Metal-coordinated phlorizin derivatives: Structural characterization and α-glucosidase inhibitory activity

This study aimed to characterize, clarify the α-glucosidase inhibitory activity and mechanism of phlorizin-Zn (II) complex (Phl-Zn) and phlorizin-Cu (II) complex (Phl-Cu). Phl-Zn and Phl-Cu were synthesized and preliminarily characterized using ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), elemental analysis (EA) and thermogravimetric analysis (TGA). The formation of stable coordination structures between phlorizin and metal ions (Cu 2+ /Zn 2+ ) through metal-oxygen bonds was confirmed. Furthermore, inhibition kinetics and multispectral techniques including ultraviolet spectrophotometry and fluorescence spectrophotometry were employed to investigate the inhibitory effects of phlorizin and its complexes on α-glucosidase. Phlorizin, Phl-Zn and Phl-Cu reversibly inhibited the enzyme activity with IC50 values of 0.766 ± 0.012, 0.470 ± 0.035 and 0.084 ± 0.019 mg/mL, respectively, demonstrating superior efficacy of the metal complexes. Both complexes exhibited mixed-type inhibition and stronger binding affinity to α-glucosidase than the enzyme-substrate interaction (K is  > K i ). Furthermore, the binding constants (Ka) of phlorizin and its two metal complexes toward the enzyme decreased with rising temperature, suggesting that their stability was weakened at higher temperatures. Additionally, both complexes bound to the single active site of α-glucosidase via hydrogen bonds and van der Waals forces, which induced fluorescence quenching, increased tryptophan polarity and reduced hydrophobicity, and finally altered the enzyme's conformation and secondary structure. It was concluded from molecular docking and molecular dynamics analysis that Phl-Cu exerted more significant stabilizing effect on the protein structure than phlorizin and exhibited favorable dynamic adaptability in its binding mode. This study provides supporting information to develop novel phlorizin-based α-glucosidase inhibitors.