Kinetics and mechanisms of chlorine dioxide oxidation of tryptophan

The reactions of aqueous ClO2 center dot and tryptophan (Trp) are investigated by stopped-flow kinetics, and the products are identified by high-performance liquid chromatography (HPLC) coupled with electrospray ionization mass spectrometry and by ion chromatography. The rates Of ClO2 center dot loss increase from pH 3 to 5, are essentially constant from pH 5 to 7, and increase from pH 7 to 10. The reactions are first-order in Trp with variable order in ClO2 center dot. Below pH 5.0, the reactions are second- or mixed-order in [ClO2 center dot], depending on the chlorite concentration. Above pH 5.0, the reactions are first-order in [ClO2 center dot] in the absence of added chlorite. At pH 7.0, the Trp reaction with ClO2 center dot is first-order in each reactant with a second-order rate constant of 3.4 x 104 M-1 s(-1) at 25.0 degrees C. In the proposed mechanism, the initial reaction is a one-electron oxidation to form a tryptophyl radical cation and chlorite ion. The radical cation deprotonates to form a neutral tryptophyl radical that combines rapidly with a second ClO2 center dot, to give an observable, short-lived adduct (k(obs) = 48 s(-1)) with proposed C(H)-OClO bonding. This adduct decays to give HOCl in a three-electron oxidation. The overall reaction consumes two ClO2 center dot per Trp and forms ClO2 center dot and HOCl. This corresponds to a four-electron oxidation. Decay of the tryptophyl-OClO adduct at pH 6.4 gives five initial products. that are observed after 2 min and are separated by HPLC with elution times that vary from 4 to 17 min (with an eluent of 6.3% CH3OH and 0.1% CH3COOH). Each of these products is characterized by mass spectrometry and UV-vis spectroscopy. One initial product with a molecular weight of 236 decays within 47 min to yield the most stable product, N-formylkynurenine (NFK), which also has a molecular weight of 236. Other products also are observed and examined.