Polymorphism and electronic structure of polyimine and its potential significance for prebiotic chemistry 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 S aturn's moon Titan is a carbon-rich, oxygen-poor world with a wide range of organic compounds, atmospheric energy sources, and alkane liquid seas-all measured by the remarkably successful Cassini-Huygens mission (1). The extreme cold puts liquid water out of reach-buried 50-100 km below a frigid ice crust (2). The lack of liquid water and presence of liquid hydrocarbons makes Titan a unique "natural laboratory" for exploring potential nonterrestrial forms of prebiotic chemistry or, more speculatively, biochemistry, whose essential biopolymers would differ profoundly from terrestrial ones (3). Regardless of the specific chemistry involved, life requires polymorphic molecules that combine flexibility with the ability to form the organized metastable structures needed for function, adaptation, and evolution. This, almost certainly, requires extended molecules capable of intermolecular and intramolecular hydrogen bonding, but such bonds need not involve oxygen; nitrogen is a potential surrogate. Although =N-H . . . N bonds are weaker than those involving oxygen, their energies are large compared with thermal energy (kT ∼ 0.18 kcal/mol at Titan's low temperature, 90-94 K) and intermolecular and intramolecular bonds need not compete with the strong -O-H . ...

Intuitive Understanding of σ Delocalization in Loose and σ Localization in Tight Helical Conformations of an Oligosilane Chain

“…In a separate publication, we will consider the application of the Ladder C model to an infinite chain in a more quantitative fashion and examine the range of parameters g and v ( ω ) within which the distinction between σ‐delocalized and σ‐localized behavior as a function of conformation applies. This work will build on prior work on helical structures in infinite solids …”

Atmospheric Circulation, Chemistry, and Infrared Spectra of Titan-like Exoplanets around Different Stellar Types

“…Given the relative abundance of M to G stars in the Galaxy, Titan may therefore represent a more prototypical world with a stable hydro-logic cycle than Earth (Lunine 2010). Titan-like exoplanets-cool terrestrial worlds with dense, hazy, and chemically complex atmosphereswould also be interesting subjects for studies of habitability, as Titan has been suggested to be an important target for astrobiology Lunine 2009Lunine , 2010Rahm et al 2016).…”

The Emergence of Life

“…Finally, even if such structures are present in Titan's lakes, it is still far from life, which requires replicating and energy-dissipating catalytic entities. Rahma et al (2016) showed by theoretical calculations that polyimines could drive photochemistry and potentially have a catalytic role on Titan's surface. Indeed, polyimine is one of the polymers of HCN.…”