Absorption Spectroscopy Study of Acid-Base and Metal-Binding Properties of Flavanones

We have used absorption spectroscopy to study the acid-base and metal-binding properties of two structurally similar flavanones: taxifolin and naringenin. We have determined the acid dissociation constants for taxifolin (pK(a1) = 7.10 +/- 0.05, pK(a2) = 8.60 +/- 0.09, pK(a3) = 8.59 +/- 0.19, pK(a4) = 11.82 +/- 0.36) and naringenin (pK(a1) = 7.05 +/- 0.05, pK(a2) = 8.85 +/- 0.09, pK(a3) = 12.01 +/- 0.38). The appearance of new absorption bands in the visible wavelength region let us determine the stoichiometric composition of the iron (II) complexes of the flavanones. We show that at pH 5, in solution there is a mixture of complexes between taxifolin and iron (II) ions in stoichiometric ratio 2:1 and 1:2, while at pH 7.4 and pH 9, we detect a 1:1 taxifolin:Fe(II) complex. We established that at these pH values, naringenin forms a 2:1 complex with iron (II) ions. We propose structures for the complexes formed. Comprehensive study of the acid-base properties and the metal-binding capability of the two structurally similar flavanones let us determine the structure-properties relation and the conditions under which antioxidant activity of the polyphenols appears, via chelation of variable-valence metal ions.