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