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，可发生还原脱卤。目前，对机理的认识是不完整的，对产品形成的认识很少，而且现有的认识很难从实验室转移到现场。为了利用多元素的同位素效应弥补这一空白，在过去的一段时间里，我们(a)除了13C/12C同位素分析外，还建立了化合物特异性的37Cl/35Cl，并探索了氯化乙烯在(b)微生物降解、(c)铁(0)脱卤和(d)与化学模型反应物（维生素B12 -所有脱卤酶的辅助因子）反应中的第一个双元素同位素分异模式。基于双元素同位素分馏（Cl vs. C）的特征不同模式，我们的研究结果首次给出了两种或更多机制在微生物还原脱卤中起作用的线索。因此，在第二阶段，我的目标是查明这些差异背后的机制，包括(a)有针对性的实验，涉及精心选择的模型反应物，以及(b)测量碳和氯之外的氢同位素效应。该方法基于以下假设：(i)之前被认为不同的两种途径——亲核取代和维生素B12的亲核加成——共享相同的初始步骤；（ii）它们可以在不同pH值的实验中反卷积；（iii）在水中和有机溶剂中均可模拟向氯化乙烯的单电子转移；（iv）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对三个机械端元进行反卷积。这种详细的机制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与无问题产物的形成，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.