Confirmation of Cyclodextrin Inclusion Complex
INQUIRY
After the formation of cyclodextrin (CD) inclusion complexes, it is necessary to use various techniques to confirm whether the complex is formed and to study the interaction between the guest molecules and CD. Common confirmation methods for CD inclusion complexes include the phase solubility method, differential scanning calorimetry (DSC) method, ultraviolet-visible (UV-Vis) and infrared (IR) spectroscopic methods, and fluorescence analysis. Here we mainly introduce the first three confirmation methods.
Phase Solubility Method
Based on the fact that the solubility of the guests differs before and after encapsulation by CD, the phase solubility method can confirm whether the product is a CD inclusion complex or a physical mixture of guests and CDs by analyzing the stability of the water-soluble inclusion complex. The stability of the complex is characterized by its equilibrium constant Ks, and Ks value can be calculated from the solubility curve derived from the interaction between the guest and the host in the solution. An increasing value of Ks indicates a more stable complex. Different types of solubility curves and different Ks values illustrate different solubility behaviors of the guest after encapsulation and different solubilizing effects of CD on the drug, which is conducive to the confirmation of CD inclusion complexes [1].
DSC Method
DSC is an analysis technique for directly assessment of the heat energy uptake, which occurs in a sample within a regulated increase or decrease in temperature. It can be used to indirectly confirm CD inclusion complexes by applying the DSC technique and observing whether endothermic peaks appear in the thermogram, based on the difference in thermal stability of the physical mixture (CD and guest molecules) and the CD inclusion complexes.
When the sample is analyzed by DSC without inclusion complex formation, two endothermic peaks can be observed at the melting or boiling points of CD molecules and guest molecules. Whereas, when the endothermic peak is not detected in the thermogram, it indicates that the guest molecule has been encapsulated within the cavity of the CD, thus confirming the formation of CD inclusion complex [2].
UV-Vis and IR Spectroscopic Methods
The UV-Vis or IR absorption spectrum (a shift or band broadening) of the formed CD-guest molecule inclusion complexes is altered compared to the original guest molecules. Thus, UV-Vis and IR spectroscopic methods can be used to determine the formation of CD inclusion complexes.
- UV-Vis spectroscopic method
UV-Vis spectroscopic method involves the determination of the absorbance of a substance in the range of near-UV (180–390 nm) or visible (390–780 nm) radiation. In order to confirm the formation of CD inclusion complex, it is necessary to measure the absorbance of a series of samples using UV-Vis spectroscopic technique, and observe the height, position, and width of the absorption peaks in UV-Vis spectroscopy [1].
IR spectroscopic method can be used to analyze guest molecules, physical mixtures of guest molecules and CD, and CD inclusion complexes. By observing the intensity of absorption bands, changes in peak shapes and peak positions in the IR spectrograms, it can be determined whether the guest molecule has entered the CD cavity, that is, whether an inclusion complex is formed.
References
- Hu, X.; et al. The Development of Cyclodextrin Inclusion Technology and Its Application in Pesticide Pollution. Advances in Material Chemistry. 2018, 6: 3.
- Cheirsilp, B.; Rakmai, J. Inclusion complex formation of cyclodextrin with its guest and their applications. Biol Eng Med. 2016, 2.
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