Kinetics and Mechanism of Oxidation of Tryptophan by Ferrate(VI)

Kinetics of the oxidation of tryptophan (Trp) and kynurenine (Kyn), precursors of nitrogenous disinfection byproducts (N-DBP), by ferrate­(VI) (Fe^VIO₄^2–, Fe­(VI)) were investigated over the acidic to basic pH range. The second-order rate constants decreased with increase in pH, which could be described by the speciation of Fe­(VI) and Trp (or Kyn). The trend of pH dependence of rates for Trp (i.e., aromatic α-amino acid) differs from that for glycine (i.e., aliphatic α-amino acid). A nonlinear relationship between transformation of Trp and the added amount of Fe­(VI) was found. This suggests that the formed intermediate oxidized products (OPs), identified by LC-PDA and LC-MS techniques, could possibly compete with Trp to react with Fe­(VI). <i>N</i>-Formylkynurenine (NFK) at pH 7.0 and 4-hydroxyquinoline (4-OH Q) and kynurenic acid (Kyn-A) at pH 9.0 were the major OPs. Tryptophan radical formation during the reaction was confirmed by the rapid-freeze quench EPR experiments. The oxygen atom transfer from Fe­(VI) to NFK was demonstrated by reacting Fe¹⁸O₄^2– ion with Trp. A proposed mechanism explains the identified OPs at both neutral and alkaline pH. Kinetics and OPs by Fe­(VI) were compared with other oxidants (chlorine, ClO₂^•, O₃, and ^•OH)