| The ability of iron and iron complexes to promote the reduction of hydrogen peroxide has been known since H. J. H. Fenton's work during the late 19 th century, yet only recently has the role of the so called Fenton reaction in biological systems been recognized. Of particular importance are the conditions surrounding activation or deactivation of the Fenton reaction. Iron complexes of ethylenediaminetetraacetate (EDTA), diethylenetriaminepentaacetate (DTPA), and citrate are thermodynamically capable of hydrogen peroxide reduction, yet the kinetics for the reaction are strongly influenced by the metal to ligand ratio (M:L), so much so in fact that the reaction nearly ceases in the presence of excess ligand. Additionally, the reaction is influenced by a peroxy-Fe-Ligand ternary complex of enhanced Fenton reactivity, the formation of which depends on pH, and relative hydrogen peroxide concentration. A detailed understanding of these observations may shed insight into the mechanism for the effect of certain antioxidants. One such class of antioxidants, the flavonoids, may exert their chemopreventive effects through the chelation of iron. Such action would depend on the stability constant for complex formation between the flavonoid and iron. Of the more than 3000 possible flavonoid compounds, the flavonol class is most often associated with metal chelation. Within this class, there are three possible binding sites. Many flavonoids possess two or three of these possible sites, making chelation studies of these compounds difficult. For this work, 3-hydroxyflavone; 5,7-dihydroxyflavone; and 3 ′4′-dihydroxyflavone; each in possession of only one of the three possible binding sites, are examined for their complexation with ferric iron via potentiometric and spectrophotometric titrations. The data are analyzed through the use of the Hyperquad computer software program, which solves for the stability constants through least squares curve fitting. Flavonoids may also exert an antioxidant effect through interaction with enzymes, in particular, the oxidoreductases. Many of these enzymes are based on the hemo-protein active site which contains a porphyrin macrocycle. The stability constant for the complexation of quercetin to the water-soluble heme-model porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H, 23H-porphyrin, both in the presence and absence of ferric iron, are measured by spectrophotometric titration and the Hyperquad software. |
