Geochemical roots of autotrophic carbon fixation:: Hydrothermal experiments in the system citric acid, H2O-(±FeS)-(±NiS)
Geochemical roots of autotrophic carbon fixation:: Hydrothermal experiments in the system citric acid, H2O-(±FeS)-(±NiS)
复制标题
DOI：
10.1016/s0016-7037(01)00674-3
复制
发表时间：
2001-10-01
影响因子：
5
通讯作者：
Yoder, HS
中科院分区：
文献类型：
--
作者：
Cody, GD;Boctor, NZ;Yoder, HS
最近的理论提出，生命起源于原始的热液环境，采用类似于还原柠檬酸循环（RCC）的化学反应作为碳固定的主要途径。据推测，这种化学物质是年轻的、生命前的地球内在地球化学的自然结果。然而，没有实验证据表明存在进入这种循环的自然途径。为此，涉及柠檬酸的水热实验的结果被用作推导这种途径的方法。在柠檬酸-H2O实验中观察到的同催化反应包括在现代代谢系统中发现的许多反应，即，水合-脱水、逆羟醛、脱羧、氢化和异构化反应。在热和水热条件下，三种主要的分解途径可降解柠檬酸。它的结论是酸催化的β-γ脱羧途径，最终导致丙烯和CO2，可能提供最有希望的反应网络逆转在自然水热条件下。在严格的水热条件下，增加压力可加速主要的脱羧反应。通过加入NaOH强制pH的效果表明，脱羧途径甚至在中等pH水平下也能起作用。网络逆转的潜力（丙烯和CO2转化为三羧酸）通过Koch在CO源的存在下用NiS异相催化促进的烯烃（加氢羧化）反应。将（1-壬烯）转化为单羧酸;将甲基丙烯酸转化为二羧酸，甲基琥珀酸;二羧酸即衣康酸转化为三羧酸即氢化乌头酸。还形成了许多令人感兴趣的含硫产物，其可以提供额外的反应。FeS和NiS的内在催化品质也探讨在没有CO的情况下。结果表明，添加NiS具有最小的影响，在产品分布，而添加FeS导致形成氢化和含硫的产品（硫醚）。这些结果指向一个简单的水热氧化还原柠檬酸合成途径，可能提供了一个地球化学的还原柠檬酸循环的燃点。版权所有（C）2001爱思唯尔科技有限公司
Recent theories have proposed that life arose from primitive hydrothermal environments employing chemical reactions analogous to the reductive citrate cycle (RCC) as the primary pathway for carbon fixation. This chemistry is presumed to have developed as a natural consequence of the intrinsic geochemistry of the young, prebiotic, Earth. There has been no experimental evidence, however, demonstrating that there exists a natural pathway into such a cycle. Toward this end, the results of hydrothermal experiments involving citric acid are used as a method of deducing such a pathway. Homocatalytic reactions observed in the citric acid-H2O experiments encompass many of the reactions found in modem metabolic systems, i.e., hydration-dehydration, retro-Aldol, decarboxylation, hydrogenation, and isomerization reactions. Three principal decomposition pathways operate to degrade citric acid under thermal and aquathermal conditions. It is concluded that the acid catalyzed beta gamma decarboxylation pathway, leading ultimately to propene and CO2, may provide the most promise for reaction network reversal under natural hydrothermal conditions. Increased pressure is shown to accelerate the principal decarboxylation reactions under strictly hydrothermal conditions. The effect of forcing the pH via the addition of NaOH reveals that the decarboxylation pathway operates even up to intermediate pH levels. The potential for network reversal (the conversion of propene and CO2 up to a tricarboxylic acid) is demonstrated via the Koch (hydrocarboxylation) reaction promoted heterocatalytically with NiS in the presence of a source of CO. Specifically, an olefin (1-nonene) is converted to a monocarboxylic acid; methacrylic acid is converted to the dicarboxylic acid, methylsuccinic acid; and the dicarboxylic acid, itaconic acid, is converted into the tricarboxylic acid, hydroaconitic acid. A number of interesting sulfur-containing products are also formed that may provide for additional reaction. The intrinsic catalytic qualities of FeS and NiS are also explored in the absence of CO. It was shown that the addition of NiS has a minimal effect in the product distribution, whereas the addition of FeS leads to the formation of hydrogenated and sulfur-containing products (thioethers). These results point to a simple hydrothermal redox pathway for citric acid synthesis that may have provided a geochemical ignition point for the reductive citrate cycle. Copyright (C) 2001 Elsevier Science Ltd.