Identification of nucleobases in extraterrestrial carbonaceous chondrites, such as Murchison indicates their formation in an abiotic condition, and supports their role in prebiotic role in early Earth. Physicochemical processes by which these complex molecules are synthesized in icy grains are not well understood. The products of UV photo-irradiation of purine and pyrimidine in H2O, NH3 and CH4 ices have been explored using fancy new density functional theory (DFT) methods and compared with laboratory experimental results. Mechanisms studied include those starting with neutral pyrimidine and purine, and their cationic counterparts, and then reacting with neutrals and radicals generated by radiation. Reaction mechanisms that involved cations on the purine or pyrimidine proved to be the ones that are most important. The calculations reveal that the formation of nucleobases is energetically and kinetically favorable. The gas phase mechanism of their formation proved ineffective, and the presence of one or several water molecules is necessary in order for the final products to form. Explicit solvent calculations using a polarized continuum model (PCM) established the effect of the ice matrix and product formation preferences. Uracil forms easily as oxidation is rather easy but the scope of thymine formation is limited due to the inefficiency of the methylation of pyrimidine, and its’ oxidized derivatives, in a mixed ice. Amine group addition to purine leading to adenine and guanine in mixed NH3 and H2O ices is facile. Many of the photoproducts in UV-irradiated H2O and pyrimidine ice mixtures are found in an experimental study. The results support the scenario in which prebiotic molecules, such as the nucleobase uracil, can be formed under abiotic processes in astrophysically relevant interstellar environments, and on surfaces of icy grains before being delivered to telluric planets such as Earth. But, it constrains the formation of thymine as well as its role in the origin of life.

Nucleobase synthesis via UV-induced oxidation of their precursors in astrophysical ices: A quantum chemical perspective

253rd National Meeting (2017)

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