L-半胱氨酸对酰胺类除草剂的离体脱氯反应与机制

研究了L-半胱氨酸与酰胺类除草剂的反应动力学、热力学及其机制.结果表明,L-半胱氨酸促进酰胺类除草剂的降解,该反应遵循二级动力学方程;除草剂降解速率同除草剂分子的亲电常数线性相关,降低顺序为甲草胺(kL-cysteine=7.65×10-3mol/(L×s))>乙草胺(kL-cysteine=7.23×10-3mol/(L×s))>丁草胺(kL-cysteine=6.01×10-3mol/(L×s))>S-异丙甲草胺(kL-cysteine=2.15×10-3mol/(L×s)),这与其土壤和高效菌降解速率顺序相一致;产物的质谱鉴定表明,L-半胱氨酸取代除草剂分子中氯原子.表明该反应为双分子亲核取代脱氯反应.热力学分析显示,该反应为焓控反应.除草剂间降解速率的差异性由熵变控制,且除草剂的降解速率与反应熵变(ΔS)具有良好的线性关系,ΔS数值越负,除草剂的降解速率越小.分子中N原子上的醚键取代基支链结构及链长度对反应速率影响较大,而芳环取代基结构没有明显影响.

Dechlorination reaction and mechanism of chloroacetanilide herbicides by L-cysteine

Dynamics, thermodynamic and mechanism of the reaction between chloroacetanilide herbicides and L-cysteine were investigated. L-cysteine could significantly promote the dissipation of four chloroacetanilide herbicides, following second-order kinetics. The degradation rates of the herbicides were linearly correlated with electrophilic index of chloroacetanilide herbicides, giving the order: alachlor (kL-cysteine=7.65×10-3mol/(L×s)) > acetochlor (kL-cysteine=7.23× 10-3mol/(L×s)) > butachlor (kL-cysteine=6.01×10-3mol/(L×s)) > S-metolachlor (kL-cysteine=2.15×10-3mol/(L×s)). This order was consistent with those of chloroacetanilide herbicide degradation in soil or by high efficient bacteria. Mass spectra identification of the products revealed that the chlorine was replaced by L-cysteine. Thus the dechlorination of these herbicides by L-cysteine followed bimolecular nucleophilic substitution reaction. Analysis of reaction thermodynamics indicated that the reaction was enthalpy-controlled and that differences in herbicide activity stemmed primarily from entropic factors(ΔS). There was good linear correlation between degradation rates of herbicides and respective ΔS values, suggesting that branched-chain structure and chain length of N-alkoxyalkyl had significant effect on reaction activity of these herbicides.