Pinpointing Chlorinated Ethylene Dehalogenation Mechanisms with Carbon, Chlorine and Hydrogen Isotope Effect Studies

氯化乙烯是地下水中普遍存在的污染物，它可能发生还原脱卤。目前，机理的见解是不完整的，产品形成的理解很少，和现有的见解是很难从实验室转移到现场。为了弥补这一差距与同位素效应的多个元素，在过去的一段时间里，我们（a）坚定地建立了化合物特异性37 Cl/35 Cl除了13 C/12 C同位素分析，并探索了第一个双元素同位素分馏模式的氯化乙烯（B）微生物降解，（c）通过Fe（0）脱卤和（d）与化学模型反应物（维生素B12 -所有脱卤酶的辅因子）的反应。基于双元素同位素分馏（Cl与C）的特征不同的模式，我们的研究结果给出了第一个热铅，两个或两个以上的机制在微生物还原脱卤的工作。因此，在第二阶段，我的目标是通过（a）涉及精心选择的模型反应物的有针对性的实验和（B）除了测量碳和氯之外还测量氢同位素效应来查明这些差异背后的机制。该方法基于以下假设：（i）先前被认为是不同的两种途径-维生素B12的亲核取代和亲核加成-共享相同的初始步骤;（ii）它们可以在不同pH值的实验中解卷积;（iii）可以在水和有机溶剂中模拟单电子转移到氯化乙烯;（iv）来自氢同位素分馏的信息作为第三可观察量允许对三个机械端元进行去卷积。这种详细的机理理解将使人们有可能更好地了解在自然和工程降解反应中有毒与无问题产品的形成，并可能有助于制定更好的污染场地补救战略。

Chlorinated ethylenes are prevalent groundwater contaminants, which may undergo reductive dehalogenation. Currently, mechanistic insight is incomplete, product formation little understood, and available insight is difficult to transfer from lab to field. To bridge this gap with isotope effects of multiple elements, in the past period we have (a) firmly established compound-specific 37Cl/35Cl besides 13C/12C isotope analysis and explored the first dual element isotope fractionation patterns of chlorinated ethylenes in (b) microbial degradation, (c) dehalogenation by Fe(0) and (d) reactions with chemical model reactants (Vitamin B12 - the cofactor of all dehalogenases). Based on characteristically different patterns in dual element isotope fractionation (Cl vs. C), our results give a first hot lead that two or more mechanisms are at work in microbial reductive dehalogenation. In the second period I therefore aim to pinpoint the mechanisms behind these differences with (a) targeted experiments involving carefully chosen model reactants and (b) measuring besides carbon and chlorine also hydrogen isotope effects. The approach pillars on the hypotheses (i) that two pathways which were previously considered distinct - nucleophilic substitution and nucleophilic addition of Vitamin B12 - share the same initial step; (ii) that they may be deconvolved in experiments at different pH; (iii) that single electron transfer to chlorinated ethylenes can be simulated both in water and in organic solvents; (iv) that information from hydrogen isotope fractionation as third observable allows deconvolving the three mechanistic endmembers. Such detailed mechanistic understanding will make it possible to better understand the formation of toxic vs. non-problematic product in natural and engineered degradation reactions and may help developing better strategies for remediation of contaminated sites.