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英國化學家13日宣布，他們開發出一個實驗模型，可展示40億年前地球生命如何以一種自我復制的分子形式起源。研究人員在早期地球環境模擬條件下首次合成了一種中間物質，該物質可純化合成RNA所需的核糖和堿基，并最終形成一種類RNA。相關研究發表在當日出版的《自然》雜志上。 眾所周知，脫氧核糖核酸（DNA）和核糖核酸（RNA）是生物體的遺傳分子，都由堿基、核糖和磷酸組成。DNA具有雙螺旋的復雜結構，而單鏈的RNA在結構上更為簡單和牢固。因此有學者認為，DNA過于復雜，不可能在瞬間突然出現。在生命最初產生時，DNA的單鏈“表親”———RNA要先于DNA出現，是地球上最早出現的攜帶遺傳信息的核酸。 不過，這種“RNA第一”理論，在現實論證中卻遇到不少問題。 RNA包括三個主要組成部分：堿基、核糖和磷酸。在傳統思路看來，這三種物質必須分別形成后再以分子的形式相結合才能形成RNA。因此與DNA相比，RNA雖然更為簡單，但仍是一個復雜分子，不大可能自行組裝成功。此前試圖通過化學方式來證明這三種化學物質可以同時產生的努力也均以失敗告終。 但英國曼徹斯特大學化學家給出了不同解釋。由約翰·薩瑟蘭教授領導的研究小組大膽提出，通過模擬地球早期環境中的一系列化學反應，可以合成一種重要的中間物質，進而合成類RNA。 實驗模型表明，從40億年前就已在地球上存在的簡單化學物質中形成RNA的所有成分是完全可能的，他們已合成了RNA的三個組成部分中的兩個，并首次成功合成出一種類RNA。試驗先由一種被稱為羥基乙醛的簡單糖類開始，而后將其與一種氰化物和氨的合成物———單氫胺以及磷酸鹽發生反應，從而產生出一種被稱為2-氨基噁唑的中間物質。研究人員發現，自然界的晝夜溫差可幫助純化2-氨基噁唑，將其變成充裕的前體，這有助于形成新核苷酸分子的糖和堿基蛋白。磷酸鹽的存在和來自太陽的紫外線則促成了合成。 美國分子生物學家杰克?索斯塔克在《自然》雜志的相關評論中，對該研究給予了高度評價,稱這將是多年來通過化學方式解釋生命起源研究的一大進展，代表了多年來“前生物化學”研究所取得的重大進展，前生物化學是研究導致地球生物出現的化學進程的術語。 推薦原始出處： Nature 459, 239-242 (14 May 2009) | doi:10.1038/nature08013 Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions Matthew W. Powner1, Béatrice Gerland1 & John D. Sutherland1 1 School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK At some stage in the origin of life, an informational polymer must have arisen by purely chemical means. According to one version of the 'RNA world' hypothesis1, 2, 3 this polymer was RNA, but attempts to provide experimental support for this have failed4, 5. In particular, although there has been some success demonstrating that 'activated' ribonucleotides can polymerize to form RNA6, 7, it is far from obvious how such ribonucleotides could have formed from their constituent parts (ribose and nucleobases). Ribose is difficult to form selectively8, 9, and the addition of nucleobases to ribose is inefficient in the case of purines10 and does not occur at all in the case of the canonical pyrimidines11. Here we show that activated pyrimidine ribonucleotides can be formed in a short sequence that bypasses free ribose and the nucleobases, and instead proceeds through arabinose amino-oxazoline and anhydronucleoside intermediates. The starting materials for the synthesis—cyanamide, cyanoacetylene, glycolaldehyde, glyceraldehyde and inorganic phosphate—are plausible prebiotic feedstock molecules12, 13, 14, 15, and the conditions of the synthesis are consistent with potential early-Earth geochemical models. Although inorganic phosphate is only incorporated into the nucleotides at a late stage of the sequence, its presence from the start is essential as it controls three reactions in the earlier stages by acting as a general acid/base catalyst, a nucleophilic catalyst, a pH buffer and a chemical buffer. For prebiotic reaction sequences, our results highlight the importance of working with mixed chemical systems in which reactants for a particular reaction step can also control other steps. (責任編輯：Doctor001) |
