Nucleobase synthesis in interstellar ices

Abstract

The synthesis of nucleobases in natural environments, especially in interstellar molecular clouds, is the focus of a long-standing debate regarding prebiotic chemical evolution. Here we report the simultaneous detection of all three pyrimidine (cytosine, uracil and thymine) and three purine nucleobases (adenine, xanthine and hypoxanthine) in interstellar ice analogues composed of simple molecules including H₂O, CO, NH₃ and CH₃OH after exposure to ultraviolet photons followed by thermal processes, that is, in conditions that simulate the chemical processes accompanying star formation from molecular clouds. Photolysis of primitive gas molecules at 10 K might be one of the key steps in the production of nucleobases. The present results strongly suggest that the evolution from molecular clouds to stars and planets provides a suitable environment for nucleobase synthesis in space.

Fig. 1

Names and structures of the products detected (except guanine) in the organic residues. The numbers in parentheses are used to identify the relevant compounds in Table 1, where the yield of each species is reported

Fig. 2

Identification of uracil in the organic residues. Mass chromatograms of a the uracil standard, b the analyte sample and c the co-injected mixture of uracil standard and analyte sample at a mass-to-charge ratio (m/z) of 113.0346. A C18 separation column was used for the analysis by HPLC/HRMS. The solid arrow indicates the presence of uracil. The inset shows an enlarged spectrum from 0 to 15 min. The intensity of the peak at ~8.6 min, which is associated with uracil (panel a), is higher in the co-injected sample (panel c) than in the analyte-only sample (panel b)

Fig. 3

Identification of hypoxanthine in the organic residues. Mass chromatograms of a the hypoxanthine standard, b the analyte sample and c the co-injected mixture of hypoxanthine standard and analyte sample at a mass-to-charge ratio (m/z) of 137.0458. A C18 separation column was used for the analysis by HPLC/HRMS. The solid arrow indicates the presence of hypoxanthine. The intensity of the peak at ~11.3 min, which is associated with hypoxanthine (panel a), is higher in the co-injected sample (panel c) than in the analyte-only sample (panel b)

Fig. 4

Identification of thymine in the isotopically labelled organic residues. Mass spectra of thymine observed in the a ¹⁵N- and b D-substituted samples at m/z from 129.02 to 129.1 and from 128.02 to 128.10, respectively. Red coloured peaks at the m/z of 129.0441 in panel (a) and at the m/z of 128.0564 in panel (b) correspond to the ¹⁵N-substituted (¹⁵N–) thymine (C₅H₆¹⁵N₂O₂: the m/z of the protonated ion is 129.0443) and the singly deuterated (d₁–) thymine (C₅H₅DN₂O₂: the m/z of the protonated ion is 128.0565), respectively. Please see the Methods section and the Supplementary Figs. 21–29 for further details on the ¹⁵N- and D-isotope probing experiments