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.