In nature, abiotically formed amino acids are usually racemic. However, this is not true for the α,α-dialkyl amino acid isovaline (Iva), which has an L-enantiomeric excess in some specimens of carbonaceous meteorites. On the early Earth and Mars, such meteorites were sources of amino acids, including Iva. Therefore, a connection may exist between the possible chiral influence of non-racemic Iva and the origin of biological homochirality. On the surface of a young terrestrial planet, amino acids can be chemically altered in many ways. For example, high temperatures from geothermal heating can lead to racemization. Four billion years ago, active volcanism and volcanic islands provided suitable conditions for such reactions and perhaps even for early microbial life on Earth. In the current study, we investigated the influence of D- and L-Iva on the thermal racemization of L-alanine (L-Ala) and L-2-aminobutyric acid (L-Abu) in a simulated hot volcanic environment. The amino acids were intercalated in the clay mineral calcium montmorillonite (SAz-1). While Iva was resistant to racemization, partial racemization was observed for Ala and Abu after 8 weeks at 150°C. The experimental results – for example, accelerated racemization in the presence of Iva and different influences of the Iva enantiomers – suggest that the amino acid molecules interacted with each other, possibly in hydrogen-bonded dimers. Accelerated racemization of amino acids could have been an obstacle to the development of homochirality. Besides, it is also detrimental to the use of homochirality as a biosignature, for example, in the search for microbial life on Mars.

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异缬氨酸加速氨基酸外消旋化：对同手性和生物印记搜索的可能影响

在自然界中，非生物形成的氨基酸通常是外消旋的。然而，对于 α,α-二烷基氨基酸异缬氨酸 (Iva) 而言，情况并非如此，它在一些碳质陨石样本中具有过量的 L-对映异构体。在早期的地球和火星上，这种陨石是氨基酸的来源，包括伊娃。因此，非外消旋 Iva 的可能手性影响与生物纯手性的起源之间可能存在联系。在年轻的类地行星表面，氨基酸可以通过多种方式发生化学变化。例如，地热加热产生的高温会导致外消旋化。40 亿年前，活跃的火山活动和火山岛为此类反应甚至地球上的早期微生物生命提供了合适的条件。在目前的研究中，我们研究了 D- 和 L-Iva 在模拟热火山环境中对 L-丙氨酸 (L-Ala) 和 L-2-氨基丁酸 (L-Abu) 的热消旋的影响。氨基酸嵌入粘土矿物钙蒙脱石（SAz-1）中。虽然 Iva 对消旋化具有抗性，但在 150°C 下 8 周后观察到 Ala 和 Abu 的部分消旋化。实验结果——例如，在 Iva 存在下加速外消旋化和 Iva 对映异构体的不同影响——表明氨基酸分子彼此相互作用，可能在氢键二聚体中。氨基酸的加速外消旋化可能是纯手性发展的障碍。此外，它也不利于使用纯手性作为生物特征，例如在火星上寻找微生物生命。