The lipid bilayer membrane, which is the foundation of life on Earth, is not viable outside of biology based on liquid water. This fact has caused astronomers who seek conditions suitable for life to search for exoplanets within the “habitable zone,” the narrow band in which liquid water can exist. However, can cell membranes be created and function at temperatures far below those at which water is a liquid? We take a step toward answering this question by proposing a new type of membrane, composed of small organic nitrogen compounds, that is capable of forming and functioning in liquid methane at cryogenic temperatures. Using molecular simulations, we demonstrate that these membranes in cryogenic solvent have an elasticity equal to that of lipid bilayers in water at room temperature. As a proof of concept, we also demonstrate that stable cryogenic membranes could arise from compounds observed in the atmosphere of Saturn’s moon, Titan, known for the existence of seas of liquid methane on its surface.
States of acrylonitrile.(A) Azotosome. Interlocking nitrogen and hydrogen atoms reinforce the structure. (B) Solid. Adjacent nitrogen atoms create some unfavorable repulsion. (C) Micelle. Adjacent nitrogen atoms make this highly unfavorable. (D) Azotosome vesicle of diameter 90 Å, the size of a small virus particle.
Terrestrial cell membranes are composed of a bilayer of phospholipids: surfactants composed of nonpolar lipid chains and oxygen-laden polar heads. The polar heads form surfaces compatible with water, allowing the membrane to separate the aqueous world outside and the aqueous life within. The lipid tails of the phospholipids aggregate by van der Waals forces, thus stabilizing the membrane. A vesicle made from such a membrane is known as a liposome.
The role of self-assembled surfactants in evolutionary biology on Earth raises the question of whether nonaqueous conditions can support any analogous structure.
In a cold world without oxygen, we suggest that the vesicles needed for compartmentalization, a key requirement for life, would be very different to those found on Earth. Rather than long-chain nonpolar molecules that form the prototypical terrestrial membrane in aqueous solution, we find membranes that form in liquid methane at cryogenic temperatures do so from the attraction between polar heads of short-chain molecules that are rich in nitrogen. We have termed such a membrane an azotosome. We find that the flexibility of such membranes is roughly the same as those of membranes formed in aqueous solutions. Despite the huge difference in temperatures between cryogenic azotosomes and room temperature terrestrial liposomes, which would make almost any molecular structure rigid, they exhibit surprisingly and excitingly similar responses to mechanical stress.
On the basis of our criteria of thermodynamic stability, or at least metastability, the azotosome appears to be a realizable cryogenic membrane. Starting from all the known molecular components in the atmosphere of such a world, Titan, we were able to select a couple of candidate molecules that were capable of exhibiting properties that appear to be important for vesicle formation. For example, an acrylonitrile azotosome has good thermodynamic stability, high energy barrier to decomposition, and area expansion modulus similar to that of phospholipid cell membranes in oxygen-rich solutions. Acrylonitrile exists in Titan’s atmosphere at a concentration of 10 ppm and could plausibly be formed on any celestial body with a nitrogen-methane atmosphere.
The availability of molecules with an ability to form cell membranes does not by itself demonstrate that life is possible. However, it does direct our search for exotic metabolic and reproductive chemistries that would be similarly compatible under cryogenic conditions. As our understanding of conditions that could nurture extraterrestrial life expands, so does our probability of finding it, perhaps within the liquid methane habitable zone.
SOURCES – Science Advances