[U-12] Biogeochemistry of CO world

[U12-P03] Abiotic synthesis of nucleosides by the solar high energy particle irradiation in the surface and atmosphere of celestial bodies

*Koharu Fukazawa¹, Tomoki Kimura¹, Kosuke Fukui¹, Yayoi Yoshimura¹ (1.Tokyo University of Science)

Keywords:origin of life, ribonucleic acid (RNA), nucleotide, abiotic synthesis

Ribonucleic acid (RNA) is one of the key molecules for elucidating the emergence process of life, but its origin is still unknown. In particular, the hypothesis has been proposed that organic materials such as nucleotides, the building blocks of RNA, were synthesized abiotically from the atmosphere and interstellar matter [Oparin, 1924]. Verifying the hypothesis is important for understanding the environment in which life was born. Nucleotides consist of ribose, phosphate, and nucleobases (e.g., adenine), which the previous studies tried to synthesize abiotically. For example, nucleobases have been synthesized by irradiating gas samples that model the atmospheres of the early Earth or Titan with spark discharges modeling lightning or with protons, X-rays, and electrons modeling cosmic rays [e.g. Ferus et al., 2017; Pilling et al., 2009]. For experiments on nucleotide synthesis on the early Earth [Ponnamperuma et al. 1963], when samples of a mixture of adenine and ribose with phosphoric acid or ethyl metaphosphate were irradiated with up to 10 J of ultraviolet photon, adenosine monophosphate (AMP), a nucleotide, was not detected in the sample with phosphoric acid, but detected in the sample with ethyl metaphosphate. A laboratory modeling of a small body environment in space reported that AMP was synthesized by irradiating adenosine, a nucleoside, and sodium dihydrogen phosphate with 2 MeV protons [Simakov et al., 2002]. On the other hand, however, adenosine and AMP have not been detected in the Murchison meteorite [e.g., George et al. 1992; Koga et al. 2023], which was found to contain adenine, ribose, and phosphate. The necessary conditions for abiotic synthesis of nucleotides in space and planetary environments are not comprehensively understood yet.

To comprehensively understand the abiotic synthesis of nucleotides in space and planetary environments, we performed experiments in which samples of different types of phosphate compounds added to adenine and ribose were irradiated with high-energy hydrogens modeling the solar particles. A powdered sample of adenine, ribose, and sodium dihydrogen phosphate or methylphosphonic acid mixed at a rate of 1:1:1 mol was irradiated with 10 keV molecular hydrogen ions. The samples were dissolved in water and subjected to high performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). The analysis results showed that irradiation produced three structural isomers of adenosine in the sample containing sodium dihydrogen phosphate, but AMP was not detected. In the sample containing methylphosphonic acid, the same three structural isomers, and an unknown product were identified, but also no AMP was detected. This is different from the results of Simakov et al. [2002], where AMPs were produced by proton irradiation of adenosine and sodium dihydrogen phosphate. This can be attributed to differences in the particle energy and flux of proton irradiation and the use of adenine and adenosine as material substances. Our results suggest that in an environment where adenine, ribose, and phosphate exist in solid form, solar high-energy particle irradiation produces a structural isomer of adenosine, while AMP is not synthesized. We are now estimating the molecular formula of the obtained unknown product in the methylphosphonate-added samples based on LC-MS. We are going to also conduct similar irradiation experiments with a new sample with metaphosphoric acid to compare the irradiation products corresponding to each phosphoric acid compound. In this presentation, the current status of our research will be reported.