Physicochemical study of floranol, its copper(II) and iron(III) complexes, and their inhibitory effect on LDL oxidation

The antioxidant activity of floranol (3,5,7,2′-tetrahydroxy-6-methoxy-8-prenylflavanone), a new flavonoid isolated from the roots of Dioclea grandiflora, was evaluated by the inhibition of human low-density lipoprotein (LDL) oxidation. Floranol increased its oxidation lag-phase significantly in a dose-dependent manner. As the antioxidant mechanism may involve metal coordination, we have undertaken a detailed study of floranol interactions with Cu(II) and Fe(III) by combination of UV–visible (UV–Vis) and mass spectrometries and cyclic voltammetry. The acidity constants of the ligand as well as the stability constants of the metal complexes were calculated. The pKa values of 6.58, 11.97 and 13.87 were determined and the following acidity order is proposed 7-OH > 5-OH > 2′-OH. The best fit between experimental and calculated spectra was obtained assuming the formation of two Cu(II) complexes: [CuL] log β = 19.34 ± 0.05 and [CuL2]2− log β = 26.4 ± 0.10 and three Fe(III) complexes: [FeL3]3− log β = 44.72 ± 0.09, [FeL2]− log β = 35.32 ± 0.08 and [FeL]+ log β = 19.51 ± 0.04. In addition, copper and iron reduction is less favorable in the presence of floranol. These results indicate that floranol can efficiently bind Cu(II) and Fe(III) ions thus preventing their effect on LDL oxidation.