Kinetics and Mechanism of Oxidation of Tryptophan
by Ferrate(VI)
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Abstract
Kinetics
of the oxidation of tryptophan (Trp) and kynurenine (Kyn),
precursors of nitrogenous disinfection byproducts (N-DBP), by ferrate(VI)
(Fe<sup>VI</sup>O<sub>4</sub><sup>2–</sup>, 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
Fe<sup>18</sup>O<sub>4</sub><sup>2–</sup> 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,
ClO<sub>2</sub><sup>•</sup>, O<sub>3</sub>, and <sup>•</sup>OH)