Abstract EANA2025-86

About 100 extraterrestrial amino acids have been identified in carbonaceous chondrites, with concentrations going from a few ppm to more than 300 ppm [1]. These molecules have likely been delivered to the surface of the early Earth hence they can have influenced the emergence of life. Several possible synthesis routes have been discussed [2,3]. Isotope compositions points to precursors formed at low temperature before parent body accretion, as they show the largest D-enrichments among organic constituents in carbonaceous chondrites [3]. Nevertheless, the magnitude of this enrichment is variable among classes of chondrites and among individual amino acid in a given chondrite; for instance, δD ranges from 360 ‰ to 3400 ‰ in the Murchison CM2 chondrite [4] whereas it ranges from 1000‰ to 7000‰ in the EET92042 CR2 chondrite [2]. Though this could be interpreted by the occurrence of different parent bodies having accreted precursors from different sources, the observed large isotopic range could result from isotope re- equilibration during the aqueous alteration that affected the parent bodies during a few millions of years after their accretion.

We have subjected six amino acids, including α- and β-alanine, isovaline and γ-aminobutyric acid (GABA) to hydrothermal conditions at 150°C and 4.7 bars for 1 and 10 days in presence of D2O [5]. Products were analyzed by gas chromatography-mass spectrometry (GC-MS) as N-ethoxycarbonyl ethyl esters allowing us to identify the site where D-H isotope exchange occurs [5]. D/H appears to be best preserved in isovaline, with almost not shift in mass spectra. In β-alanine, on the other hand, 4 H-atoms may undergo isotope exchange whereas only one H-atom exhibits isotope exchange in α-alanine. Interestingly, only 2 H-atoms are prone to exchange in GABA, despites its similarity with β-alanine (one more -CH2 group in a straight chain). In any cases, the H atoms at the α-position relative to the carboxylic group are prone to fast exchange. Ab-initio modeling pinpoints the higher acidity of the carbon in α position leading to a site-specific preferential D- H exchange [5]. In contrast, long-chain amino acids and branched-amino acids without α-hydrogen, but with -CH3 groups, should better preserve their D/H than short ones. Our experimental study shows that the carbon skeleton strongly influences the tendency to exchange H-isotopes between D-rich amino acids and D-poor water on the parent body. The different behavior of amino acids under aqueous alteration may partly explain the large range of isotope compositions reported within single meteorites. 

1. Elsila et al. 2016. ACS Central Science, doi : 10.1021/acscentsci.6b00074

2. Elsila et al. 2012. MAPS, doi: 10.1111/j.1945-5100.2012.01415.x

3. Sephton 2014. Treatise in Geochemistry, doi :10.1016/B978-0-08-095975-7.01002-0

4. Pizzarello et al. 2005. Geochim. Cosmochim. Acta, doi:10.1016/j.gca.2004.07.031

5. He et al. 2025. Geochim. Cosmochim. Acta, doi: 10.1016/j.gca.2025.03.017