Challenging the racemic amino acid content of asteroid Ryugu through the reliable enantioselective analysis of the Orgueil meteorite

Because life evolved as an asymmetric phenomenon, any comprehensive theory of its emergence on Earth must explain the origin of biological homochirality—the predominant presence of specific chiral configurations in biopolymers, such as d-sugars in nucleic acids (DNA and RNA) and l-amino acids in proteins. The systematic reports of an excess of l-amino acids in several carbonaceous meteorites prompted the hypothesis that interstellar chemistry may have played a role in the origin of homochiral biomolecules by biasing the presumably racemic abiotic pool on early Earth through the delivery of extraterrestrial debris containing enantioenriched molecules [1]. Asymmetric photochemistry by circularly polarized light (CPL) constitutes the leading explanation for the observed l-excesses of amino acids in meteorites due to its proved capability of inducing enantiomeric excesses (ee) in amino acids via asymmetric photolysis [2] and the discovery of CP radiation in infrared star-forming regions [3].

However, recent findings concerning the enantioselective analyses of amino acids from asteroid Ryugu [4], and more recently, from asteroid Bennu [5], seem to contradict the CPL scenario. All amino acids found in these pristine samples, which theoretically should be free from terrestrial contamination, have been reported as racemic. We aim to challenge these findings by comparing them with the results of our comprehensive methodology for the reliable enantioselective analyses of extra-terrestrial chiral amino acids. Our study focuses on the Orgueil meteorite, a CI chondrite known to resemble the mineralogy and amino acid composition of the asteroid Ryugu [6]. We propose that the observed racemic contents in Ryugu, and possibly Bennu, could potentially stem from the limited quantity of samples analysed and the resulting constraints in the reliability of the available analytical methodologies. This limitation, especially in accurately quantifying small ees (<5%) [7], as those expected to be produced by CPL asymmetric photolysis [2], could contribute to the reported results.

Robust enantioselective methodologies combining high sensitivity, resolution, and reliable determination of small ees are decisive to understand the distribution and characteristics of chiral compounds in astro-physical samples, as well as comprehending the potential role of symmetry-breaking interstellar processes. In our investigation, we focussed on minimizing degradation and racemization processes throughout all stages of the extraction-chromatographic protocol. Particular attention was devoted towards assessing and reducing the impact of the mineral matrix, as well as minimizing potential contamination sources. Through analysing several fragments of the Orgueil meteorite, we will highlight the critical impact of analyte concentration, enantiomeric resolution, and a rigorous statistical treatment of the chromatographic data to reduce quantification uncertainties when measuring and reporting ees.

References

• [1] Glavin D. P., et al. Rev. 120, 4660–4689 (2020)
• [2] a) Meinert, C., et al. Chem. Int. Ed. 53, 210 (2014); b) Bocková, J., et al. Nat. Commun. 14, 3381 (2023)
• [3] a) Bailey J., et al. Science 281, 672–674 (1998); b) Chrysostomou A., et al. Nature 450, 71–73 (2000), c) Kwon J., et al. J. Lett. 765, L6 (2013).
• [4] a) Naraoka H. et al., Science 379, eabn9033 (2023). b) Parker E.T. et al. Cosmochim. Acta, 347, 42–57 (2023)
• [5] Glavin D. P., et al. 55th Lunar and Planetary Science Conference (LPSC), 1640 (2024)
• [6] Yokoyama T., et al., Science 379, eabn7850 (2023).
• [7] Pepino R., Sep. Sci. 45, 4416–4426 (2022).

Acknowledgement: Supported by the European Research Council under the European Union’s Horizon 2020 research and innovation programme [grant agreement 804144].