NASA

Water plays unique role in supporting biomolecular and cellular self-organization

A number of biochemistries alternative to the terrestrial CHON biochemistry are possible in non-aqueous environments that might exist elsewhere in space.

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Evidence (1)

eDue to hydrophobic effects, water is unique in its ability to support self-organization of organic matter

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Due to hydrophobic effects, water is unique in its ability to support self-organization of organic matter

To form life, molecules have to self-organize to form ordered polymers supramolecular and cellular structures, such as folded proteins, double helices of nucleic acids or membranes. As far as we know, this is possible only in water because of hydrophobic effects. If other liquids are postulated as solvents for life, one needs to take special care to explain not only chemical synthesis in these liquids, but also self-organizing capabilities.

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Pohorille A,Pratt LR

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Molecular dynamics simulations suggest that membranes composed of acrylonitrile could form vesicle-like structures in liquid methane.

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eMembrane alternatives in worlds without oxygen: Creation of an azotosome

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Membrane alternatives in worlds without oxygen: Creation of an azotosome

Using molecular dynamics simulations with the Optimized Potential for Liquid Simulation (OPLS) force field, Stevenson et al. investigate the potential of organic compounds to form membrane-bounded compartments in liquid hydrocarbon solvents under cryogenic conditions. Several simulated nitrogen-containing compounds are found to be capable of self-assembly into vesicle-like structures, termed "azotosomes," that exhibit properties similar to those of phospholipid vesicles in aqueous room-temperature conditions.

Authors

James Stevenson, Jonathan Lunine, Paulette Clancy

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.

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eMembranes in liquid ammonia environments.

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Membranes in liquid ammonia environments.

As compared with water, ammonia’s increased ability to dissolve hydrophobic organic molecules suggests an increased difficulty in using the hydrophobic effect to generate compartmentalization in ammonia, relative to water. However, the formation of ammonia-phobic and ammonia-philic phases, which would be useful for compartmentalization, would be conceivable in liquid ammonia at temperatures well below its boiling point at standard pressures. (Baross, J.A. (Chair) The Limits of Organic Life in Planetary Systems. (The National Academies Press, Washington, DC, 2007).pp. 69-79.)

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Barros, J.A. (Chair)

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Non-aqueous media can extend the "habitable zone"

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Solvents other than water have different temperature ranges in which they exist in the liquid phase. This property has the potential of extending the “habitable zone” to distances other than that available to aqueous environments.

Chemical species in liquid phase over different temperature ranges than water.

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eAmmonia, Ammonia-water, Formamide, cryosolvents

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Ammonia, Ammonia-water, Formamide, cryosolvents

Non-aqueous media have the major advantage of extending the “habitable zone” for potential novel life forms (Baross, JA (Chair) The Limits of Organic Life in Planetary Systems. pp. 43-52 and 69-79 (The National Academies Press, Washington, DC, 2007)). Although ammonia is liquid at lower temperatures than water, the temperature range over which ammonia is liquid for relevant planetary surface pressures is greater than for water. Water-ammonia eutectics, which are liquid even at the temperature of Titan, are a potential biosolvent. Formamide (HCONH2), formed by the reaction of hydrogen cyanide with water, is another potential biosolvent. Like water, formamide has a large dipole moment and is an excellent solvent for almost anything that dissolves in water, including polyelectrolytes. In particular, formamide is able to dissolve RNA, DNA, and proteins, as well as their precursors. Formamide is not reactive like water. Indeed, many species that are thermodynamically unstable in water with respect to hydrolysis, are stable in formamide. Formamide is itself hydrolyzed by water, meaning that it persists only in a relatively dry environment, such as a desert. Cryosolvents (liquids at low temperatures) cannot be summarily dismissed as potential biosolvents. However, a major problem is the relative insolubility of substances in cryosolvents.

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Barros, J.A. (Chair)

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Liquid water necessary for life

Other liquids could be the medium for life

Background

Water plays unique role in supporting biomolecular and cellular self-organization

Non-aqueous media can extend the "habitable zone"