Identifying Initial Transformation Products during Chlorination of the Indole Moiety and Unveiling Their Formation Mechanisms

Abstract

To identify toxicity drivers within the poorly characterized high-molar-weight disinfection by-products (DBPs), the relatively stable high-yield initial transformation products generated from aromatic amino acids and peptides and humic substances have drawn much attention. In this study, the initial transformation products in chlorination of the indole moiety in tryptophan (Trp) were proposed and their formation mechanisms were investigated by quantum chemical computational method. The results indicate that 3-Cl-Trp+ is initially formed after the Cl+ of HOCl attacking on the indole moiety, then the nucleophilic addition with nucleophilic agents (H2O and OCl‾) is preferable over the deprotonation in thermodynamics to generate 2-X-3-Cl-Indoline moiety (X=OH and OCl), which is in contrast to indole. Over 25 kinds of initial transformation products were proposed from 2-X-3-Cl-Indoline moiety and two ring opening ways were found occurred at the N1–C2 and C2–C3 bonds. Significantly, most structures of the initial transformation products proposed based on the experimental detection m/z values were confirmed by quantum chemical calculations and some new products were also proposed by this work. The results are helpful to expand our understanding of the intrinsic reactivity of aromatic ring towards chlorination by hypochlorous acid.