Adenine Synthesis in Interstellar Space: Mechanisms of Prebiotic Pyrimidine-Ring Formation of Monocyclic HCN-Pentamers

Abstract

The question whether the nucleobases can be synthesized in interstellar space is of fundamental significance in considerations of the origin of life. Adenine is formally the HCN pentamer, and experiments have demonstrated that adenine is formed under certain conditions by HCN pentamerization in gas, liquid, and condensed phases. Most mechanistic proposals invoke the intermediacy of the HCN tetramer AICN (4), and it is thought that adenine synthesis is completed by addition of the 5(th) HCN to 4 to form amidine 5 and subsequent pyrimidine cyclization. In this context, we have been studying the mechanism for prebiotic pyrimidine-ring formation of monocyclic HCN-pentamers with ab initio electronic structure theory. The calculations model gas phase chemistry, and the results primarily inform discussions of adenine synthesis in interstellar space. Purine formation requires tautomerization of 5 to the conjugated amidine 6 (via hydrogen-tunneling, thermally with H(+) -catalysis, or by photolysis) or to keteneimine 7 (by photolysis). It was found that 5-(N'-formamidinyl)-1H-imidazole-4-carbonitrile (6) can serve as a substrate for proton-catalyzed purine formation under photolytic conditions and N-(4-(iminomethylene)-1H-imidazol-5(4H)-ylidene)formamidine (7) can serve as a substrate for uncatalyzed purine formation under photolytic conditions. The absence of any sizeable activation barrier for the cyclization of 7 to the (Z)-imino form of 9H-adenine (Z)-2 is quite remarkable, and it is this feature that allows for the formation of the purine skeleton from 7 without any further activation.

Chemical evolution: The mechanism of the formation of adenine under prebiotic conditions

“…During the review of this article, Glaser et al (32) proposed and analyzed mechanisms of pyrimidine-ring formation of isomeric monocyclic HCN pentamers. Pyrimidine-ring formation from a monocyclic HCN pentamer, either by proton-catalyzed or by uncatalyzed cyclization, was found not to be viable thermochemically.…”

Ammonia elimination from protonated nucleobases and related synthetic substrates

“…One can also envision the formation of 8h by direct NH 3 elimination from 63 (consider resonance form 63-B) via 68 or from 65 via 67 if 65 does not reclose to 16= [51]. These options depend on the accessibilities of 63 and 65.…”

Quantum-chemical approach to serine formation in the interstellar medium: A possible reaction pathway

“…Glaser (2007) reported that the DFT calculations are widely used in the chemistry community because it succeeds in reproducing the experimental data with high reliability. The geometries of the reactants, products, intermediates, and transition states were calculated using the DFT with the B3LYP functional with a 6 − 311G+(2df, 2p) basis set, and we improved the energy description even more by single-point calculations with hybrid meta exchange correlation DFT functionals M06 and M06 − 2X to calibrate the high computational cost and low-level calculations.…”