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Transport Properties of Ibuprofen Encapsulated in Cyclodextrin Nanosponge Hydrogels: A Proton HR-MAS NMR Spectroscopy Study
Published on: August 15, 2016
Deprotonation of beta-cyclodextrin in alkaline solutions.
Ernestas Gaidamauskas1, Eugenijus Norkus, Eugenijus Butkus
1Department of Chemistry, Colorado State University, Fort Collins, CO 80523-1872, USA.
Carbohydrate Research
|December 17, 2008
Summary
Beta-cyclodextrin (beta-CD) remains undeprotonated until pH 12.0. Alkaline conditions above this pH trigger deprotonation, primarily at C-2 and C-3 hydroxyl groups, with a pKa of 13.5.
Area of Science:
- Supramolecular Chemistry
- Carbohydrate Chemistry
- Analytical Chemistry
Background:
- Beta-cyclodextrin (beta-CD) is a cyclic oligosaccharide widely used in various applications due to its ability to form inclusion complexes.
- Understanding the behavior of beta-CD in solution, particularly its ionization state under different pH conditions, is crucial for optimizing its use.
- Previous studies have explored beta-CD's properties, but detailed spectroscopic analysis in highly alkaline aqueous solutions is less common.
Purpose of the Study:
- To investigate the deprotonation behavior of beta-cyclodextrin in alkaline aqueous solutions.
- To identify which hydroxyl groups on the beta-CD molecule are susceptible to deprotonation at elevated pH.
- To determine the pKa values associated with the deprotonation of beta-CD's hydroxyl groups.
Main Methods:
- Solution pH was systematically varied in alkaline aqueous media.
- (13)C Nuclear Magnetic Resonance ((13)C NMR) spectroscopy was employed to monitor changes in the carbon backbone.
- (1)H Nuclear Magnetic Resonance ((1)H NMR) spectroscopy was used to observe proton environments, particularly hydroxyl groups.
Main Results:
- Beta-cyclodextrin (beta-CD) showed no significant deprotonation below pH 12.0.
- Above pH 12.0, deprotonation occurred, predominantly affecting the hydroxyl groups adjacent to the C-2 and C-3 carbon atoms of the glucopyranose units.
- Deprotonation of hydroxyl groups adjacent to C-6 carbons was observed to a lesser extent.
- The pKa values for the deprotonation of OH-groups adjacent to C-2 and C-3 were determined to be approximately 13.5 ± 0.2 at 22.5 degrees C.
Conclusions:
- Beta-cyclodextrin exhibits distinct deprotonation sites in alkaline solutions, with C-2 and C-3 hydroxyl groups being more acidic than C-6 hydroxyl groups.
- The determined pKa values provide quantitative insights into the ionization of beta-CD under alkaline conditions.
- These findings are important for understanding beta-CD's behavior in applications involving high pH environments, such as drug delivery and catalysis.
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