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Research Article

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

Martin Rahm, Jonathan I. Lunine, David A. Usher, and David Shalloway
  1. aDepartment of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853;
  2. bDepartment of Astronomy, Cornell University, Ithaca, NY 14853;
  3. cCarl Sagan Institute, Cornell University, Ithaca, NY 14853;
  4. dDepartment of Molecular Biology and Genetics, Cornell University, Ithaca, NY 14853

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PNAS first published July 5, 2016; https://doi.org/10.1073/pnas.1606634113
Martin Rahm
aDepartment of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853;
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  • For correspondence: jlunine@astro.cornell.edu martinr@kth.se
Jonathan I. Lunine
bDepartment of Astronomy, Cornell University, Ithaca, NY 14853;
cCarl Sagan Institute, Cornell University, Ithaca, NY 14853;
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  • For correspondence: jlunine@astro.cornell.edu martinr@kth.se
David A. Usher
aDepartment of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853;
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David Shalloway
dDepartment of Molecular Biology and Genetics, Cornell University, Ithaca, NY 14853
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  1. Contributed by Jonathan I. Lunine, May 20, 2016 (sent for review April 26, 2016; reviewed by Erich Karkoschka and Miklos Kertesz)

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Significance

Titan is the only place in the solar system, except Earth, where rainfall and seasonally flowing liquids erode the landscape. Whereas the surface pressure is similar to that of Earth, the temperature is extremely low and the dominant liquids are methane and ethane. This makes Titan a test case for exploring the environmental limits of prebiotic chemistry and addressing the question of whether life can develop without water. Experimental and observational data suggest that hydrogen cyanide, the most abundant hydrogen-bonding molecule in Titan’s atmosphere, may polymerize on the surface to polyimine. Using quantum mechanical calculations, we show that polyimine has interesting electronic and structural properties that could potentially facilitate prebiotic chemistry under cryogenic conditions akin to those on Titan.

Abstract

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.

  • hydrogen cyanide
  • prebiotic chemistry
  • Titan
  • polymers

Footnotes

  • ↵1To whom correspondence may be addressed. Email: jlunine{at}astro.cornell.edu or martinr{at}kth.se.
  • Author contributions: M.R., J.I.L., and D.S. designed research; M.R. and D.S. performed research; J.I.L. and D.A.U. analyzed data; and M.R., J.I.L., D.A.U. and D.S. wrote the paper.

  • Reviewers: E.K., University of Arizona; and M.K., Georgetown University.

  • The authors declare no conflict of interest.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1606634113/-/DCSupplemental.

Freely available online through the PNAS open access option.

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Polyimine on Titan
Martin Rahm, Jonathan I. Lunine, David A. Usher, David Shalloway
Proceedings of the National Academy of Sciences Jul 2016, 201606634; DOI: 10.1073/pnas.1606634113

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Polyimine on Titan
Martin Rahm, Jonathan I. Lunine, David A. Usher, David Shalloway
Proceedings of the National Academy of Sciences Jul 2016, 201606634; DOI: 10.1073/pnas.1606634113
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