Degradation Mechanisms And Substituent Effects Of N-chloro-?-amino Acids:A Computational Study

N-chloro-?-amino acids can form not only in living organisms,through the reaction of amino acid with the chlorine produced from oxidation of chloride ion by hydrogen peroxide generated by myeloperoxidase in neutrophils;but also in the disinfection of water or wastewater associated with chlorine-containing disinfectant.Previous research has shown that N-chloro-?-amino acids and their degradation products have high cytotoxicity and genotoxicity,therefore,the research regrading N-chloro-?-amino acids and their degradation product has attracted extensive attention.However,understanding of degradation mechanism of N-chloro-?-amino acids is still insufficient due to the present limited analytical and detecting methods.In this dissertation,glycine,alanine,and valine were chosen as model amino acids,and degradation mechanisms of N-monochloro-?-amino acids and N,N-dichloro-?-amino acids under neutral and alkaline conditions were systematically studied by using quantum chemistry method.Additionally,more than ten substituents were selected,and the substituent effects on the degradation of N-chloro-?-amino acids at different sites were investigated.The detailed research results are as follows:1.Validation of calculation methods and establishment of the LEFR of pK_aIn this dissertation,six functionals including M06-2X,B3LYP,CAM-B3LYP,B1B95,BMK and M05-2X as well as the high accurate CCSD(T)ab initio method,along with 6-31+G(d),6-311+G(d)and 6-311++G(d,p)three basic sets,two solvent models including CPCM and SMD were chosen to investigate the concerted Grob fragmentation(CGF)reaction of N-monochloro-glycine(N-Cl-Gly).The calculation results indicate that the activation free energy(?G^?)calculated at the CCSD(T)-SMD/6-311+G(d)//M06-2X/6-311+G(d)level is 23.4 kcal/mol,which agrees well with experimental value(approximately 25 kcal/mol).Hence,the above method was selected to explore the degradation mechanism of N-chloro-?-amino acids.For the calculations of pK_a,CBS-QB3 and M05-2X-SMD/6-31G(d)//CBS-QB3were used to obtain gas phase reaction free energy(?G*)and solvation reaction free energy(?G*_solv),respectively.Then solution phase reaction free energy(?G*_soln)was obtained through thermodynamic cycle.The correlation between the experimental pK_avalues and the uncorrected quantum chemical pK_avalues for the-NH⁺moiety in seven amines were established,and the pK_avalues of intermediates in the system were predicted.2.Degradation mechanisms of N-chloro-?-amino acidsInvestigations of degradation pathways of three model N-chloro-?-amino acids,i.e.,N-chloro-glycine,N-chloro-alanine and N-chloro-valine were performed.The results indicate that,under neutral condition,N-monochloro-?-amino acids undergo CGF and hydrolysis reactions to produce aldehydes and ammonia compounds,and the CGF reaction is the rate-limiting step with the?G^?values in the range of 19?23kcal/mol.Under alkaline condition,N-monochloro-?-amino acids first react with OH?to form carboxyl imines through a?-C elimination reaction of(?-E).Then carboxyl imines undergo two-step hydrolysis reaction to form?-ketone acids.The?G^?values of the rate-limiting step are in the range of 19?23 kcal/mol,and the estimated rate constants k are in the range of 5.6×10^-5?1.5×10^-2M^-1s^-1,which is consistent with the experimental rate constants k of 10^-4?10^-3M^-1s^-1.With respect to N,N-dichloro-?-amino acids,under neutral or Cl/N?2 conditions,they mainly generate N-chloroaldimines through CGF reaction(with the?G^?values in the range of 15?19 kcal/mol).Subsequently,N-chloroaldimines undergo hydrolysis to form aldehydes(the?G^?value being 40 kcal/mol)or elimination to generate nitriles(the?G^?value being 32 kcal/mol).The relatively high?G^?values of the hydrolysis and elimination reactions indicate that N-chloroaldimines are stable and can be the main products under neutral or Cl/N?2 conditions.This is consistent with the experimental results.Under alkaline or Cl/N>2 conditions,with the assistance of the OH?or OCl?base,N,N-dichloro-?-amino acids can rapidly degrade into carboxyl chlorimides through?-E reaction.Then carboxyl chlorimides proceed through a rapid CGF reaction to generate nitriles.In addition,it was found that OH?and OCl?bases can also significantly promote the elimination of N-chloroaldimines to produce nitriles(the?G^?value being less than 10 kcal/mol).This shows that nitriles are the main degradation products in the degradation of N,N-dichloro-?-amino acids under alkaline or Cl/N>2 condition,which is consistent with the experimental observations.Moreover,it also explains the experimental phenomenon that the yield of nitriles increases while that of N-chloroaldimines decreases,when the ratio of Cl/N increases from 2.8 to 4.4.Additionally,the formation mechanism of chloronitriles in the chlorination of nitriles was found to be promoted by hypochlorite,and the chlorination activity was found to decrease or increase with the hydrogen atom of the nitrile substituted by the methyl group or Cl atoms,respectively.3.Substituent effects on the degradation of N-chloro-?-amino acidsIn this section,N-Cl-Gly and 13 kinds of electron-withdrawing groups(EWG)and electron-donating groups(EDG)were chosen as model N-chloro-?-amino acid and substituents,respectively.The substituent effects of above groups substituted at the?-C and N-terminal sites of N-Cl-Gly on CGF and?-E reactions were systematically investigated.For CGF reaction,the?G^?values for all the substituents substituted ate the?-C or N-terminal sites are all lower than that of N-Cl-Gly.When the conjugated EDG(-NH₂,-NHCH₃and-OCH₃)are substituted at the N-terminal site,the?G^?values are in the range of 5.7-7.7 kcal/mol far below N-Cl-Gly(21.8kcal/mol),which are consistent with the rate constants k detected by experiments.This implies that the reaction activities for the conjugated EDG substituted N-Cl-Gly are higher than that of N-Cl-Gly,which can significantly promote the CGF reaction.For?-E reaction,the?G^?value for the-CN substituted at?-C site of N-Cl-Gly is 7.4kcal/mol.Such a low?G^?value indicates that-CN not only increases the acidity of C-H bond,but also stabilizes the intermediate anion.When-OCH₃group substituted at?-C site,the?G^?value is only 4.9 kcal/mol.All the above results indicate that conjugated EWG substituted at?-C site and conjugated EDG substituted at the N-terminal site can significantly promote the?-E reaction.Moreover,the calculation results of the di-substituents and tri-substituents of-CH₃and-OCH₃demonstrate that the CGF reaction becomes easier with the increase of the number of substituted methyl and methoxy groups in N-Cl-Gly.In this dissertation,the degradation mechanisms of N-chloro-?-amino acids were disclosed,and the effects of different reaction conditions and substituents on the degradation mechanism and products were explored.The results not only provided a theoretical basis for understanding the degradation mechanisms of N-chloro-?-amino acids under physiological and environmental conditions,but also provided a theoretical guidance for identifying precursors of toxic degradation products and developing water treatment processes to prevent and control the formation of toxic by-products,which are of great value to human health.