Haemin-catalyzed oxidation of ascorbic acid by H2O2. Evidence for a second haemin–H2O2complex

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Abstract
A study of the haemin-catalyzed oxidation of ascorbic acid by H2O2 shows that two intermediate complexes between haemin and H2O2 are formed in two consecutive steps. The second complex can react either with H2O2 to give oxygen or with ascorbic acid to give dehydroascorbic acid. The respective rate constants are k4=(4.5 ± 0.4)× 102 M–1 min–1 and k5=(5 ± 2)× 103 M–1 min–1 at 0.0°C and pH = 7.21. The mechanism is similar to that of the Fe3++H2O2 reaction.