Irradiation of Pyrimidine in Pure H2O Ice

The detection of nucleobases, the informational subunits of DNA and RNA, in several meteorites suggests that these compounds of biological interest were formed via astrophysical, abiotic processes. This hypothesis is in agreement with recent laboratory studies of irradiation of pyrimidine in H2O-rich ices with vacuum UV photons emitted by an H2-discharge lamp in the 6.9–11.3 eV (110–180 nm) range at low temperature, shown to lead to the abiotic formation of several compounds including the nucleobases uracil, cytosine, and thymine. In this work, we irradiated H2O:pyrimidine ice mixtures under astrophysically relevant conditions (14 K, £ 10- 9 torr) with high-energy UV photons provided by a synchrotron source in three different ranges: the 0th order light (4.1–49.6 eV, 25–300 nm), the He i line (21.2 eV, 58.4 nm), and the He ii line (40.8 eV, 30.4 nm). The photo-destruction of pyrimidine was monitored with IR spectroscopy, and the samples recovered at room tem-perature were analyzed with liquid and gas chromatographies. Uracil and its precursor 4(3H)-pyrimidone were found in all samples, with absolute and relative abundances varying significantly from one sample to another. These results support a scenario in which compounds of biological interest can be formed and survive in environments subjected to high-energy UV radiation fields. Key Words: Pyrimidine—Nucleo-bases—Interstellar ices—Cometary ices—High-energy photons—Molecular processes—Prebiotic chemistry. Astrobiology 14, 119–131. 1