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可能导致第一个有机分子的过程(附原文)

已有 951 次阅读 2020-9-18 21:22 |个人分类:新观察|系统分类:科普集锦| 生命的起源, 有机物分子, 二氧化碳

可能导致第一个有机分子的过程(附原文)

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Fig. 1. Proposed mechanism of pH-driven CO2 reduction with H2 across a conducting Fe(Ni)S barrier, and schematic of the reactor. (A) Under alkaline-vent conditions, the oxidation of H2 (Left) is favored by the alkaline pH due to the presence of hydroxide ions (OH) that react exergonically in the production of water. Released electrons would travel across the micrometers- to centimeters-thick Fe(Ni)S network (53) (Center) to the more oxidizing acidic solution on the ocean side. There they meet dissolved CO2 and a relatively high concentration of protons (H+), favoring the production of formic acid (HCOOH) or formate (HCOO). This electrochemical system enables the overall reaction between H2 and CO2, which is not observed under standard reaction conditions. (B) Diagram of the reactor, with embedded micrograph of a reaction run with precipitate at the interface. Further details are provided in the main text and SI Appendix, Fig. S2.

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Fig. 3. Alternative CO2 reduction mechanisms. (A–C) Classical hydrogenations with CO2 permeability (A), H2 permeability (B), or passage of dissociatively adsorbed atomic H (C) are all unlikely: isotopic labeling (Table 1) indicates that the formyl H derives from ocean-side water rather than H2 from the vent side. (D) Localized redox cycling with CO2 and permeability of H+ (either through a pore or anhydrously through the precipitate) are both unlikely, because in our fully hydrated system, H+/D+ would exchange with local vent water, giving a mixed or exchanged isotopic signal that we do not observe. (E) H2 permeability is also an unlikely mechanism here, because H2 oxidation is much less favored on the acidic (ocean) side compared with the alkaline (vent) side, as demonstrated by the pH exploration data in Table 2. More details are provided in the text and SI Appendix, Figs. S22–S27.

孙学军老师在2006年曾经就写过“水是生命之母,氢气是生命之父”的博文,文末指出:

很多人认为,地球曾经存在一个最后普遍性的共同祖先,地球上现存和已经灭绝的所有生物都是自它衍生而来,它被称为最后一个共同祖先(the last universal common ancestor简称LUCA)。原始共同祖先思想是一种朴素唯物主义哲学思辨的产物,可追溯至2000多年前的古希腊哲学家亚里士多德。也是进化论推演的一个产物,在19世纪博物学家和进化论的先驱达尔文那里得到了清晰的表述,即地球上的一切生物都源于一个共同的祖先。但是,为何这样的共同祖先得以诞生?在什么环境下诞生?这个神秘生命始祖到底是什么?这些都还是未解之谜。但是,美国自然史博物馆American Museum of Natural History202098日提供的消息,或许对于解开此谜有所帮助。

美国自然史博物馆牵头,并由美国国家航空航天局(NASA)资助的一项新研究,发现了一种可能是生命起源之前大约40亿年前,在地球上产生第一批有机分子的关键过程。该过程与某些古代水下热液喷口中可能发生的过程相似,也可能与寻找宇宙其他地方的生命有关。这项研究的细节于202098日已经在美国国家科学院学报》(Proceedings of the National Academy of Sciences, PNAS上发表——Reuben Hudson, Ruvan de Graaf, Mari Strandoo Rodin, Aya Ohno, Nick Lane, Shawn E. McGlynn, Yoichi M. A. Yamada, Ryuhei Nakamura, Laura M. Barge, Dieter Braun, Victor Sojo. CO2 reduction driven by a pH gradientPNAS, September 15, 2020; 117(37): 22873-22879.9 DOI: 10.1073/pnas.2002659117. First published September 8, 2020. ( 22873.full.pdf此新研究可能与寻找地球外生命,绿色化学有关。

生物学是由有机分子构成的,有机分子主要来源于通过几种减少CO2固碳途径。其中只有Wood Ljungdahl乙酰辅酶A途径(Wood Ljungdahl acetyl-CoA pathway)总体上是有益的,并且存在于古生菌和细菌中,这使得它与生命起源的研究相关。研究者他们在新沉积的Fe(Ni)沉淀上使用了与地质相关的、类似生命的微流体pH梯度,以证明这一途径的第一步:CO2H2中产生甲酸盐(HCOO-),否则将是不利的。通过将CO2H2分离成酸性和碱性条件,就像它们在早期地球碱性热液喷口中一样,研究者证明了pH驱动的碳固定的温和间接电化学机制与生命的出现、工业和环境化学有关。

地球上的所有生命都是由有机分子构成的,有机分子是由碳原子与氢,氮和氧等其他元素的原子结合而成的化合物。在现代生活中,大多数这些有机分子源自通过几种途径(例如植物的光合作用)减少CO2。但是这些途径中的大多数要么需要细胞的能量才能起作用,要么被认为进化相对较晚。那么,第一个有机分子是如何在生命起源之前出现的呢?

