Mechanistic insight from rapid-scan stopped-flow spectrophotometry into the autoxidation of ascorbate catalysed by an iron porphyrin complex

Abstract

The reaction of the water-soluble complex Fe^III(tpps) [tpps = 5,10,15,20-tetrakis(p-sulfonatophenyl)porphyrinate] with ascorbic acid has been investigated in aqueous solution by a combination of rapid-scan and stopped-flow techniques. The spectral changes recorded for the reaction between the Fe^III(tpps) monomer and ascorbic acid in a weakly acidic medium suggest the formation of the Fe^III(tpps^˙+) radical cation and the partial formation of Fe^II(tpps), which depend on the concentration ratio of ascorbic acid and oxygen. In a basic medium the Fe^III(tpps) dimer is reduced to Fe^II(tpps), showing an induction period which depends on the oxygen concentration of the solution. Measurements with an oxygen-sensitive electrode clearly demonstrated that during the induction period oxygen is used up by ascorbate and that the decrease in oxygen concentration is accelerated by the presence of Fe^III(tpps), thus demonstrating the catalytic effect of the metalloporphyrin on the autoxidation of ascorbate. This suggests that in the presence of Fe^III(tpps) a redox cycle takes place [Fe^II/III(tpps)] which consumes oxygen, with the result that no spectral changes are observed during the induction period.