Combinatorial effects on clumped isotopes and their significance in biogeochemistry
Combinatorial effects on clumped isotopes and their significance in biogeochemistry
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聚集同位素的组合效应及其在生物地球化学中的意义
DOI：
10.1016/j.gca.2015.09.020
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发表时间：
2016
影响因子：
5
通讯作者：
Yeung, Laurence Y.
中科院分区：
文献类型：
--
作者：
Yeung, Laurence Y.
同位素在分子集合中的排列记录了它们的物理和化学历史。聚集同位素分析质疑这些安排，即，稀有同位素结合在一起的频率，这在许多情况下可以通过平衡和/或动力学同位素分馏来解释。然而，纯粹的组合效应，植根于一个封闭系统中配对原子的统计数据，也是相关的，并没有得到很好的理解。在这里，我表明，当两个相同的原子是同一分子上的邻居时，组合同位素效应是最重要的（例如，O2、N2和D-D在CH 4中的聚集）。当一个原子对的两个半体以不同的同位素偏好组装或从不同的水库中抽取时，组合效应导致聚集同位素丰度的耗尽，最有可能在0和-1 ‰之间，尽管对于D-D对，它们可能是-10 ‰或更大。这些损耗与许多小分子的低温平衡凝聚同位素效应具有相似的量级，但符号相反。酶促同位素配对反应可以具有位点特异性同位素分馏因子和原子库，应该表达这类组合同位素效应，尽管它不限于生物反应。化学动力学的同位素效应，这是一个键形成的过渡态，独立出现，并表示有关的稀有同位素丰度的二阶组合效应。异质性（例如，Csingle bond O和Csingle bond H）对直接组合影响不敏感，但次级组合影响明显。一般来说，组合因子和化学动力学因子对于计算和解释动力学控制反应的聚集同位素特征都很重要。我应用这个分析框架，同位素配对反应有关的地球化学的氧，碳，氮循环，可能会受到组合成团的同位素效应。这些同位素特征，表现为直接结合的同位素“团块”或分子的同位素解剖特征，与分子机制有关，并可能最终提供有关环境相关空间尺度上的地球化学循环的额外信息。
The arrangement of isotopes within a collection of molecules records their physical and chemical histories. Clumped-isotope analysis interrogates these arrangements, i.e., how often rare isotopes are bound together, which in many cases can be explained by equilibrium and/or kinetic isotope fractionation. However, purely combinatorial effects, rooted in the statistics of pairing atoms in a closed system, are also relevant, and not well understood. Here, I show that combinatorial isotope effects are most important when two identical atoms are neighbors on the same molecule (e.g., O2, N2, and D–D clumping in CH4). When the two halves of an atom pair are either assembled with different isotopic preferences or drawn from different reservoirs, combinatorial effects cause depletions in clumped-isotope abundance that are most likely between zero and −1‰, although they could potentially be −10‰ or larger for D–D pairs. These depletions are of similar magnitude, but of opposite sign, to low-temperature equilibrium clumped-isotope effects for many small molecules. Enzymatic isotope-pairing reactions, which can have site-specific isotopic fractionation factors and atom reservoirs, should express this class of combinatorial isotope effect, although it is not limited to biological reactions. Chemical-kinetic isotope effects, which are related to a bond-forming transition state, arise independently and express second-order combinatorial effects related to the abundance of the rare isotope. Heteronuclear moeties (e.g., Csingle bondO and Csingle bondH), are insensitive to direct combinatorial influences, but secondary combinatorial influences are evident.In general, both combinatorial and chemical-kinetic factors are important for calculating and interpreting clumped-isotope signatures of kinetically controlled reactions. I apply this analytical framework to isotope-pairing reactions relevant to geochemical oxygen, carbon, and nitrogen cycling that may be influenced by combinatorial clumped-isotope effects. These isotopic signatures, manifest as either directly bound isotope “clumps” or as features of a molecule’s isotopic anatomy, are linked to molecular mechanisms and may eventually provide additional information about biogeochemical cycling on environmentally relevant spatial scales.
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