Novel Chlorination Byproducts of Tryptophan: Initial High-Yield Transformation Products versus Small Molecule Disinfection Byproducts

Because of the difficulty of carbon–carbon bond cleavage needed to liberate the small molecule chlorine disinfection byproducts (DBPs; e.g., trihalomethanes), they typically form at <1% yields. By using N-acetyl-tryptophan (NacTrp) as a model reactive peptide-bound monomer within natural organic matter (NOM), this study illustrates the characterization of its initial chlorine transformation products as an alternative approach to identify high-yield DBPs. When NacTrp was treated at 1–10 chlorine/NacTrp molar ratios, seven novel initial transformation products were purified and characterized by high-resolution mass spectrometry, including two chlorine-containing DBPs. The total concentrations of these novel DBPs accounted for 55–100% conversion of the parent NacTrp at 1–5 chlorine/NacTrp molar ratios over 24 h, compared to ≤5% for the total concentrations of small molecule DBPs. At a 10:1 molar ratio, the novel DBP concentrations declined over 24 h to account for 25% molar conversion, while small molecule DBP concentrations increased, yet still accounted for only 20% molar conversion. A reaction pathway consistent with these initial transformation products was proposed. The high yields of these novel DBPs are attributable to the lack of carbon–carbon bond cleavage. This study illustrates the identification of initial transformation products of important monomers as an efficient option to characterize the unknown DBPs.