Theoretical Study On The Reaction Mechanism Of Glycine/cysteine Degradation And Transformation Of Halogenated Benzoquinone

Over the last century,water disinfection has become a critical step in the treatment of drinking water to prevent water-borne diseases.However,disinfectants react with organic matters in the source of water and then generate various disinfection byproducts(DBPs).DBPs have been reported to be associated with human health risks,such as bladder cancer,colorectum cancer,and cardiac birth defects.Recent studies have shown that many unregulated DBPs may be causative agents that drive DBPs toxicity.Thus,besides regulated DBPs,researches on some emerging DBPs of high toxicity will have a positive impact on understanding and improving the safety of drinking water.As an emerging class of unregulated DBPs,halobenzoquinones(HBQs)have been proven to be widely distributed in chlorinated drinking water and recreational water.Generally,HBQs can undergo spontaneous hydrolysis reactions in neutral or alkaline conditions.Recently,amino acids(AAs)and their derivatives have good application prospects in the transformation and degradation of HBQs,but the detailed transformation and degradation mechanism is not clear.To better understand the transformation and degradation behavior of HBQs by AAs,in this study,the reaction mechanism between two AAs including glycine(Gly)and cysterine(Cys)and HBQs has been investigated detailedly employing density functional theory(DFT).The stepwise transformation pathway to form the final product has been proposed.In addition,the reliability of the condensed Fukui function in predicting the regional reactivity of HBQs is also investigated,and the distortion-interaction model was used to investigate the key factors affecting the reactivity.Expectedly,the present results will not only provide new insights for in-depth understanding of the transformation mechanism of HBQs induced by AAs,but also provide important references for the future researches on the transformation and degradation of HBQs induced by other AAs.In the first part,the transformation behavior of chloro-and bromo-benzoquinones in the presence of Gly has been systematically investigated theoretically.It was found that the amino group of glycine can attack the C atom of quinone ring through Michael addition and nucleophilic substitution mechanism and the products were connected by C-N-C covalent bonds.For the Michael addition mechanism,it is predominant in the first step of the reaction for the most HBQs except for tetrahalogenated benzoquinones and consists of four steps,i.e.,nucleophilic attack,enolate protonation,proton transfer,and keto-enol isomerization,where water molecules play a positive catalytic role in the keto-enol isomerization process.As for the nucleophilic substitution mechanism,it consists of two steps,i.e.,nucleophilic attack and proton transfer,which is crucial for the dehalogenation transformation of tetrahalogenated benzoquinones.By analyzing the reaction mechanism for the stepwise transformation processes,we found that the C atom at the para-position of the introduced Gly fragment is the most active site for further nucleophilic reaction.Further experiments involving the higher concentration ratios of HBQs to Gly are called for to confirm the final products containing three or four glycine fragments.In addition,the reliability of the condensed Fukui function in predicting the regional reactivity of HBQs has been examined.Expectedly,the proposed intermediates or products with specific structures and molecular weights in the transformation process can provide an important theoretical basis for the experimental identification of the transformation products using mass spectra technology experimentally.In the second part,the transformation behavior of chloro-and bromo-benzoquinones in the presence of Cys has been systematically investigated theoretically.It was found that the sulfhydryl functional group was the active site for Cys and the products were connected by C-S-C covalent bonds.The transformation of HBQs by Cys occur via Michael addition and nucleophilic substitution mechanisms.The introduced Cys fragments tend to exist in the form of zwitterion.The possible stepwise transformation pathways for HBQs in the presence of Cys have been proposed.Similarly,the proposed intermediates or products with specific structures and molecular weights can provide a theoretical basis for the experimental identification of the transformation products of HBQs using mass spectra technology experimentally.In the third part,the transformation behavior of the hydrolysis product of 2,6-dichloro-1,4-benzoquinone(2,6-DCBQ)in the presence of Gly/Cys has been systematically investigated theoretically.It was found that OH~–is the active nucleophile in the natural hydrolysis of benzoquinone.The competition between OH~–and Gly~–has been observed.On the contrary,no competition between OH~–and Cys~–has been observed.The possible stepwise transformation pathways for the hydrolysis of 2,6-DCBQ were proposed.The C atom at the para-position of the introduced OH~–fragment is the most active site for further nucleophilic reaction.Moreover,the possible stepwise transformation pathways for the hydrolysis product of 2,6-DCBQ in the presence of Gly/Cys have been proposed.Expectedly,the present results can provide new insights into the transformation of hydrolysis proudcts of HBQs by AAs experimantlly.