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Polymorphism and electronic structure of polyimine and its potential significance for prebiotic chemistry on Titan

Rahm, Martin and Lunine, Jonathan and Usher, David and Shalloway, David (2016) Polymorphism and electronic structure of polyimine and its potential significance for prebiotic chemistry on Titan. Proceedings of the National Academy of Sciences of the United States of America, 113 (29). pp. 8121-8126. ISSN 0027-8424. PMCID PMC4961190. doi:10.1073/pnas.1606634113.

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The chemistry of hydrogen cyanide (HCN) is believed to be central to the origin of life question. Contradictions between Cassini–Huygens mission measurements of the atmosphere and the surface of Saturn’s moon Titan suggest that HCN-based polymers may have formed on the surface from products of atmospheric chemistry. This makes Titan a valuable “natural laboratory” for exploring potential nonterrestrial forms of prebiotic chemistry. We have used theoretical calculations to investigate the chain conformations of polyimine (pI), a polymer identified as one major component of polymerized HCN in laboratory experiments. Thanks to its flexible backbone, the polymer can exist in several different polymorphs, which are relatively close in energy. The electronic and structural variability among them is extraordinary. The band gap changes over a 3-eV range when moving from a planar sheet-like structure to increasingly coiled conformations. The primary photon absorption is predicted to occur in a window of relative transparency in Titan’s atmosphere, indicating that pI could be photochemically active and drive chemistry on the surface. The thermodynamics for adding and removing HCN from pI under Titan conditions suggests that such dynamics is plausible, provided that catalysis or photochemistry is available to sufficiently lower reaction barriers. We speculate that the directionality of pI’s intermolecular and intramolecular =N–H…N hydrogen bonds may drive the formation of partially ordered structures, some of which may synergize with photon absorption and act catalytically. Future detailed studies on proposed mechanisms and the solubility and density of the polymers will aid in the design of future missions to Titan.

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Lunine, Jonathan0000-0003-2279-4131
Additional Information:© 2016 National Academy of Sciences. Freely available online through the PNAS open access option. Contributed by Jonathan I. Lunine, May 20, 2016 (sent for review April 26, 2016; reviewed by Erich Karkoschka and Miklos Kertesz) We thank Roger Clark for useful comments on the observability of compounds on Titan’s surface and acknowledge valuable comments from Roald Hoffmann and discussions with participants of the “Don’t Follow (Just) the Water: Does Life Occur in Nonaqueous Media?” Workshop organized by the W. M. Keck Institute for Space Studies. This work was supported in its early stages by a grant from the John Templeton Foundation and in its final stages by NSF support from Grant CHE-1305872. Calculations presented in this work used the Extreme Science and Engineering Discovery Environment (49), which is supported by NSF Grant ACI-1053575
Group:Keck Institute for Space Studies
Issue or Number:29
PubMed Central ID:PMC4961190
Record Number:CaltechAUTHORS:20160831-133208376
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:70077
Deposited By: Iryna Chatila
Deposited On:07 Sep 2016 03:54
Last Modified:11 Nov 2021 04:24

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