Photosynthesis and photo-stability of nucleic acids in prebiotic extraterrestrial environments.
Photosynthesis and photo-stability of nucleic acids in prebiotic extraterrestrial environments.
复制标题
前地外环境中核酸的光合作用和光稳定性。
DOI：
10.1007/128_2013_499
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发表时间：
2015
影响因子：
8.6
通讯作者：
M. Nuevo
中科院分区：
文献类型：
--
作者：
S. Sandford;P. Bera;Timothy J. Lee;C. Materese;M. Nuevo
实验室实验表明，具有天体物理学意义的低温冰的紫外光照射导致有机分子的形成，包括对生物学重要的分子，如氨基酸，醌和两亲物。当嘧啶被引入这些冰中时，辐射的产物包括核碱基尿嘧啶、胞嘧啶和胸腺嘧啶，DNA和RNA的信息亚单位，以及它们的一些异构体。这些化合物的形成，已经在实验和理论上进行了研究，需要在嘧啶上连续添加OH，NH +4和CH +4基团。结果表明，H2O冰在核碱基的形成中起着关键作用，作为氧化剂，作为反应可以发生的基质，以及作为催化剂，有助于从中间体化合物中提取质子。由于H2O也是大多数寒冷天体物理环境中最丰富的冰成分，它可能在太空中形成生物相关化合物方面发挥同样的作用。结果还表明，虽然尿嘧啶和胞嘧啶的形成从嘧啶在冰是相当简单的，胸腺嘧啶的形成不是。这主要是由于甲基化是其形成的限制步骤，特别是在富含H2O的冰中，甲基化必须与氧化竞争。胸腺嘧啶的非生物形成相对于尿嘧啶和胞嘧啶的非生物形成的效率相对较低，再加上在陨石中没有检测到胸腺嘧啶的事实，与RNA世界假说并不矛盾。事实上，早期地球缺乏非生物产生的胸腺嘧啶，这可能迫使人们选择RNA世界，其中只需要尿嘧啶和胞嘧啶，而不需要胸腺嘧啶。
Laboratory experiments have shown that the UV photo-irradiation of low-temperature ices of astrophysical interest leads to the formation of organic molecules, including molecules important for biology such as amino acids, quinones, and amphiphiles. When pyrimidine is introduced into these ices, the products of irradiation include the nucleobases uracil, cytosine, and thymine, the informational sub-units of DNA and RNA, as well as some of their isomers. The formation of these compounds, which has been studied both experimentally and theoretically, requires a succession of additions of OH, NH₂, and CH₃groups to pyrimidine. Results show that H₂O ice plays key roles in the formation of the nucleobases, as an oxidant, as a matrix in which reactions can take place, and as a catalyst that assists proton abstraction from intermediate compounds. As H₂O is also the most abundant icy component in most cold astrophysical environments, it probably plays the same roles in space in the formation of biologically relevant compounds. Results also show that although the formation of uracil and cytosine from pyrimidine in ices is fairly straightforward, the formation of thymine is not. This is mostly due to the fact that methylation is a limiting step for its formation, particularly in H₂O-rich ices, where methylation must compete with oxidation. The relative inefficiency of the abiotic formation of thymine to that of uracil and cytosine, together with the fact that thymine has not been detected in meteorites, are not inconsistent with the RNA world hypothesis. Indeed, a lack of abiotically produced thymine delivered to the early Earth may have forced the choice for an RNA world, in which only uracil and cytosine are needed, but not thymine.