Hydroxypropyl Gamma Cyclodextrin

The selection of the most suitable cyclodextrin for a certain application can be made by using numerous analytical methods such as spectroscopic methods in solutions (UV spectrophotometry, NMR, fluorescence, ESR), separation techniques (CE, HPLC, TLC), and electroanalytical methods. In case of solid phase samples, IR, Raman, X-ray, and thermal methods are the most commonly used, however, these approaches are not capable of determining the liquid phase (equilibrium) stability constant.

Uv Spectroscopy

It is a relatively straightforward method having low material needs, but it provides very little information about the structure of the complex. In this method, the change in the transitions of the chromophore group signals the formation of a complex. As a result of the formation of an inclusion complex and the chromophore getting into a more apolar environment, the UV absorbance spectrum might become more structured than in an aqueous solution. This interaction might result in either a hyper- or hypochromic and hypso- or bathocromic shift of the absorption spectrum. The two disadvantages of the method are that (a) other UV-active compounds may interfere with the spectrum, (b) the complexation does not necessarily result a characteristic change in the spectrum.

Nmr Spectroscopy

It has several advantages. Presence of a chromophore in the guest molecule is not required and, unlike in CD and UV spectroscopy, it provides information about the structure of the complex and it allows for quantification from the changes in the chemical shifts. This method’s drawback is its low sensitivity and the occasional need for deuterated solvents. If the guest molecule has low solubility, long acquisition times may be needed for achieving adequate signal–noise ratios in the case of 2D NOESY or ROESY techniques, which are necessary in structure analysis. The use of other solvents might be required to ensure the proper concentration of the compound. However, this additional solvent might affect the stability and the structure of the complex. In case of native cyclodextrins—due to their symmetry—the interpretation of the spectra can be simple, but in the case of modified cyclodextrins, the presence of structural isomers (different substitution degree) complicates the structural analysis.

Fluorescence Spectroscopy

It is the quantum yield effect, making it more sensitive and usable in a wider concentration range, compared to other techniques. It has to be noted that the signal intensity, despite being in correlation with the concentration, is not linear. When using modified cyclodextrins containing fluorophore groups, the fluorescence spectrum changes due to interactions with the guest molecule. Commonly, the fluorophore is displaced from the cavity by the guest molecule in a competitive process. Based on this principle, cyclodextrins can be applied as molecular sensors.

Capillary Electrophoresis

It requires small quantities of material, but either the guest molecule or the cyclodextrin must have a charged functional group. Neutral molecules and cyclodextrins cannot be tested directly. If the cyclodextrin is dissolved in the mobile phase, it might function as a chiral selector in case of enantiomer pairs.

Hplc Methods

They are not restricted to charged molecules and apply cyclodextrins two ways: first, the cyclodextrin is covalently bonded to the solid phase; second, the cyclodextrin is dissolved in the mobile phase. When applying covalently bonded cyclodextrins, the separation can be performed using normal (NP) or reverse phase (RP) mode. Testing different cyclodextrins require purchasing a column of each type. This is costly and time-consuming, especially if optimisation of the chromatographic parameters also requires addressing mobile phase composition and temperature parameters.

Circular Dichroism

Alongside the methods mentioned above, circular dichroism can come to mind as a good alternative. Its sensitivity and material needs are on par with UV spectroscopy but it provides more detailed structural information. Upon analysis of CD spectra, the formation of inclusion complexes can be confirmed and the structure of the complex can be assessed using theoretical calculations. Another benefit of this method is that, like UV spectroscopy, it can rapidly provide results, resulting in shorter experiment times. Furthermore, CD spectroscopy is less expensive than NMR or separation techniques.