为了解决这个问题,美国缅因州大西洋学院(College of the Atlantic in Maine)的博物馆格斯特纳学者(Museum Gerstner Scholar维克托·索霍(Victor Sojo)和鲁宾·哈德森(Reuben Hudson)设计了一种基于微流体反应器的新颖装置,这是一个很小的独立实验室,可以让科学家们研究流体的行为-在这种情况下,气体-也处于微观态。先前版本的反应器试图将氢气和CO2的气泡在液体中混合,但未发生还原反应,这可能是因为高挥发性氢气在它有机会发生反应之前就逸出了。解决方案来自维克托·索霍和鲁宾·哈德森之间的讨论,后者在日本琦玉的理化学研究所(RIKEN)可持续资源科学中心( RIKEN Center for Sustainable Resource Science in Saitama, Japan)有一个共享实验室。但是,最终反应器还是美国在缅因州鲁宾·哈德森的实验室建造的。

鲁宾·哈德森说:新反应器不是在反应之前使流体中的气体鼓泡,而是使流体由气体本身驱动,因此它们逸出的机会很小。研究人员使用他们的设计将H2CO2结合在一起,产生了一种称为甲酸(HCOOH)的有机分子。该合成过程类似于唯一不需要总能量供应的已知CO2固定途径(CO2-fixation pathway),被称其为Wood-Ljungdahl乙酰-辅酶A途径(Wood-Ljungdahl acetyl-CoA pathway)。反过来,此过程类似于古代海洋热液喷口中可能发生的反应。

维克托·索霍说:此结果(The consequences)远远超出了我们自己的生物圈。” “今天类似的热液系统(hydrothermal systems)可能存在于太阳系的其它地方,最明显的分别是在土星(Saturn)的土卫二(Enceladus)和木星(Jupiter)的木卫二(Europa)中,在整个宇宙的其它水土世界(water-rocky worlds)中也是如此。

美国NASA喷气推进器实验室(NASA's Jet Propulsion Laboratory)的劳里·巴奇(Laurie Barge),也是此项研究的共同作者补充说:了解如何在温和的地质条件下减少CO2对于评估其它世界生命起源的可能性很重要,这有助于理解宇宙中普遍存在或稀有生命。

研究人员在相对温和的条件下,将CO2转化为有机分子,这意味着这些发现也可能与环境化学有关。面对持续的气候危机,人们一直在寻找减少CO2的新方法。

该研究的作者之一,也是日本东京理工学院(Tokyo Institute of Technology)的肖恩·美格林(Shawn E. McGlynn)补充说:本文的结果涉及多个主题:从理解新陈代谢的起源,到支撑地球上氢和碳循环的地球化学,再到绿色化学应用,其中以生物地理为灵感的工作可以帮助促进温和条件下的化学反应。”

这项研究的其他作者除了包括来自美国大西洋学院(College of the Atlantic)的研究人员之外,还有来自日本理化学研究所(RIKEN)、东京工业大学、英国伦敦大学学院(University College London)以及德国慕尼黑路德维希马克西米利安斯大学(Ludwig-Maximilians University in Munich)的研究人员。关于Wood-Ljungdahl途径的介绍,也可以浏览水是生命之母,氢气是生命之父。更多信息请注意浏览原文(22873.full.pdf )或者相关报道

Significance

Biology is built of organic molecules, which originate primarily from the reduction of CO2 through several carbon-fixation pathways. Only one of these—the Wood–Ljungdahl acetyl-CoA pathway—is energetically profitable overall and present in both Archaea and Bacteria, making it relevant to studies of the origin of life. We used geologically pertinent, life-like microfluidic pH gradients across freshly deposited Fe(Ni)S precipitates to demonstrate the first step of this pathway: the otherwise unfavorable production of formate (HCOO) from CO2 and H2. By separating CO2 and H2 into acidic and alkaline conditions—as they would have been in early-Earth alkaline hydrothermal vents—we demonstrate a mild indirect electrochemical mechanism of pH-driven carbon fixation relevant to life’s emergence, industry, and environmental chemistry.

Abstract

All life on Earth is built of organic molecules, so the primordial sources of reduced carbon remain a major open question in studies of the origin of life. A variant of the alkaline-hydrothermal-vent theory for life’s emergence suggests that organics could have been produced by the reduction of CO2 via H2 oxidation, facilitated by geologically sustained pH gradients. The process would be an abiotic analog—and proposed evolutionary predecessor—of the Wood–Ljungdahl acetyl-CoA pathway of modern archaea and bacteria. The first energetic bottleneck of the pathway involves the endergonic reduction of CO2 with H2 to formate (HCOO), which has proven elusive in mild abiotic settings. Here we show the reduction of CO2 with H2 at room temperature under moderate pressures (1.5 bar), driven by microfluidic pH gradients across inorganic Fe(Ni)S precipitates. Isotopic labeling with 13C confirmed formate production. Separately, deuterium (2H) labeling indicated that electron transfer to CO2 does not occur via direct hydrogenation with H2 but instead, freshly deposited Fe(Ni)S precipitates appear to facilitate electron transfer in an electrochemical-cell mechanism with two distinct half-reactions. Decreasing the pH gradient significantly, removing H2, or eliminating the precipitate yielded no detectable product. Our work demonstrates the feasibility of spatially separated yet electrically coupled geochemical reactions as drivers of otherwise endergonic processes. Beyond corroborating the ability of early-Earth alkaline hydrothermal systems to couple carbon reduction to hydrogen oxidation through biologically relevant mechanisms, these results may also be of significance for industrial and environmental applications, where other redox reactions could be facilitated using similarly mild approaches.



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