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Published on: March 4, 2017
Heat-capacity changes in host-guest complexation by Coulomb interactions in aqueous solution
Koji Kano1, Yoshiyuki Ishida, Kohei Kitagawa
1Department of Molecular Science and Technology, Faculty of Engineering, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan. kkano@mail.doshisha.ac.jp
Chemistry, an Asian Journal
|August 3, 2007
Summary
Heat capacity changes reveal insights into cyclodextrin complexation. Smaller guests fitting cyclodextrin cavities show negative heat capacity changes, while smaller guests interacting via Coulomb forces exhibit positive changes.
Area of Science:
- Supramolecular Chemistry
- Physical Chemistry
- Biophysical Chemistry
Background:
- Cyclodextrins (CDs) are widely used in host-guest complexation.
- Protonated amino-CDs offer unique binding properties due to charge interactions.
- Understanding thermodynamic parameters like heat capacity is crucial for characterizing binding events.
Purpose of the Study:
- To investigate heat-capacity changes (ΔCp⁰) during the complexation of 1-alkanecarboxylates with protonated alpha- and beta-cyclodextrins.
- To correlate ΔCp⁰ with binding constants (K) and elucidate the driving forces behind complexation.
- To apply these findings to the electrostatic binding of proteins with anionic ligands.
Main Methods:
- Isothermal titration calorimetry (ITC) was used to determine heat-capacity changes.
- Binding constants (K) were measured for various guest molecules with modified cyclodextrins.
- Thermodynamic parameters (ΔH⁰, ΔS⁰, ΔCp⁰) were analyzed in relation to guest size and binding interactions.
Main Results:
- ΔCp⁰ decreased with increasing binding constant (K), plateauing at 4000 M⁻¹.
- Complexes with well-fitting guests (e.g., pentanoate, hexanoate, heptanoate with per-NH₃⁺-α-CD) showed negative ΔCp⁰, attributed to van der Waals forces and dehydration.
- Complexes with small guests and large cavities (Coulombic interactions) exhibited positive ΔCp⁰, indicating endothermic processes and entropy-driven binding.
Conclusions:
- The sign and magnitude of ΔCp⁰ provide insights into the binding mechanism and the nature of interactions (van der Waals vs. Coulombic).
- Negative ΔCp⁰ is associated with hydrophobic dehydration and guest-cavity fit.
- Positive ΔCp⁰ is linked to Coulombic interactions, dehydration of charged groups, and water structuring, relevant for protein-ligand binding.
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