Carbonaceous chondrites contain amino acids, with variable abundances and isotope compositions between and within carbonaceous chondrites. The parent body processes, and the presence of clay minerals may explain those differences. Here, we experimentally investigate the evolution of 6 amino acids (glycine, β-alanine, α-alanine, 2-aminoisobutyric acid, γ-aminobutyric acid, and isovaline) exposed to hydrothermal conditions in the presence or absence of silicates. We determined the chemical nature and isotopic composition of the organic compounds of the soluble and solid fractions of the residues using X-ray diffraction, spectroscopy, and mass-spectrometry methods. Glycine and α-alanine exhibit a rather high stability, which is consistent with the measured abundances of α-alanine and glycine in chondrites having experienced various degrees of aqueous alteration. In the meantime, the evolution of β-alanine under hydrothermal conditions leads to the formation of a new compound, which likely results from the decarboxylation and deamination of β-alanine followed by recombination. More than 95 % of γ-ABA was transformed into 2-pyrrolidione though self-cyclization during the aqueous alteration. The solid residues of the experiments conducted in the presence of clay minerals contain organic material, with abundances varying depending on the amino acid used for the experiments (TOC isovaline > 2-aminoisobutyric acid > γ-aminobutyric acid > glycine > α-alanine > β-alanine). Clay minerals thus preferentially trap branched amino acids over chained amino acids, likely within their interlayer spaces as suggested by XRD data. The δ13C values of amino acids have not changed significantly during the experiments, even with the presence of silicates. Thus, the δ13C values of amino acids reported in CR and CM chondrites likely relate to synthetic conditions or the origin of their precursors (i.e. inherited from the pre-accretion processes).

中文翻译：

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小行星水蚀变下氨基酸的演变


碳质球粒陨石含有氨基酸，在碳质球粒陨石之间和内部具有不同的丰度和同位素组成。母体过程和粘土矿物的存在可以解释这些差异。在这里，我们实验研究了在存在或不存在硅酸盐的情况下暴露于热液条件下的 6 种氨基酸（甘氨酸、β-丙氨酸、α-丙氨酸、2-氨基异丁酸、γ-氨基丁酸和异缬氨酸）的演变。我们使用 X 射线衍射、光谱学和质谱法测定了残基的可溶性和固体组分的有机化合物的化学性质和同位素组成。甘氨酸和 α-丙氨酸表现出相当高的稳定性，这与在经历了不同程度的水蚀变的球粒陨石中测得的 α-丙氨酸和甘氨酸丰度一致。同时，β-丙氨酸在水热条件下的进化导致新化合物的形成，这可能是由于 β-丙氨酸的脱羧和脱氨反应随后进行重组的结果。超过 95% 的 γ-ABA 在水改变过程中通过自环化转化为 2-吡咯二酮。在粘土矿物存在下进行的实验的固体残留物含有有机物质，丰度根据实验中使用的氨基酸而变化（TOC 异缕氨酸 > 2-氨基异丁酸 > γ-氨基丁酸 > 甘氨酸 > α-丙氨酸 > β-丙氨酸）。因此，粘土矿物优先捕获支链氨基酸而不是链状氨基酸，正如 XRD 数据所示，可能在它们的层间空间内。氨基酸的 δ13C 值在实验过程中没有显着变化，即使存在硅酸盐也是如此。 因此，CR 和 CM 球粒陨石中报告的氨基酸的 δ13C 值可能与合成条件或其前体的来源有关（即从吸积前过程继承）。