A Quantitative Approach to the Free Radical Interaction Between Alpha-tocopherol or Ascorbate and Flavonoids

Abstract

Despite numerous previous studies, the mechanism of the free radical interaction between alpha-tocopherol (VE), or ascorbate and flavonoids, as coantioxidants remains unclear. The synergistic antioxidant effects of VE or L-ascorbyl 2,6-dibutyrate (ASDB, an ascorbate derivative) with the flavonoids (-)-epicatechin (EC), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECG) and (-)-epigallocatechin gallate (EGCG) and methyl gallate (MG), were investigated by the induction period method in the polymerization of methyl methacrylate (MMA), initiated by thermal decomposition of 2,2′-azobis(isobutyronitrile) (an alkyl radical, R •), under nearly anaerobic conditions. For VE, a synergistic antioxidant effect was observed with MG, EC, EGC and ECG, whereas this activity was decreased by the addition of EGCG. For ASDB, a synergistic antioxidant effect was observed with EGC and ECG, whereas this activity was decreased by the addition of EGCG or MG. A synergistic antioxidant effect (regeneration of VE) appears to be feasible even though the BDE (phenolic O-H bond dissociation entropy) of the coantioxidants is significantly higher than that of VE. The driving force for the regeneration process may be the removal of the semiquinone radical from the flavonoids MG, EC, EGC and ECG by the VE radical. In the ASDB/flavonoid mixture, flavonoid radicals are scavenged by ASDB. The partial regeneration of flavonoids by ASDB may follow a similar recycling mechanism to that of the well-known VE/ascorbate mixture. The free radical interaction between EGCG and VE or ASDB decreased the antioxidant effect. Such enhancement of prooxidation in EGCG/VE or EGCG/ASDB mixtures oxidized by R • may increase their cytotoxic effects.