Thymine and Other Prebiotic Molecules Produced from the Ultraviolet Photo-Irradiation of Pyrimidine in Simple Astrophysical Ice Analogs

Abstract

The informational subunits of RNA or DNA consist of substituted N-heterocyclic compounds that fall into two groups: those based on purine (C₅H₄N₄) (adenine and guanine) and those based on pyrimidine (C₄H₄N₂) (uracil, cytosine, and thymine). Although not yet detected in the interstellar medium, N-heterocycles, including the nucleobase uracil, have been reported in carbonaceous chondrites. Recent laboratory experiments and ab initio calculations have shown that the irradiation of pyrimidine in ices containing H₂O, NH₃, or both leads to the abiotic production of substituted pyrimidines, including the nucleobases uracil and cytosine. In this work, we studied the methylation and oxidation of pyrimidine in CH₃OH:pyrimidine, H₂O:CH₃OH:pyrimidine, CH₄:pyrimidine, and H₂O:CH₄:pyrimidine ices irradiated with UV photons under astrophysically relevant conditions. The nucleobase thymine was detected in the residues from some of the mixtures. Our results suggest that the abundance of abiotic thymine produced by ice photolysis and delivered to the early Earth may have been significantly lower than that of uracil. Insofar as the delivery of extraterrestrial molecules was important for early biological chemistry on early Earth, these results suggest that there was more uracil than thymine available for emergent life, a scenario consistent with the RNA world hypothesis.

The Photochemistry of Pyrimidine in Realistic Astrophysical Ices and the Production of Nucleobases

“…The gas mixtures used were prepared in a glass line evacuated to ∼10 −6 mbar by mixing H 2 O vapor, CH 3 OH vapor, CH 4 gas, NH 3 gas, and pyrimidine vapor in 2.1 L glass bulbs. Purities of these compounds are given elsewhere (Nuevo et al 2009(Nuevo et al , 2012Materese et al 2013). Ratios between the components of these mixtures were determined by their partial pressure in the bulb (±0.05 mbar).…”

“…The chromatographic analyses of the residues obtained in this study follow the same protocol used for our earlier study of UV irradiation of pyrimidine in H 2 O-rich ices (Nuevo et al 2009(Nuevo et al , 2012Materese et al 2013). Briefly, 100 μL of each sample was dried under vacuum for 2 hr and derivatized with MTBSTFA.…”

“…Briefly, 100 μL of each sample was dried under vacuum for 2 hr and derivatized with MTBSTFA. Then 1 μL of the derivatized fraction of each residue was injected into a gas chromatograph coupled to a mass spectrometer (GC-MS) equipped with the same Restek Rxi-5ms column and applying the same temperature gradient as for our previous studies (Nuevo et al 2009(Nuevo et al , 2012Materese et al 2013). This column and protocol are optimized to search for the presence of small aromatic photoproducts containing OH and NH 2 groups, including the nucleobases uracil, cytosine, and thymine.…”

“…The addition of PAHs to these ices leads to the formation of aromatic molecules that contain a wide range of functional groups on their peripheral carbon atoms (Bernstein et al 1999(Bernstein et al , 2002b. Similarly, the addition of pyrimidine to H 2 O-rich binary ice mixtures that contain NH 3 , CH 3 OH, or CH 4 leads to the formation of the nucleobases uracil, cytosine, and thymine (Nuevo et al 2009(Nuevo et al , 2012Materese et al 2013). In these experiments, thymine was only formed in much smaller quantities, and only when using UV photon doses three times higher than those to form uracil and cytosine.…”

“…In these experiments, thymine was only formed in much smaller quantities, and only when using UV photon doses three times higher than those to form uracil and cytosine. The low level of thymine production may be a result of the larger number of chemical substitutions required for its formation relative to uracil and cytosine (Materese et al 2013). In addition, it also appears that methyl group addition does not compete well with oxidation (Sandford et al 2014).…”