A combined experimental and theoretical study on the formation of the amino acid glycine (NH2CH2COOH) and its isomer (CH3NHCOOH) in extraterrestrial ices
A combined experimental and theoretical study on the formation of the amino acid glycine (NH2CH2COOH) and its isomer (CH3NHCOOH) in extraterrestrial ices
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DOI：
10.1086/430106
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发表时间：
2005-06-20
影响因子：
4.9
通讯作者：
Kaiser, RI
中科院分区：
文献类型：
--
作者：
Holtom, PD;Bennett, CJ;Kaiser, RI
我们已经研究了最简单的氨基酸，甘氨酸，由银河宇宙射线粒子在外星冰的合成。实验室实验和电子结构计算表明，甲胺分子[CH_3NH_2（X（1）A '）]可以通过与宇宙射线粒子轨道上的高能电子相互作用解离成氢原子和自由基CH_2NH_2（X（2）A '）和CH_3NH（X（2）A '）。具有足够动能的氢原子可以克服入口势垒与二氧化碳分子[CO2（X（1）Sigma（g）（+））]加成，产生反式羟基羰基自由基HOCO（X（2）A '）。具有正确几何取向的相邻基团然后重组形成甘氨酸，NH2CH2COOH（X（1）A），以及其异构体，CH 3NHCOOH（X（1）A）。这些发现首次揭示了最简单的氨基酸甘氨酸及其异构体如何通过星际和彗星冰中的非平衡化学合成的详细反应机制。我们的研究结果提供了一个重要的替代水和光子诱导形成的氨基酸在彗星和分子云。这些结果还预测星际介质中存在迄今为止未检测到的甘氨酸异构体，表明应该在土星卫星泰坦上观察到甘氨酸，并有助于解释默奇森和奥尔盖尔陨石中更复杂的氨基酸的合成。
We have investigated the synthesis of the simplest amino acid, glycine, by Galactic cosmic-ray particles in extraterrestrial ices. Laboratory experiments combined with electronic structure calculations showed that a methylamine molecule [CH3NH2(X(1)A ')] can be dissociated through interaction with energetic electrons in the track of a cosmic-ray particle to form atomic hydrogen and the radicals CH2NH2(X(2)A ') and CH3NH(X(2)A '). Hydrogen atoms with sufficient kinetic energy could overcome the entrance barrier to add to a carbon dioxide molecule [CO2(X(1)Sigma(g)(+))], yielding a trans-hydroxycarbonyl radical, HOCO(X(2)A '). Neighboring radicals with the correct geometric orientation then recombine to form glycine, NH2CH2COOH(X(1)A), and also its isomer, CH3NHCOOH(X(1)A). These findings expose for the first time detailed reaction mechanisms of how the simplest amino acid glycine and its isomer can be synthesized via nonequilibrium chemistry in interstellar and cometary ices. Our results offer an important alternative to aqueous and photon-induced formation of amino acids in comets and in molecular clouds. These results also predict the existence of a hitherto undetected isomer of glycine in the interstellar medium, suggest that glycine should be observable on Saturn's moon Titan, and help to account for the synthesis of more complex amino acids in the Murchison and Orgueil meteorites.