Degradation Mechanisms and Substituent Effects of N-Chloro-α-Amino Acids: A Computational Study

N-Chloro-α-amino acids formed in the chlorination disinfection treatment of water or wastewater and in living organisms have attracted extensive attention due to the potential toxicities of themselves and their decomposition products. The degradation mechanisms of three N-chloro-α-amino acids, i.e., N-chloro-glycine, N-chloro-alanine, and N-chloro-valine, have been systematically investigated using quantum chemical computations. The results indicate that N-chloro-α-amino acid anions undergo two competitive degradation pathways: a concerted Grob fragmentation (CGF) and β-elimination (β-E). Generally, the former predominates over the latter under neutral conditions and finally generates amines and carbonyls, while the latter is preferred under base-promoted conditions and mainly produces the respective α-keto acid anions or nitriles in the end. To gain deeper insights into the substitution effects, in view of the advantages of quantum chemical computations, a number of real or designed N-chloro-α-amino acids with traditional electron-donating groups (EDG) or electron-withdrawing groups (EWG) have been studied. All of the substituted N-chloro-α-amino acids, regardless of the type and position of substituents, are kinetically more favorable than N-monochloro-glycine for degradation via the CGF pathway. Moreover, conjugated EDG substituted on the N-terminal facilitate both CGF and β-E reactions, whereas conjugated EDG and EWG on the α-carbon are only favorable for the CGF and β-E reactions, respectively. These results are expected to expand our understanding of organic N-chloramine degradation mechanisms and chlorination reaction characteristics.