The Photochemistry of Purine in Ice Analogs Relevant to Dense Interstellar Clouds

Abstract

The aromatic nitrogen heterocyclic compound purine is the core structural framework of many important biomolecules, particularly nucleobases. Purine and purine derivatives have been observed in carbonaceous chondrites, and it has been hypothesized that the exogenous delivery of these compounds, along with many other biologically relevant compounds, may have played a role in the emergence of life. Numerous experiments in our laboratory have demonstrated that the nucleobases used by life to encode genetic material could have been produced abiotically under astrophysically relevant conditions. Specifically, the UV photoprocessing of pyrimidine and purine in simple ices of astrophysical interest has resulted in the production of all five biological nucleobases, namely, uracil (RNA), cytosine (RNA and DNA), thymine (DNA), adenine (RNA and DNA), and guanine (RNA and DNA). Additionally, follow-up work has examined the photochemistry of pyrimidine in more complex astrophysical ice mixtures to better understand the formation of these compounds under realistic conditions. In this work, we examine the photochemistry of purine in more complex ices of astrophysical interest and compare our results with those from simpler ice mixtures. We also examine the effects of competing parallel synthesis of organic compounds in the ices (unrelated to purine). Finally, we discuss the astrophysical and astrobiological implications of our findings.

Spectroscopic identification of fragment ions of DNA/RNA building blocks: the case of pyrimidine

“…40 Related photon processing of H 2 O:NH 3 :purine ices results in the formation of adenine, guanine, and other substituted purine derivatives. 41,47 Interestingly, apart from photon irradiation of simple H 2 O:Pym ices leading to thymine formation, all other photon processing results in the production of RNA nucleobases and thus supports the hypothesis of RNA being the first type of genetic material used by early life. 48 The Pym nucleobases inherit certain properties like their geometry and core and valence electronic structure from the parent Pym moiety, 49,50 which renders this N-heterocycle a subject of demanding attention.…”

“…37,38 It has been proposed that such N-containing macromolecules may be synthesized through complex reactions between abundant N-bearing species such as NH 3 or HCN and carbonaceous precursors in interstellar ice grains under the influence of UV photons and cosmic ray analogs (e.g., protons and electrons). [39][40][41][42][43] Recent laboratory experiments under interstellar-like conditions demonstrate the formation of uracil and cytosine upon UV irradiation of H 2 O:Pym and H 2 O:NH 3 :Pym ices, respectively. 44,45 These experiments triggered theoretical investigations to gain further insight into the involved chemical conversion scheme.…”

Nucleobase synthesis in interstellar ices

“…near or above room temperature), there are no reports on the formation of nucleobases from abundant molecules in interstellar ices through the combination of photolysis at astrophysically relevant low temperatures and subsequent thermal processes. In particular, due to the limited number of related studies 14 , the mechanism of formation of nitrogen-containing heterocyclic rings including purine and pyrimidine, which are the basic structures of nucleobases and have been utilised in previous studies for nucleobase production 21–24 , from a mixture of abundant molecules in interstellar ices remains controversial. To understand the prebiotic evolution of biologically important chemical species in molecular clouds, the development of a synthetic pathway mimicking that process would be of pivotal importance.…”

Deoxyribose and deoxysugar derivatives from photoprocessed astrophysical ice analogues and comparison to meteorites

“…Over the last 25 years, laboratory experiments simulating the photo-irradiation or particle bombardment of mixtures of astrophysical ice analogs at low temperatures (10‒80 K) have shown that organic compounds can form under astrophysical, non-biological conditions. In particular, the ultraviolet (UV) irradiation of ice mixtures consisting of H 2 O, CH 3 OH, CO, CO 2 , CH 4 , and/or NH 3 leads to the formation of compounds of astrobiological interest such as amino acids 14–16 , nucleobases 17,18 , functionalized polycyclic aromatic hydrocarbons (PAHs) 19,20 , amphiphiles 21 , as well as urea, hydantoin, and small aldehydes 22–24 . A variety of organic compounds have also been shown to form at 5 K when ice mixtures containing CH 3 OH were UV irradiated or bombarded with energetic electrons 25,26 .…”