The Challenging Detection of Nucleobases from Pre-accretional Astrophysical Ice Analogs

Amino acids, sugars, and nucleobases are considered as the so-called molecular bricks of life, the major subunits of proteins and genetic materials. All three chemical families have been previously detected in meteorites. In dense molecular cloud ice analogs, the formation of a large set of amino acids and sugars (+derivatives) has been observed. In this contribution, we demonstrate that similar ices (H2O:13CH3OH:NH3 ices, 2:1:1) can also lead to the formation of nucleobases. Using combined UPLC-Orbitrap mass spectrometric and UPLC-SRM-triple quadrupole mass spectrometric analyses, we have unambiguously detected cytosine in these primitive, realistic astrophysical ice analogs. Additionally, a huge variety of nucleobase isomers was observed. These results indicate that all central subunits of biochemical materials may have already been present at early stages of chemical evolution of the protosolar nebula, before accretion toward planetesimals. Consequently, the formation of amino acids, sugars, and nucleobases does not necessarily require secondary alteration processes inside meteoritic parent bodies. They might have been supplied from dense molecular cloud ices toward post-accretional objects, such as nonaqueously modified comets, and subsequently delivered onto the early Earth's surface, potentially triggering the emergence of prebiotic chemistry leading to the first living systems.