Antioxidant activity of an inclusion complex between rutin and β-cyclodextrin: experimental and quantum chemical studies

Abstract

This study aims to synthesize a guest-host complex derived from rutin (Rut) and β-cyclodextrin (β-CD) (denoted as [Rut⊂β-CD]). The obtained substance was characterized by the FT-IR and DSC methods, signifying the formation of an inclusion complex between Rut and β-CD. Complex formation increased the antioxidant activity of rutin corresponding to the decrease of EC₅₀ values from 1.547 × 10^-5 mol L^-1 to 1.227 × 10^-5 mol L^-1 according to the DPPH free radical scavenging test. The rutin-β-CD interaction energies were calculated in the vacuum and various solvents (e.g., water, ethanol, and dimethylsulfoxide) utilizing an accurate and broadly parametrized self-consistent tight-binding quantum chemical method (GFN2-xTB). The calculation results reveal the influence of solvent on the structural formation of the rutin-β-CD complex. In both the vacuum and aqueous solution, rutin can enter into the small-sized empty cavity of β-CD, albeit through different terminals, resulting in distinct preferential structures. The presence of organic solvents appears to reduce the interaction between rutin and β-CD, with the interaction strength following the order: water > ethanol > dimethyl sulfoxide.

Fig. 1. Interaction modes between rutin and β-CD. Color codes: grey – C, red – O, white – H, and numbering scheme of phenolic –OH groups in Rut molecule.

Fig. 2. Fourier infrared spectra of Rut, β-CD and [Rut⊂β-CD] complex.

Fig. 3. ATR-FTIR spectra of saturated solutions of Rut (blue line) and [Rut⊂β-CD] complex (red line), 50 mM solution of β-CD (green line). 0.02 M sodium – phosphate buffer solution, pH 7.4., 22 °C. Spectra are min–max normalized for better representation.

Fig. 4. ATR-FTIR mapping for [Rut⊂β-CD] complex powder. Mapping area is 302 × 302 microns. 15× ATR objective.

Fig. 5. DSC curves of rutin, β-CD and complex.

Fig. 6. PXRD patterns for Rut (blue), CD (green) and [Rut⊂β-CD] complex (red).

Fig. 7. Fragment of the 2D ROESY spectrum of a solution of the rutin complex with β-CD in D₂O, measured at 278 K and 600 MHz ¹H resonance frequency. Signals with positive and negative intensity are shown in red and blue, respectively. The yellow oval highlights the cross peaks between the protons H6 and H8 of rutin and the protons of glucose residues β-CD, indicating their spatial proximity.

Fig. 8. The correlation equations between the percentage of DPPH radical scavenging activity of rutin (a) or [Rut⊂β-CD] (b) and their concentrations.

Fig. 9. Scatter graph of depence of RDG ot sign λ₂ (ρ) function (a); the RDG isosurface depicted at an isovalue of 0.8 of IIB complex (b).

Fig. 10. HOMO and LUMO of the studied systems in the vacuum.

Fig. 11. HOMO and LUMO of the studied systems in water.