidate the nature of the forces stabilizing the AD-gamma-CD complex.

Main Methods:

• Temperature-dependent ultrasonic velocity and density measurements.
• Calculation of thermodynamic parameters including volume, expansibility, and adiabatic compressibility.
• Comparative analysis of data for AD-gamma-CD and AD-beta-CD complexes.

Main Results:

• AD forms a loose complex with gamma-CD, characterized by significant void space within the cavity.
• The AD-gamma-CD complex exhibits high compressibility, with an intrinsic compressibility coefficient of 37x10(-6) bar(-1) at 18°C, decreasing to 23x10(-6) bar(-1) at 55°C.
• In contrast, the AD-beta-CD complex is tightly bound with minimal void space.

Conclusions:

• The significant void space and high compressibility of the AD-gamma-CD complex indicate weak interactions between the guest and host.
• The AD-gamma-CD complex is predominantly stabilized by the hydrophobic effect, with minor contributions from van der Waals forces.
• The AD-beta-CD complex is stabilized by both hydrophobic effects and strong host-guest van der Waals interactions.