Enhancement of flavonols water solubility by cyclodextrin inclusion complex formation-case study

The phenomenon of solubility increasing of nonpolar flavonol antioxidant – quercetin (QC) in water solutions containing β-cyclodextrin (CD) was studied. Complexation of QC by CD was investigated using phase solubility study. It allows to estimate the apparent formation constant for 1:1 host-guest complex. Additionally thermodynamic properties were evaluated as well showing spontaneous exothermic process of complex formation. Two solubility models were also employing to verify their applicability to predict QC concentration in solution. Those models included modified Apelbalt and Buchowski-Ksiazczak equations. In order to investigate the antioxidant properties of ternary systems QC/CD/water a radical scavenging activity using stable radical of DPPH was performed. Experiments indicated that antioxidant activity is temperature and CD concentration-dependent. It was shown that complexation may inhibit the radical scavenging by QC or changing the scavenging stoichiometry.

are commercialized as dietary supplements either as pure compounds (e.g. quercetin), extracts or mixtures.

Figure 1. Molecular structure of flovonols (left) and quercetin (right)
Quercetin is a most common in flavonols group. Quercetin and its sugar-bound, or glycosylated, forms represent 60-75% of flavonoid intake 4 . Quercetin is known to have the ability for the oxidation of low-density lipoproteins prevention by means of free radicals scavenging as well as transition metal chelating. According to that, quercetin could help in the diseases prevention including cancer, atherosclerosis, etc. 5 . In plants, as most of flavonols quercetin is bound to sugars molecules forming O-glycosides. The most common quercetin glycosides have a sugar group at the 3-position such as quercetin-3-O-β-glycoside. According to that quercetin is poorly absorbed in human body because no enzyme is known to split the glycosidic bond. Aglycones, sugar-free flavonoids can easily pass through the gut wall, but flavonoids are rarely found as aglycones in plants 6 .
According to that the inclusion complexes of flavonols including quercetin with cyclodextrin may increase the bioavailability of this important compound. In literature several reports can be found on quercetin/cyclodextrin complexes synthesis. There is however only a few information on complexation thermodynamics. The presented paper focuses on finding the complexation constant as well as basic thermodynamic parameters. Solubility models are also applied to explain the quercetin transfer between solid state and water solution.

Experimental:
All the studies were carried out using food grade  -cyclodextrin -Roquette (France) . Other

Results and discussion:
As a result of investigation the basic knowledge on inclusion complex formation between quercetin and -cyclodextrin has been obtained. Complexation of quercetin is a multistage process that includes: quercetin transfer from the solid state to the aqueous phase and introducing the molecule into the internal cavity of cyclodextrin. The overall scheme of the process is presented on Figure 2.

. Free radical scavenging activity of QC aqueous solutions
In order to clarify the problem some thermodynamics calculation should be done. According to that the stability constant for quercetin/cyclodextrin complex -K F as well as standard enthalpy and entropy of complexation has been estimated. Additionally Gibbs free energy has been evaluated as well. The results obtained based on Higuchi-Connors method for K F calculation 7 .
Thermodynamic parameters have been evaluated according to dell Valle 8 . Results of the study were presented in Table 1.

Figure 5. Solubility plot for QC in aqueous CD solutions
On the basis of the calculation one can note that the complexation constant K F decreases with temperature by about one and a half order of magnitude. This may mean reducing the effects of a host -guest interaction with temperature. As a consequence hydrophobic interactions are strengthened at the cost of hydrogen bonding. The obtained results allow to conclude that the complexation of quercetin on the CD is an exothermic process (H 0 <0). The values of enthalpy changes indicate that during complexation some energy is releasing. This behavior is typical of hydrophobic interactions in the interior of the molecule where the CD is exchanged polar water molecules by the nonpolar guest molecules.
The entropy changes for the complex formation process is rather low, but the value of this change is positive, similar to that observed in many other systems containing cyclodextrin 8  As one can see from Table 2

Conclusion:
A solubility and antioxidant activity study on ternary system quercetin/CD/water has been carried out. According to obtained results it may be stated that CD presented in the solution enhance the quercetin solubility by means of host-guest complexation. The obtained data suggest 1:1 complex to be formed and the process is spontaneous (G O < 0). The modified Apelblat and Buchowski-Ksiazczak solubility models works well for the investigated system however their correlation to experimental data depends on CD concentration. The obtained solutions posses some antioxidant activity. Independently on quantitative results the complexation of quercetin by -cyclodextrin has been proved. It opens new possibilities to apply the nonpolar insoluble flavonols also in water solutions.