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 FeIII(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 FeIII(tpps) monomer and ascorbic acid in a weakly acidic medium suggest the formation of the FeIII(tpps˙+) radical cation and the partial formation of FeII(tpps), which depend on the concentration ratio of ascorbic acid and oxygen. In a basic medium the FeIII(tpps) dimer is reduced to FeII(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 FeIII(tpps), thus demonstrating the catalytic effect of the metalloporphyrin on the autoxidation of ascorbate. This suggests that in the presence of FeIII(tpps) a redox cycle takes place [FeII/III(tpps)] which consumes oxygen, with the result that no spectral changes are observed during the induction period.