A New Mechanism of Guanine-Isomer Formation from Species Previously Observed in the Interstellar Medium

Abstract

Adenine and guanine have been observed in meteor material. Because of this, they are thought to have been formed in the interstellar medium. However, while there exist several theoretical works exploring the mechanisms of formation for adenine, guanine has received only minor attention. In this work, a retrosynthetic analysis strategy was employed to suggest a new mechanism of guanine formation from species previously observed in the interstellar medium: HCN, CCO, HNCNH, and its isomer H 2 NCN. MP2/6-311++G(2d,2p) calculations were used to evaluate the Gibbs free energy of the minimum and transition states involved in a simple reaction mechanism, involving only five steps. The intermediate species are highly polar (dipole moment range of 4.35−7.72 D), which make them candidates for observation in the rotational spectra. In terms of energy, this new mechanism is very exergonic, and the energy barriers are monotonically decreasing. The value of the energy barrier for the first step in this mechanism was substantially reduced when water molecules were explicitly introduced as a way to represent water-ice-coated grain surfaces.

Self‐catalytic mechanism of prebiotic reactions: From formamide to purine bases

“…Later, several theoretical papers on the mechanism of adenine, cytosine, thymine, uracil, and guanine formation under extraterrestrial nonaqueous scenarios have been also published. [ 16–19 ] Gupta et al [ 16 ] report results from B3LYP/6‐31G** calculations for the formation of adenine via successive reactions, starting from hydrogen cyanide, cyanocarbene, cyanamide, and cyanide radical. Тhe reaction path is found to be exothermic and barrier free.…”

“…Using CCSD(T)/6‐311++G(d,p) calculated energies of structures optimized at B3LYP/6‐311++G(d,p) level, Wang and Bowie [ 18 ] suggest a carbene intermediate based mechanism for the formation of cytosine, thymine, and uracil from cyanoacetylene and urea. Da Silva and de Araujo [ 19 ] presented a nine‐step mechanism for the formation of two isomers of guanine from diimide, ketenylidene, and hydrogen cyanide. The energy barriers in this mechanism, calculated at MP2/6‐311++G(2d,2p) level, are reduced when water molecules are explicitly involved as a way to represent water‐ice coated grain surfaces.…”

Gas Phase Reactivity of the CN Radical with Methyl Amines at Low Temperatures (23–297 K): A Combined Experimental and Theoretical Investigation

“…As a consequence of this observational activity, a large literature developed about the potential pathways and mechanisms leading to the production/destruction of biomolecules (or their precursors) either within the gas phase or at the analog interstellar ice and dust surfaces. Despite the significant efforts carried out using chemical modeling, − experiments, − or quantum theories − in order to identify the major mechanisms, the chemistry of nitrogen-bearing species is still poorly understood in the harsh and extreme conditions reigning in the ISM. This is mainly due to a serious lack of fundamental data such as the rate constants and the branching ratios for the various elementary chemical processes suspected to play a role in the formation of more complex molecules, especially at the very low temperatures (∼10 K) prevailing in molecular clouds.…”

Formation of Cytosine and Uracil from Cyanoacetylaldehyde and Guanidine: A Computational Study

“…Prebiotic syntheses of building blocks of protein, RNA, and DNA, such as amino acids and nucleobases, from simple molecules have been extensively studied since Miller's experiment. [1][2][3] Theoretical studies to elucidate the synthetic mechanisms have focused mainly on adenine [4][5][6][7][8][9][10][11] and glycine, [12][13][14][15] whereas those for the synthesis of guanine [16][17][18] and pyrimidines such as cytosine (Cyt), uracil (Ura), and thymine 11,19 have been limited. Miller et al synthesized pyrimidines by the reaction of guanidine (GD) hydrochloride with cyanoacetaldehyde (CAA).…”