Membrane alternatives in worlds without oxygen: Creation of an azotosome

Abstract

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.

Keywords: Titan; abiogenesis; computational chemistry; exobiology; liposome; molecular dynamics; physical chemistry; quantum chemistry.

Fig. 1. Liposomes and azotosomes.

(A) Liposome in polar solvent. Polar heads are braced by nonpolar lipid tails. (B) Azotosome in nonpolar solvent. Nonpolar tails are braced by polar nitrogen–rich heads.

Fig. 2. Stretching a hexanenitrile azotosome and a hexane bilayer.

The slope of the linear fit is proportional to the area modulus K_a.

Fig. 3. 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.

Fig. 4. Nitrogen head positions in selected azotosomes.

(A) Initial grid. (B) Aminopentane (amorphous). (C) Pentanenitrile (hexagonal). (D) Acrylonitrile (close packed hexagonal).

Fig. 5. Umbrella sampling of the azotosome decomposition process.

The test molecule is incrementally withdrawn from the membrane in the z direction.

Fig. 6. Stretching the azotosomes.

The slope of the fit line is proportional to the area modulus K_a.

Fig. 7. Potential energy profile for the decomposition of acrylonitrile.

The largest instantaneous energy barrier is the activation energy to decompose the azotosome.