Penning Holger, Cramer Christopher J, Elsner Martin
Institute of Groundwater Ecology, Helmholtz Zentrum München, German Research Center for Environmental Health, Ingolstädter Landstr. 1, 85764 Neuherberg, Germany.
Environ Sci Technol. 2008 Nov 1;42(21):7764-71. doi: 10.1021/es801101c.
Stable isotope fractionation permits quantifying contaminant degradation in the field when the transformation reaction is associated with a consistent isotope enrichment factor epsilon. When interpreted in conjunction with dual isotope plots, isotope fractionation is also particularly useful for elucidating reaction mechanisms. To assess the consistency of epsilon and dual isotope slopes in a two-step reaction, we investigated the abiotic hydrolysis of the herbicide isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea) using a fragmentation method that allows measuring isotope ratios in different parts of the molecule. Carbon and nitrogen position-specific isotope fractionation, as well as slopes in dual isotope plots, varied linearly with rate constants k(obs) depending on the presence of buffers that mediate the initial zwitterion formation. The correlation can be explained by two consecutive reaction steps (zwitterion formation followed by dimethylamine elimination) each of which has a different kinetic isotope effect and may be rate-limiting. Intrinsic isotope effects for both steps, extracted from our kinetic data using a novel theoretical treatment, agree well with values computed from density functional calculations. Our study therefore demonstrates that more variable isotope fractionation may be observed in simple chemical reactions than commonly thought, but that consistent epsilon or dual isotope slopes may nonetheless be encountered in certain molecular fragments.
当转化反应与一致的同位素富集因子ε相关时，稳定同位素分馏能够在现场对污染物降解进行定量。当与双同位素图结合解释时，同位素分馏对于阐明反应机制也特别有用。为了评估两步反应中ε和双同位素斜率的一致性，我们使用一种能够测量分子不同部分同位素比率的碎片化方法，研究了除草剂异丙隆（3-(4-异丙基苯基)-1,1-二甲基脲）的非生物水解。碳和氮的位置特异性同位素分馏以及双同位素图中的斜率，根据介导初始两性离子形成的缓冲液的存在情况，随速率常数k(obs)呈线性变化。这种相关性可以由两个连续的反应步骤（两性离子形成，随后是二甲胺消除）来解释，每个步骤都有不同的动力学同位素效应，且可能是限速步骤。使用一种新颖的理论处理方法从我们的动力学数据中提取的两个步骤的固有同位素效应，与密度泛函计算得出的值非常吻合。因此，我们的研究表明，在简单化学反应中可能会观察到比通常认为的更多变的同位素分馏，但在某些分子片段中仍可能遇到一致的ε或双同位素斜率。
