Transformation of adenine and cytosine in chlorination - An ESI-tqMS investigation

The interaction of chlorine with nitrogenous constituents in water is being concerned due to the formation of relatively toxic nitrogenous disinfection byproducts during chlorine disinfection. In this study, the transformation pathways of the chlorination of adenine and cytosine are proposed based on the products analysis using a collision-energy-dependent method on triple quadrupole mass spectrometry coupled with electrospray ionization. Products with multiple chlorine addition on the heterocyclic ring and on the aliphatic amine were observed during the chlorination of adenine and cytosine. The primary amine functional group in adenine and cytosine can undergo chlorine substitution to form N-chloramine and undergo hydrolysis of the C-N bond to form carbonyl derivative. The transformation of adenine and cytosine depends on pH and the chlorine to precursor (Cl/P) ratio. An 8-chloro derivative of adenine was observed at pH 4, but not at pH 7. Substitution of 1-2 chlorine atoms for the hydrogen atoms in the N-heterocyclic ring was observed during adenine chlorination compared to substitution of 1-4 chlorine atoms during cytosine chlorination. Chlorination of adenine also led to ring cleavage products. Both 5-chlorocytosine and 4-N-chlorocytosine were identified as cytosine transformation products. At pH 7 and a Cl/P molar ratio of 2, the major products of chlorination of cytosine were found to be aromatic chloro-compounds, not aliphatic N-chloramine. The results of this study are significant for understanding the transformation mechanisms of compounds containing both N-heterocyclic and primary amines due to chlorination.