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

氯代乙烯是地下水中普遍存在的污染物，可进行还原脱卤化。目前，机械论的洞察力还不完全，对产品形成的了解很少，现有的洞察力很难从实验室转移到领域。为了弥补这一差距与多种元素的同位素效应，在过去的一段时间里，我们已经(A)除了13C/12C同位素分析之外，牢固地建立了化合物特有的37Cl/35Cl，并探索了氯化乙烯在(B)微生物降解、(C)Fe(0)脱卤化和(D)与化学模型反应物(维生素B12--所有脱卤酶的辅因子)的反应中的第一个双元素同位素分馏模式。基于双元素同位素分馏(Cl和C)的不同特征模式，我们的结果首次提出了一个热线索，即在微生物还原脱卤化过程中有两种或两种以上的机制在起作用。因此，在第二阶段，我的目标是通过(A)对精心选择的模型反应物进行有针对性的实验，以及(B)除了测量碳和氯之外，还测量氢同位素效应，以查明这些差异背后的机制。该方法的基础是假设(1)以前被认为是不同的两条途径--维生素B12的亲核取代和亲核加成--具有相同的初始步骤；(2)它们可以在不同的pH下在实验中去卷积；(3)可以在水和有机溶剂中模拟到氯化乙烯的单电子转移；(4)来自氢同位素分馏的信息作为第三个可观察到的信息，允许去卷积三个机械末端成员。这种详细的机理理解将使人们有可能更好地理解在自然和工程降解反应中有毒和无问题产品的形成，并可能有助于制定更好的污染场地修复策略。

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.