Kinetics of ironoxidation upon polyphenol binding

Abstract

Polyphenol prevention of iron-mediated DNA damage occurs primarily through iron binding. Once bound, iron in the Fe²⁺-polyphenol complex autooxidizes to Fe³⁺ in the presence of O₂. To determine the correlation between the rate of Fe²⁺-polyphenol autooxidation and polyphenol antioxidant ability, kinetic studies at pH = 6.0 in the presence of oxygen were performed using UV-vis spectrophotometry. Initial rates of iron-polyphenol complex oxidation for epigallocatechin gallate (EGCG), methyl-3,4,5-trihydroxybenzoate (MEGA), gallic acid (GA), epicatechin (EC), and methyl-3,4-dihydroxybenzoate (MEPCA) were in the range of 0.14–6.7 min⁻¹. Polyphenols with gallol groups have faster rates of iron oxidation than their catechol analogs, suggesting that stronger iron binding results in faster iron oxidation. Concentrations of polyphenol, Fe²⁺, and O₂ were varied to investigate the dependence of the Fe²⁺-polyphenol autooxidation on these reactants for MEGA and MEPCA. For these analogous gallate and catecholate complexes of Fe²⁺, iron oxidation reactions were first order in Fe²⁺, polyphenol, and O₂, but gallate complexes show saturation behavior at much lower Fe²⁺ concentrations. Thus, gallol-containing polyphenols promote iron oxidation at a significantly faster rate than analogous catechol-containing compounds, and iron oxidation rate also correlates strongly with polyphenol inhibition of DNA damage for polyphenol compounds with a single iron-binding moiety.