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 5th 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'-forinamidinyl)-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.

Department(s)

Chemistry

Keywords and Phrases

Adenine; Proton; Biogenesis; Biology; Chemical Model; Chemistry; Microclimate; Synthesis; Thermodynamics; Biogenesis; Exobiology; Extraterrestrial Environment; Models; Chemical; Protons; Thermodynamics; Ab Initio Theory; Adenine Synthesis; DNA; Gas Phase Chemistry; RNA

International Standard Serial Number (ISSN)

1531-1074; 1557-8070

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2007 Mary Ann Liebert, All rights reserved.

Publication Date

01 Jun 2007

PubMed ID

17630841

Link to Full Text

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