Meierhenrich Uwe J, Thiemann Wolfram H P
University of Bremen, Dept. Physical Chemistry, FB 02, Leohener Str., D-28359 Bremen, Germany.
Orig Life Evol Biosph. 2004 Feb;34(1-2):111-21. doi: 10.1023/b:orig.0000009832.71546.1d.
Biopolymers like DNA and proteins are strongly selective towards the chirality of their monomer units. The use of homochiral monomers is regarded as essential for the construction and function of biopolymers; the emergence of the molecular asymmetry is therefore considered as a fundamental step in Chemical Evolution. This work focuses on physicochemical mechanisms for the origin of biomolecular asymmetry. Very recently two groups, one from Allamandola at NASA Ames and the other from our Inter-European team, demonstrated simultaneously the spontaneous photoformation of a variety of chiral amino acid structures under simulated interstellar conditions. Since both groups used unpolarized light for the photoreaction the obtained amino acids turned out racemic as expected. The obtained experimental data support the assumption that tiny ice grains can furthermore play host to important asymmetric reactions when irradiated by interstellar circularly polarized ultraviolet light. It is possible that such ice grains could have become incorporated into the early cloud that formed our Solar System and ended up on Earth, assisting life to start. Several lines of evidence suggest that some of the building blocks of life were delivered to the primitive Earth via (micro-) meteoroids and/or comets. These results suggest that asymmetric interstellar photochemistry may have played a significant part in supplying Earth with some of the enantioenriched organic materials needed to trigger life. The search for the origin of biomolecular homochirality leads to a strong interest in the fields of asymmetric photochemistry with special emphasis on absolute asymmetric synthesis. We outline here the theoretical background on asymmetric interstellar ice photochemistry, summarize recent concepts and advances in the field, and discuss briefly its implications. The obtained data are crucial for the design of the enantioselective COSAC GC-MS experiment onboard the ROSETTA spacecraft to a comet to be launched in the very near future.
像DNA和蛋白质这样的生物聚合物对其单体单元的手性具有很强的选择性。使用纯手性单体被认为是生物聚合物构建和功能的关键；因此，分子不对称性的出现被视为化学进化中的一个基本步骤。这项工作聚焦于生物分子不对称性起源的物理化学机制。最近，两个研究小组，一个来自美国国家航空航天局艾姆斯研究中心的阿拉曼多拉团队，另一个来自我们的欧洲跨团队，同时证明了在模拟星际条件下各种手性氨基酸结构的自发光形成。由于两个小组在光反应中都使用了非偏振光，所以得到的氨基酸如预期那样是外消旋的。获得的实验数据支持这样一种假设，即微小的冰粒在受到星际圆偏振紫外光照射时，还能成为重要不对称反应的宿主。这些冰粒有可能被纳入形成我们太阳系并最终落到地球上的早期星云，从而助力生命的起源。几条证据线索表明，一些生命的构成要素是通过（微）流星体和/或彗星被输送到原始地球的。这些结果表明，不对称星际光化学可能在为地球提供引发生命所需的一些对映体富集有机物质方面发挥了重要作用。对生物分子同手性起源的探索引发了人们对不对称光化学领域的浓厚兴趣，特别强调绝对不对称合成。我们在此概述不对称星际冰光化学的理论背景，总结该领域的最新概念和进展，并简要讨论其意义。所获得的数据对于在不久的将来发射的前往彗星的罗塞塔号航天器上进行的对映选择性气相色谱 - 质谱联用实验（COSAC GC - MS）的设计至关重要。
