Anti -Oxidant Activity of Trolox: Understanding the Mechanistic Pathway of Oxidation of Trolox by µ-Oxo-Diiron(III) Complex, a Model of Non-Heme Metalloprotein

The reaction of Trolox-C (TXC), a water-soluble vitamin E analog, with µ-oxo-diiron complex [Fe(III)₂(µ-O)(phen)₄(H₂O)₂]Cl₄⋅5H₂O(1, phen = 1,10-phenanthroline) entailed one-electron oxidations of the phenolic antioxidant with intermediate formation of a phenoxyl radical and accumulation of a quinone end product. These oxidation reactions were linked to individual reductions of complex 1 in acidic buffer media (pH 4.0–5.8), Trolox quantitatively reduces 1 following a first-order reaction kinetics. Each molecule of 1 accepts 2e^– from Trolox and is reduced to ferroin, [Fe(phen)₃]²⁺. On oxidation, Trolox produces Trolox quinone. In reaction media, due to successive deprotonations, 1 exists in equilibrium with the species [Fe(III)₂(µ-O)(phen)₄(H₂O)(OH)]³⁺ (1a) and [Fe(III)₂(µ-O)(phen)₄(OH)₂]²⁺ (1b) (pK_a = 3.71 and 5.28 respectively). The kinetic analyses suggest for an unusual reactivity order as 1 < 1a < 1b. The mechanistic possibilities suggest that while 1, 1a, and 1b are reduced by an ET mechanism. We believe that the initial activation of the bridging µ-oxo group by a proton abstraction results in the higher reactivity of 1b. No variations in media ionic strength suggests for the reaction between one cationic species with other neutral charge species. Positive kinetic isotope effect (k_H/k_D < 1) indicates for a pre-equilibrium protonation step occur in complex 1.