Subsequently, the products obtained are analysed using a high-resolution mass spectrometer (HRMS) coupled with a high-performance liquid chromatograph (HPLC). With this aim, here we show experimental results on the formation of nucleobases under astrophysically relevant conditions, whereby a mixture of simple molecules (H 2O:CO:NH 3:CH 3OH = 5:2:2:2) is exposed to UV photons on a reaction substrate at 10 K. 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. 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, 22, 23, 24, from a mixture of abundant molecules in interstellar ices remains controversial. 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. By contrast, although several studies reported the prebiotic synthesis of nucleobases from formamide (NH 2CHO) 17, 18, ammonium cyanide (NH 4CN) 2 and urea (CO(NH 2) 2) 19, 20 under relatively warm conditions (i.e. This evidence suggests that these processes might be involved in the prebiotic synthesis of complex organic molecules in space 15, 16. protons and electrons) followed by heating to room temperature 8, 9, 10, 11, 12, 13, 14, 15. Moreover, larger, more complex organic molecules such as amino acids and sugars can be produced from a mixture of those simpler molecules after exposure to ultraviolet (UV) photons and cosmic ray analogues (e.g. As a result of these studies, it is known that many abundant molecules in interstellar ices such as H 2O, CH 3OH and NH 3 can be effectively produced by non-energetic surface reactions that occur in the typical conditions found in molecular clouds 7. A number of laboratory studies have been performed to gain a better understanding of the molecular evolution that takes place during the process of formation of stars ( T > 100 K) from molecular clouds ( T ~ 10 K). Given that the exogenous delivery of organic molecules to Earth by meteorites and comets during the late heavy bombardment period before 3.8 billion years ago might have played a role in constraining the breadth of the initial inventory of organic molecules present on the early Earth 6, understanding the formation process of extraterrestrial molecules is of special importance for deciphering the chemical evolution before the birth of life on Earth. The formation of nucleobases can also take place even in extraterrestrial environments, as evidenced by the detection of these species in carbonaceous meteorites 4, 5. From the viewpoint of origins of life on the Earth, therefore, they have been the target for laboratory experiments on the prebiotic synthesis in terrestrial environments 1, 2, 3. Nucleobases play an essential role in the biology of terrestrial organisms since they are the basic units used to record genetic information.
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