建構生命的重要元件是來自宇宙深處嗎？
所有生命皆需要細胞和能量來複製。地球上的有機生命體如果沒有執行這些功能的基本元件，便無法進行複製而完全無法存在。
67P/楚留莫夫－格拉希門克彗星。圖片來源：ESA/Rosetta/NAVCAM
科學家目前對組成生命元件的重要成分之一――磷酸鹽的瞭解還是不多。不過，美國夏威夷大學馬諾阿分校的研究人員跟法國和台灣的科學家合作之下，得出了新的有力證據顯示磷酸鹽是在外太空生成，並在地球形成的最初十億年內搭乘隕石或彗星來到地球。之後含磷化物成為所有地球生物的細胞中可以發現的生物分子之一。
這篇突破性的研究發表在九月號的《自然通訊》(Nature
Communications)，標題為「一種在星際空間合成磷的含氧酸的方法」(“An Interstellar Synthesis of
Phosphorus Oxoacids.”)，作者為夏威夷大學馬諾阿分校的研究生Andrew
Turner(現為美國派克維爾大學的助理教授)以及化學教授Ralf
Kaiser。
研究指出磷酸鹽和焦磷酸是分子生物學基本元件中的兩種重要零件。它們是染色體的主要成分，其中的DNA攜帶了遺傳訊息。連同細胞膜裡的磷脂，以及在細胞中負責運送能量的三磷酸腺苷，這些零件組裝成所有有機生命體進行自我複製時所需的材料。
在美國夏威夷大學馬諾阿分校的W.M.凱克天文化學實驗室，團隊把超高真空艙冷卻到絕對溫度5度(-268.15℃)，並在裡面複製出星際空間中的冰晶微粒。冰晶表面覆有低溫分子雲中常見的二氧化碳和水，另外還有磷化氫。當團隊利用高能電子形成的游離輻射來模擬太空中的宇宙射線照射冰晶，發生的非平衡反應可以合成許多種磷的含氧酸，像是磷酸和焦磷酸。
「磷化氫對地球上的所有生物來說都相當致命。」主要作者Turner表示。「但在星際介質中，磷化氫的特殊化學性質可以促成罕見的化學反應途徑，使生物相關的分子得以形成，比方說磷的含氧酸。這些分子最後或許可以點燃分子演化使我們所知的生命形成。」
Kaiser補充：「我們實驗用的精密分析技術結合了雷射、質譜儀、氣相層析法，結果偵測出磷的含氧酸。科學家認為67P/楚留莫夫－格拉希門克彗星的磷來源為磷化氫，因此這類彗星的冰晶中或許也會形成磷的含氧酸。」Kaiser表示他們的技術也可以用來偵測極為微量的爆炸物和藥物。
法國尼斯大學的Cornelia
Meinert表示：「由於彗星含有的物質至少有一部份是原行星盤形成太陽系後殘留下來的，因此磷化合物的來源或許可以追溯至星際介質中的冰晶含有足夠磷化氫進行反應的時候。」
磷的含氧酸搭乘隕石或彗星來到地球之後，或許參與了有生命之前的含磷化學反應。因此如果想要解開在生命出現之前溶於水的磷化合物如何形成，以及它們如何變成地球、甚至可能是宇宙其他生物體的構成要素之一，了解磷的含氧酸的簡易合成過程是必要的。
跟Turner和Kaiser一起進行這項計畫的還有Meinert以及台灣東華大學的張秀華。
Did key building blocks for life come
from deep space?
All living beings need cells and energy
to replicate. Without these fundamental building blocks, living organisms on
Earth would not be able to reproduce and would simply not exist.
Little was known about a key element in the building
blocks, phosphates, until now. University of Hawaiʻi at Mānoa researchers, in
collaboration with colleagues in France and Taiwan, provide compelling new
evidence that this component for life was found to be generated in outer space
and delivered to Earth in its first one billion years by meteorites or comets.
The phosphorus compounds were then incorporated in biomolecules found in cells
in living beings on Earth.
The breakthrough research is outlined in “An
Interstellar Synthesis of Phosphorus Oxoacids,” authored by UH Mānoa graduate
student Andrew Turner, now assistant professor at the University of Pikeville,
and UH Mānoa chemistry Professor Ralf Kaiser, in the September issue of Nature Communications.
According to the study, phosphates and diphosphoric
acid are two major elements that are essential for these building blocks in
molecular biology. They are the main constituents of chromosomes, the carriers
of genetic information in which DNA is found. Together with phospholipids in
cell membranes and adenosine triphosphate, which function as energy carriers in
cells, they form self-replicating material present in all living organisms.
In an ultra-high vacuum chamber cooled down to 5 K
(-450°F) in the W.M. Keck Research Laboratory in Astrochemistry at UH Mānoa,
the Hawaiʻi team replicated interstellar icy grains coated with carbon dioxide
and water, which are ubiquitous in cold molecular clouds, and phosphine. When
exposed to ionizing radiation in the form of high-energy electrons to simulate
the cosmic rays in space, multiple phosphorus oxoacids like phosphoric acid and
diphosphoric acid were synthesized via non-equilibrium reactions.
“On Earth, phosphine is lethal to living beings,”
said Turner, lead author. “But in the interstellar medium, an exotic phosphine
chemistry can promote rare chemical reaction pathways to initiate the formation
of biorelevant molecules such as oxoacids of phosphorus, which eventually might
spark the molecular evolution of life as we know it.”
Kaiser added, “The phosphorus oxoacids detected in
our experiments by combination of sophisticated analytics involving lasers,
coupled to mass spectrometers along with gas chromatographs, might have also
been formed within the ices of comets such as 67P/Churyumov-Gerasimenko, which
contains a phosphorus source believed to derive from phosphine.” Kaiser says
these techniques can also be used to detect trace amounts of explosives and
drugs.
“Since comets contain at least partially the remnants
of the material of the protoplanetary disk that formed our solar system, these
compounds might be traced back to the interstellar medium wherever sufficient
phosphine in interstellar ices is available," said Cornelia Meinert of the
University of Nice (France).
Upon delivery to Earth by meteorites or comets, these
phosphorus oxoacids might have been available for Earth’s prebiotic phosphorus
chemistry. Hence an understanding of the facile synthesis of these oxoacids is
essential to untangle the origin of water-soluble prebiotic phosphorus
compounds and how they might have been incorporated into organisms not only on Earth,
but potentially in our universe as well.
Turner and Kaiser worked with Meinert and Agnes Chang
of National Dong Hwa University (Taiwan) on this project.
原始論文：Andrew
M. Turner, Alexandre Bergantini, Matthew J. Abplanalp, Cheng Zhu, Sándor Góbi, Bing-Jian
Sun, Kang-Heng Chao, Agnes H. H. Chang, Cornelia Meinert, Ralf I. Kaiser. An
interstellar synthesis of phosphorus oxoacids. Nature Communications, 2018; 9
(1) DOI: 10.1038/s41467-018-06415-7
引用自：University
of Hawaii at Manoa. "Did key building blocks for life come from deep
space?"
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